Opticall~y Stimulated Luminescence Dosimetry
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Optically Stimulated Luminescence Dosimetry
L. Botter-Jensen Riso National Laboratory Radiation Research Department DK-4000 Roskilde Denmark
S. W. S. McKeever Department of Physics Oklahoma State University Stillwater, OK 74078-0444 USA
A. G. Wintle Institute of Geography and Earth Sciences University of Wales Aberystwyth, SY23 3DB UK
2003
ELSEVIER Amsterdam - Boston - Heidelberg - London - New York - Oxford P a r i s - San D i e g o - San F r a n c i s c o - S i n g a p o r e - S y d n e y - T o k y o
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Dedicated to our spouses and families, who supported us and put up with us during this long process:
To Marja, Malene, Kristine and Caroline To Joan, Katie and Alison To Jack
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TABLE OF C O N T E N T S
PREFACE
.................................................................
C H A P T E R 1: I N T R O D U C T I O N .......................................... 1.1 Optically s t i m u l a t e d l u m i n e s c e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 H i s t o r i c a l d e v e l o p m e n t o f O S L d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 O S L d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Personal dosimetry ............................................... 1.3.2 E n v i r o n m e n t a l d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 M e d i c a l d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 R e t r o s p e c t i v e d o s i m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 This b o o k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C H A P T E R 2:
OPTICALLY STIMULATED LUMINESCENCE THEORY ..................................................
1 1
2 5 7 9 9 9 11
15
2.1
Stimulated luminescence
2.2
G e n e r a l i s e d m a t h e m a t i c a l d e s c r i p t i o n o f optically s t i m u l a t e d l um i ne sc e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T h e p h o t o i o n i s a t i o n cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 O p t i c a l transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 W a v e l e n g t h dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19 19 21
2.3.3 M e a s u r e m e n t o f the p h o t o i o n i s a t i o n cross-section . . . . . . . . . . . . . . . . . CW-OSL .............................................................. 2.4.1 M o d e l s a n d rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 The one-trap/one-centre m o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 M o d e l s containing m u l t i p l e - t r a p s a n d centres . . . . . . . . . . . . . . . . . . . . . 2.4.4 A m o r e g e n e r a l i s e d m o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 T e m p e r a t u r e dependence effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 T h e r m a l quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LM-OSL .............................................................. 2.5.1 First- a n d general-order-kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Relationship between L M - O S L and C W - O S L .................... 2.5.3 W a v e l e n g t h dependence o f L M - O S L ............................. 2.5.4 P h o t o c o n d u c t i v i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23 27 27 27 30 34 37 44 47 47 52 52 54
2.3
2.4
2.5
...............................................
xv
15 17
Table o f Contents
viii 2.6
2.7
2.8
Pulsed OSL
56
2.6.1
............................................................ Principles o f pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Delayed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P h o t o t r a n s f e r r e d effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M a t h e m a t i c a l description and typical data . . . . . . . . . . . . . . . . . . . . . . . . Radiophotoluminescence ...............................................
2.7.1
60
2.7.2
61 65
2.8.1
Procedure
C H A P T E R 3: 3.1
3.2
3.3
59 60
......................................................
O S L P R O P E R T I E S OF S Y N T H E T I C M A T E R I A L S
65
......
71 71
3.1.2 Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 O S L stimulation and emission characteristics o f A l 2 0 s C ......... 3.1.4 The O S L response o f A l 2 0 3 : C to radiation exposure . . . . . . . . . . . . . . 3.1.5 The temperature dependence o f O S L f r o m A I 2 0 s C . . . . . . . . . . . . . . . 3.1.6 Zeroing o f the O S L signal f r o m A I 2 0 3 : C . . . . . . . . . . . . . . . . . . . . . . . . . Halides ................................................................ 3.2.1 KCl ............................................................ 3.2.2 KBr ............................................................
71 73 75 77 79 81 81 82
NaCl ........................................................... RbI ............................................................ 3.2.5 CaF2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 B a F X ( X = Br, Cl, I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphates ..............................................................
3.2.3
84
3.2.4
85
MgS04 ......................................................... GAS04 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 A S ( A = M g , Sr, Ca, Ba) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 BeO ............................................................ 3.5.2 Fused quartz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 3.3.2
3.4 3.5
C H A P T E R 4: 4.1
71
A1203:C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PASSIVE OPTICALLY STIMULATED LUMINESCENCE DOSIMETRY .............................................
Personal dosimetry
...................................................
86 87 90 90 90 90 90 92 92 95
101 101
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Landauer's L u x e U M personal dosimetry system . . . . . . . . . . . . . . . . . . Landauer's InLight T M personal dosimetry system . . . . . . . . . . . . . . . . .
101
4.1.2 4.1.3 4.1.4 4.1.5
Beta dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P O S L imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
4.1.1
102 102 104
Table o f Contents 4.2
Environmental 4.2.1 4.2.2
OSL dosimetry using A1203:C
ix .........................
107
M e a s u r e m e n t o f the natural terrestrial background radiation . . . . . Measurement o f the natural space background radiation . . . . . . . . .
110
4.3
UV dosimetry
4.4
O S L a n d R L r e m o t e optical fibre d o s i m e t r y in m e d i c a l a p p l i c a t i o n s 4.4.1 4.4.2
........................................................
5.1.1 5.1.2
5.1.3
5.1.4
5.1.5
5.1.6
....
Real-time ( R T ) in vivo monitoring o f doses during radiotherapy .. Opticalfibre dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C H A P T E R 5: O S L P R O P E R T I E S O F N A T U R A L M A T E R I A L S 5.1 Quartz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
107 107
........
Crystal structure and point defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decay curve shapes obtained under continuous s t i m u l a t i o n - CW-OSL ...................................................... 5.1.2.1 Stimulation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.2 Effect o f the l l O ~ trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.3 Dependence on power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.4 Three components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.5 Effect o f stimulation wavelength . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.6 Effect o f stimulation temperature . . . . . . . . . . . . . . . . . . . . . . Linear modulation O S L - - L M - O S L ............................. 5.1.3.1 L M - O S L at 160~ with 470 nm stimulation . . . . . . . . . . . . 5.1.3.2 L M - O S L at different temperatures with 526 nm stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3.3 L M - O S L f r o m single grains using 532 nm . . . . . . . . . . . . . Pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4.1 Time resolved luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4.2 Delayed optically stimulated luminescence or optically stimulated afterglow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Excitation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.1 Bleaching response spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.2 Excitation spectra after bleaching by 514 + 25 nm light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5.3 Continuous scanning o f stimulation wavelengths . . . . . . . . 5.1.5.4 Excitation using interference filters and xenon lamp . . . . 5.1.5.5 Excitation using laser lines f r o m 458 to 645 nm . . . . . . . . 5.1.5.6 Stimulation in the infra-red 7 8 0 - 9 2 0 nm . . . . . . . . . . . . . . . Emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.1 O S L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2 T L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2.1 3 6 0 - 4 2 0 nm (near U V to violet) . . . . . . . . . . . 5.1.6.2.2 4 2 0 - 4 9 0 nm (blue) . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6.2.3 5 9 0 - 6 5 0 nm (orange-red) . . . . . . . . . . . . . . . . . . 5.1.6.3 Radioluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
112 112 112
119 119
119 123 123 123 125 126 127 130 130 130
135 135 136 137 140
141 141 143 143 145 147 147 149 149 150 150 153 153
155
x
Table o f Contents 5.1.7
Dose dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1 Fast component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.1 Multiple aliquot data . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.2 Single aliquot data . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.1.3 Single grain data . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7.2 Low doses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8 Effects o f previous thermal treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.1 High temperature annealing--above 500~ ............ 5.1.8.1.1 Comparison o f L M - O S L , TL, R L and E P R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.1.2 C W - O S L growth curves after annealing . . . . . 5.1.8.2 Low temperature annealing--160 to 280~ ............ 5.1.8.3 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.3.1 Isothermal decay . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.3.2 Pulse annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.8.4 Irradiation at elevated temperatures . . . . . . . . . . . . . . . . . . . 5.1.8.5 Thermal transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9 Raised temperature OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9.1 Thermal quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.9.2 Thermal assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10 The slow component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.1 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.2 Growth curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.3 Optical bleaching . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.10.4 TRL ...................................... 5.1.11 Photoionisation cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.12 Modelling processes giving rise to OSL in quartz . . . . . . . . . . . . . . . . 5.1.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Feldspars 5.2.1 5.2.2
5.2.3 5.2.4
5.2.5
.............................................................
157 157 157 159 160 160 162 162
162 165 167 169 169
170 173 174 177 177 179 180
181 183 184 184 184 186 188 188
Crystalstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Decay curve shape obtained under continuous stimulation--CW-OSL and C W - I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.1 Stimulation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2 Effect o f stimulation temperature . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2.1 Initial part o f signal . . . . . . . . . . . . . . . . . . . . . . . . 189 5.2.2.2.2 Decay curve shape . . . . . . . . . . . . . . . . . . . . . . . . . 194 Linear modulation I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Pulsed O S L and I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.1 Pulsed O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.2 Pulsed I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.2.4.3 Optically stimulated afterglow . . . . . . . . . . . . . . . . . . . . . . . . . 197 Excitation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 5.2.5.1 Direct measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
xi
Table of Contents
5.2.6
5.2.7
5.2.5.2 Bleaching response spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . Emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.1 I R S L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.1.1 2 8 0 - 2 9 0 n m (near U V ) . . . . . . . . . . . . . . . . . . . . 5.2.6.1.2 3 2 0 - 3 4 0 n m (near U V ) . . . . . . . . . . . . . . . . . . . . 5.2.6.1.3 3 9 0 - 4 4 0 n m (violet/blue) . . . . . . . . . . . . . . . . . . 5.2.6.1.4 5 5 0 - 5 7 0 n m (yellow~green) . . . . . . . . . . . . . . . . 5.2.6.1.5 6 0 0 - 7 5 0 n m (red/far red) . . . . . . . . . . . . . . . . . . 5.2.6.2 T L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3 R L emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.1 Under X - r a y stimulation at low temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.2 Under X - r a y stimulation above room temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6.3.3 Under beta stimulation f r o m a lSTCs source 5.2.6.4 Photoluminescence emission spectra . . . . . . . . . . . . . . . . . . . Effects o f previous optical treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.7.1 Bleaching at ambient temperature . . . . . . . . . . . . . . . . . . . . . 5.2.7.2 I R bleaching at elevated temperature . . . . . . . . . . . . . . . . . .
5.2.8
Effects o f previous thermal treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.1 Pre-heating o f laboratory and naturally irradiated samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.2 Pulse annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.8.3 Irradiation at elevated temperature . . . . . . . . . . . . . . . . . . . . 5.2.9 R a i s e d temperature I R S L and O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.1 T h e r m a l quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.2 T h e r m a l assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.9.2.1 A b o v e room temperature . . . . . . . . . . . . . . . . . . . 5.2.9.2.2 Below room temperature . . . . . . . . . . . . . . . . . . . 5.2.9.2.3 Wavelength dependence . . . . . . . . . . . . . . . . . . . . 5.2.9.2.4 L i n k to anomalous f a d i n g . . . . . . . . . . . . . . . . . . 5.2.10 A n o m a l o u s f a d i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.10.1 T L , O S L and I R S L .................................. 5.2.10.2 A t t e m p t s to remove anomalous f a d i n g . . . . . . . . . . . . . . . . . 5.2.10.2.1 Using a p r e h e a t . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.10.2.2 Using an optical treatment . . . . . . . . . . . . . . . . . 5.2.10.3 A t t e m p t s to avoid anomalous f a d i n g . . . . . . . . . . . . . . . . . . . 5.2.10.3.1 Using time-resolved m e a s u r e m e n t s . . . . . . . . . . 5.2.10.3.2 Using different detection wavelengths . . . . . . . 5.2.10.4 5.2.10.5
C L and T L spectra o f f a d i n g f e l d s p a r s . . . . . . . . . . . . . . . . . L o w temperature phosphorescence . . . . . . . . . . . . . . . . . . . . .
5.2.10.6 5.2.10.7
Single grain I R S L f a d i n g and f a d i a plots . . . . . . . . . . . . . . . L o g a r i t h m i c signal decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
201 201 201 201 202 203 203 203 203 203 203
.
205 205 205 207 207 208 211 211 212
215 215 215 216 216 216 217 218 219 219
219 219 220 220 220 220 220 221 223 224
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Table of Contents 5.2.10.8 Correcting f o r anomalous f a d i n g . . . . . . . . . . . . . . . . . . . . . . . 5.2.11 Radioluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.1 A new dating m e t h o d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.2 Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.3 M e t h o d s o f De determination . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.4 Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.11.5 Single grain m e a s u r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12 M o d e l s f o r I R S L , O S L , I R - R L in feldspars . . . . . . . . . . . . . . . . . . . . . 5.2.12.1 I R S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.2 O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.3 I R - R L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.12.4 Comparison o f l R - R L and I R S L (or O S L ) . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER
Part 6.1 6.2 6.3 6.4
6:
RETROSPECTIVE
OSL DOSIMETRY
..................
I: R E T R O S P E C T I V E A C C I D E N T D O S I M E T R Y . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample preparation and experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of the accident dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Retrospective assessment o f environmental dose rates . . . . . . . . . . . . 6.4.2 E stimati o n o f the accident dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Analytical protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Multiple-aliquot protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3 The single aliquot regeneration and added dose p r o t o co l . . . . . . . . . . 6.5.4 True single-aliquot protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4.1 Introduction ......................................... 6.5.4.2 Variation o f O S L signal with pre-heat . . . . . . . . . . . . . . . . . . 6.5.4.3 Choice o f O S L signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4.4 Sensitivity changes with regeneration cycles . . . . . . . . . . . . 6.5.4.5 The S A R protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Evaluation of dose-depth profiles in bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Continuous O S L scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 Determination o f d o s e - d e p t h profiles f r o m Chernobyl bricks . . . . . 6.6.3 Absolute errors and e s t i m a t e d precision o f the equivalent dose in bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Retrospective OSL dosimetry using unheated quartz . . . . . . . . . . . . . . . . . . . 6.7.1 Dose distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7.2 T h e r m a l transfer and sensitivity changes . . . . . . . . . . . . . . . . . . . . . . . .
224 227 227 229 229 229 229 230 230 231 231 233 234
245
245 245 246 247 247 247 249 250 250 250 250 252 252 253 253 255 255 257 258 259 259 260 261 263
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6.8 6.9
Retrospective OSL dosimetry using household and workplace chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Retrospective OSL dosimetry using porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.2 The origin o f O S L in porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9.2.1 Time-decaying dose-dependent O S L signals . . . . . . . . . . . . 6.9.2.2 Time-steady P L emission spectra f r o m porcelain . . . . . . . 6.9.2.3 O S L stimulation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O S L dose response o f porcelain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.9.3 6.9.4 D o s e - d e p t h profiles in porcelain and the effect o f transparency 6.9.5 O S L dosimetry using porcelain dental crowns . . . . . . . . . . . . . . . . . . . . 6.10 Retrospective accident dosimetry--conclusions . . . . . . . . . . . . . . . . . . . . . . . .
...
Part II: G E O L O G I C A L A N D A R C H A E O L O G I C A L D A T I N G . . . . . . . . . . . 6.11 M e a s u r e m e n t procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.1 Multiple-aliquot m e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2 Single-aliquot m e t h o d s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1 Feldspars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1.1 Additive dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.1.2 Regenerative dose . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2 Q u a r t z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2.1 Additive dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.2.2 Regenerative dose . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.3 L u m i n e s c e n c e sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.2.4 Reliability o f O S L monitoring o f sensitivity change . . . . . 6.11.3 Dose distributions f o r single aliquots . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.3.1 H i s t o g r a m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.3.2 6.11.3.3 6.11.3.4
Probability density plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radial plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calculation o f De . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.12 Single grains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1 M e a s u r e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1.1 Feldspars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.1.2 Q u a r t z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2 Dose distributions f o r single grains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.1 H i s t o g r a m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.2 Probability density plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.3 Radial plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12.2.4
Calculation o f De . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.13 Geological and archaeological d a t i n g - c o n c l u s i o n s
.....................
265 267 267 267 267 270 271 271 272 273 275
276 276 277 280 280 281 281 281 281 285 287 291 293 293 295 296 297 298 298 298 299 299 299 300 300 301 302
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7: O S L M E A S U R E M E N T T E C H N O L O G Y ................. Stimulation modes .................................................... 7.1.1 CW-OSL ...................................................... 7.1.2 L M - O S L ...................................................... 7.1.3 POSL ......................................................... 7.2 T h e light de te c t i o n system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 P h o t o m u l t i p l i e r tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 I m a g i n g p h o t o n d e t e c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Solid-state detectors ............................................ 7.3 A u t o m a t e d O S L r e a d e r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 D e v e l o p m e n t o f optical s t i m u l a t i o n sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Laser stimulation .............................................. 7.4.2 I R L E D s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.3 I R laser diode s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.4 B r o a d - b a n d light s t i m u l a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.5 Optimisation of OSL detection .................................. 7.4.6 Green L E D stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.7 Blue LED stimulation .......................................... 7.4.8 Blue LED and cut-off filter characteristics ....................... 7.4.9 R a m p i n g the L E D s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.10 P u l s e d a n d t i m e - r e s o l v e d O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 W a v e l e n g t h resolved O S L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Stimulation spectrometry ....................................... 7.5.2 E m i s s i o n s p e c t r o m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 I m a g i n g systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Single gr a in O S L systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.1 Introduction ................................................... 7.7.2 CCD luminescence imaging systems ............................. 7.7.3 S i n g l e g r a i n laser O S L s y s t e m s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 O S L scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9 P o r t a b l e systems for O S L m e a s u r e m e n t s in the field . . . . . . . . . . . . . . . . . . . 7.10 T h e m e a s u r e m e n t o f R L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11 C o m m e r c i a l l y available O S L a p p a r a t u s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.12 F u t u r e d e v e l o p m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
320 321 323 325 325 326 330 330 332 334 334 334 335 335 338 340 340 343 345
SUBJECT
351
CHAPTER
7.1
INDEX
........................................................
311 311 311 311 311 312 312 313 314
315 316 316 316 317 318
PREFACE
Optically stimulated luminescence (OSL) has become the technique of choice for many areas of radiation dosimetry. The technique is finding widespread application in a variety of radiation dosimetry fields, including personal monitoring, environmental monitoring, retrospective dosimetry (including geological dating and accident dosimetry), space dosimetry, and many more. In this book we have attempted to synthesise the major advances in the field, covering both fundamental understanding and the many applications. The latter serve to demonstrate the success and popularity of OSL as a dosimetry method. The book is designed for researchers and radiation dosimetry practitioners alike. Chapter 1 sets the stage with an overview of the process and its uses. Chapter 2 then delves into the detailed theory of the process from the point of view of stimulated relaxation phenomena, describing the energy storage and release processes phenomenologically and developing detailed mathematical descriptions to enable a quantitative understanding of the observed phenomena. The various stimulation modes (continuous wave, pulsed, or linear modulation) are introduced and compared. Chapter 3 discusses the most important synthetic OSL materials beginning with the dominant carbon-doped A1203, and moving through discussions of other, less-well studied but nevertheless important, or potentially important, materials. Chapter 4 is the first of the applications chapters and deals with the use of OSL from synthetic materials in personal, environmental, medical and UV dosimetry. Chapter 5 discusses in detail the OSL properties of the two most important natural OSL dosimetry material types, namely quartz and feldspars. These discussions originate primarily from the use of these materials in geological dating and this leads naturally into Chapter 6, dealing with all aspects of retrospective dosimetry. The division of retrospective dosimetry into accident dosimetry (Part I) and dating (Part II) is a natural one, and the inclusion of both parts under one chapter heading is appropriate since the detailed methodologies are similar in many respects, with many advances in one field being transferred to the other. Finally, Chapter 7 gives the reader an overview of the developments in instrumentation that have occurred over the past decade or more. These instrumentation developments have themselves led to new experimental methodologies, particularly in the field of geological dating where the ability to analyze large numbers of small sample aliquots, and even single
xvi
Preface
grains, has led to new capabilities and possibilities undreamt of at the beginning of OSL dosimetry. We hope that the book will find use in those laboratories within academia, national institutes and the private sector where research and applications in radiation dosimetry using luminescence are being conducted. Potential readers include personnel involved in radiation protection practice and research, hospitals, nuclear power stations, radiation clean-up and remediation, food irradiation and materials processing, security monitoring, geological and archaeological dating, luminescence studies of minerals, etc. We are grateful to the various authors (as indicated in the figure captions) and the following publishers for kind permission to reproduce copyrighted or trade-marked material (in alphabetical order): the American Institute of Physics (for figures 2.5, 2.9, 2.21, 2.25, 2.26), Ancient TL (for figure 6.40), Blackwell Publishing (for figures 5.29 and 6.41), Taylor and Francis (for figure 7.15), Elsevier, Geologos (for figure 5.32a), the Institute of Physics (for figures 5.10, 5.13a, 5.34, 5.35a,b, 5.41, 5.68 and 5.88), the International Atomic Energy Agency (for figures 4.11, 4.12 and 4.13), Landauer Inc. (for figures 4.1(c) and 4.2), the National Research Council of Canada (for figures 5.46, 5.54 and 5.84), the Nature Publishing Group (for figure 5.30), Nuclear Technology Publishing (for figures 1.4, 2.29, 3.3, 3.4, 3.6-3.10, 3.12, 3.15a,b, 3.16, 3.20, 3.21, 4.6, 4.7, 4.9, 5.57, 5.60, 5.71, 5.73, 5.74, 5.83, 6.2, 6.6-6.8, 6.11-6.15, 6.17-6.21, 6.35, 6.36, 6.38, 7.3, 7.6a, 7.7-7.9, 7.13 and 7.17), and Springer-Verlag (for figures 2.16, 5.1, 5.58 and 5.78). We each thank our respective institutions for allowing us the time and facilities to work on the book (Riso National Laboratory, Denmark; The University of Wales, Aberystwyth, UK; Oklahoma State University, USA) and one of us (AGW) also thanks the Swedish Natural Science Research Council for funding a six month visiting professorship at the University of Uppsala during the book's preparation. No work of this size takes place in isolation and particular thanks need to go to several individuals. First among these is our long-suffering friend and colleague, Finn Jorgensen, who with professionalism, infinite patience and a permanent smile drew, re-drew and re-re-drew countless numbers of figures. Similar humble thanks are due to Antony Smith for skill and patience in rescuing several of our ill-copied figures and transforming them into works of graphic art. Others whose friendship was stretched beyond the bounds of decency include several of our colleagues who read sections and chapters of the text at various stages of completion, and with grace, tact and delicacy pointed out our numerous errors. All remaining deficiencies in the book are
Preface
xvii
ours and ours alone. Finally, the three authors wish to thank their students and professional colleagues from all over the globe who have enriched our research, and improved our understanding with insight, originality and common sense. We are in debt to the whole community.
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Chapter 1
Introduction 1.1. Optically stimulated luminescence Optically stimulated luminescence (OSL) is the luminescence emitted from an irradiated insulator or semiconductor during exposure to light. The OSL intensity is a function of the dose of radiation absorbed by the sample and thus can be used as the basis of a radiation dosimetry method. The process begins with irradiation causing ionisation of valence electrons and the creation of electron/hole pairs. Pre-existing defects within the material then localise the free electrons and holes through non-radiative trapping transitions. Subsequent illumination of the irradiated sample with light leads to absorption of energy by the trapped electrons and transitions from the localised trap into the delocalised conduction band. Recombination of the freed electrons with the localised holes results in radiative emission and luminescence. This is the OSL signal, the intensity of which is proportional to the dose of absorbed radiation. OSL signals are often accompanied by photoconductivity phenomena. OSL is not to be confused with the related phenomenon of photoluminescence (PL) that can be stimulated from similar materials, but which is generally not dependent upon irradiation of the sample. PL is the excitation, via the absorption of light, of an electron in a crystal defect within the material, resulting in excitation of the electron from the defect' s ground state to an excited state. Relaxation back to the ground state results in the emission of luminescence, the intensity of which is proportional to the concentration of excited defects. Ionisation of the electron from the defect (i.e., a transition from a localised to a de-localised state) does not generally occur, however, and there is no associated photoconductivity. As a consequence of the above mechanism, the wavelength of the emitted luminescence is longer than that of the excitation light (Stokes' shift). Exceptions to that latter rule (the so-called "anti-Stokes" phosphors) may be found in which energy transfer mechanisms dominate. If the defect being excited is itself created by irradiation of the sample, a PL signal that is dependent on absorbed dose may be obtained. This is termed radiophotoluminescence (RPL) and the RPL signal may be utilised in dosimetry, but the mechanism is PL, not OSL. OSL is one of a class of measurements known as stimulated phenomena. Such phenomena may be stimulated thermally (thermally stimulated phenomena or TSP) or optically (optically stimulated phenomena or OSP). TSP include thermoluminescence (TL), thermally stimulated conductivity (TSC), thermally stimulated exo-electron emission (TSEE), thermally stimulated capacitance (TSCap), deep level transient spectroscopy (DLTS), thermogravimetry (TG), differential thermal analysis (DTA)
Optically Stimulated Luminescence Dosimetry
Fig. 1.1. Schematic representation of several popular thermally and optically stimulated phenomena. Capacitance techniques (DLTS and TSCap) measure signals proportional to the concentration of charges when they reside in the traps. Conductivity techniques (TSC and PC) monitor the charges after release from the traps as they transit through the conduction band. Luminescence techniques (TL and OSL) monitor the charges as they undergo radiative recombination with charge of the opposite sign. Exo-electron processes (TSEE and OSEE) monitor the charges if they are emitted from the surface of the material. Although not the same type of stimulated phenomenon, PL is also indicated.
and others. Likewise, OSP include OSL, photoconductivity (PC) and optically stimulated exo-electron emission (OSEE). The relationship between these different phenomena is illustrated in Fig. 1.1 using a schematic energy band diagram. The reader is referred to works by Br/iunlich (1979), Chen and Kirsh (1981) and Chen and McKeever (1997) for general texts on TSPs and related phenomena. See McKeever (1998, 2001) for reviews of OSL and its use in dosimetry.
1.2. Historical development of OSL dosimetry In recent years, OSL has become a popular procedure for the determination of environmental radiation doses absorbed by archaeological and geological materials in an attempt to date those materials. In this procedure, the target samples (usually natural grains of quartz and/or feldspar) are exposed in the laboratory to a steady source of light of appropriate wavelength and intensity, and the luminescence stimulated from the mineral during this procedure is monitored as a function of the stimulation time. The integral of the luminescence emitted during the stimulation period is a measure of the dose of radiation absorbed by the mineral since it was last exposed to light. Through calibration of the signals against known doses of radiation, the absorbed dose can be obtained and through
Introduction
3
a separate determination of the environmental dose rate, the age of the sample can be determined. Huntley et al. (1985) first used the method, now known as "continuouswave-OSL" (CW-OSL), for this purpose and the latest developments in this field have been described in the triennial conferences on luminescence and ESR dating (Fain et al., 1991; Bailiff et al., 1994; McKeever, 1997, 2000). The first OSL measurements on quartz and feldspar were made using an argon ion laser (Huntley et al., 1985). However, the development of cheaper stimulation systems based, first on filtered lamps, and then on light emitting diodes (LEDs), have led to a massive expansion in dating applications. Feldspars, particularly sand-sized potassium-rich feldspars that could be isolated using heavy liquids, were the first to be investigated. Htitt et al. (1988) showed that luminescence signals could be stimulated from feldspars using near infra-red wavelengths around 880 nm, where a resonance in the stimulation spectrum had been observed. This led to the measurement of infra-red stimulated luminescence (IRSL) using clusters of inexpensive diodes (Spooner et al., 1990). Green light from filtered halogen lamps was used for quartz (BCtter-Jensen and Duller, 1992) until sufficiently powerful blue (470 nm) LEDs became available (BCtter-Jensen et al., 1999b). Since diodes can be used to give short stimulation pulses, and have far longer working lives than the lamps, it was possible to construct laboratory procedures to determine the equivalent dose (De) for single aliquots of sample. Duller (1991) developed an additive dose method for feldspars and this has been widely adopted. A similar procedure was developed for quartz using the filtered lamp system (Murray et al., 1997). More recently, following a five-year study of the OSL properties of quartz, Murray and Wintle (2000) developed the single aliquot regenerative dose (SAR) protocol that has been used in both dating and accident dosimetry. In this method, the sensitivity of all OSL measurements used to obtain De is monitored by the OSL response to a test dose. For sedimentary quartz, the method has been shown to be reliable by the accurate dating of 50 samples, for which there is independent age information (Murray and Olley, 2002). The SAR protocol has now been used for single quartz grains (Duller et al., 2000) when stimulated using a focussed solid-state laser as the stimulation source (Duller et al., 1999). This has opened up a whole new level of investigation for sedimentary deposits (Duller and Murray, 2000). The use of OSL as a personal dosimetry technique, however, is not yet so widespread, despite the fact that its use in this field has a much longer genesis. It was first suggested for this application several decades ago by Antonov-Romanovskii et al. (1956) and was later used by Br~iunlich et al. (1967) and Sanborn and Beard (1967). Since these early developments, however, the use of OSL in radiation dosimetry has not been extensively reported, perhaps due to the lack of a good luminescent material, which was both highly sensitive to radiation, and had a high optical stimulation efficiency, a low effective atomic number and good fading characteristics (i.e., a stable luminescence signal at room temperature). MgS, CaS, SrS and SrSe doped with different rare earth elements such as Ce, Sm and Eu were among the first phosphors suggested for OSL dosimetry applications (Br~iunlich et al., 1967; Sanborn and Beard, 1967; Rao et al., 1984). They possess a high sensitivity to radiation and a high efficiency under infra-red stimulation at a wavelength around 1 I~m, but they suffer from significant fading of the luminescence at room temperature. These phosphors also have a very high effective atomic number and, as a
Optically Stimulated Luminescence Dosimetry
result, exhibit strong photon energy dependence, which is unacceptable for use in personal dosimetry. Several research groups have tried to use optical stimulation as a dosimetric tool by using light to transfer trapped charge carriers from deep traps to shallow traps and then monitoring the phosphorescence at room temperature as the charge leaked away from the shallow traps. This approach was suggested for fast neutron dosimetry for which one can mix the phosphor with polyethylene to measure the absorbed dose from recoil protons and perform the luminescence measurements at room temperature. Several phosphors such as BeO (Tochilin et al., 1969; Rhyner and Miller, 1970), CaF2:Mn (Bernhardt and Herforth, 1974) and CaSO4:Dy (Pradhan and Ayyanger, 1977; Pradhan and Bhatt, 1981) were used in this mode but they each exhibited relatively low sensitivity. This OSL readout mode is often described as "Delayed" OSL (DOSL) (Yoder and Salasky, 1997). A new modification, called pulsed OSL (POSL), was introduced by McKeever, Akselrod and colleagues (Markey et al., 1995; McKeever et al., 1996; Akselrod and McKeever, 1999) using crystalline A1203:C as the luminescent material. Here, one exposes irradiated A1203:C to a pulsed light source and synchronously detects the emitted luminescence between pulses, but not during the pulse. This synchronous arrangement allows one to use less optical filtration than with CW-OSL, which is used in the latter method to discriminate between the stimulation light and the luminescence. At the same time, the POSL method allows one to bias against the slow phosphorescence processes, which make up the main signal in DOSL measurements. These features grant the POSL technique both a high sensitivity and weaker temperature dependence compared with the DOSL method. The high sensitivity and rapid readout features also allow use of the method for imaging the distribution of the dose over large area detectors (Akselrod et al., 2000). Some authors use the fact that irradiation of the detector material induces stable radiation-induced defects, and subsequent illumination of the sample with light stimulates PL from those defects. The emission is termed "radiophotoluminescence" (RPL), and the intensity is proportional to the absorbed dose. This approach is significantly different from the other OSL methods as the stimulation with light does not result in the ionisation of the defect, but only in its excitation. Thus, the dose can be read multiple times without destroying the signal. Disadvantages of this approach are that the signal cannot be reduced to zero by this procedure, and the sensitivity of the technique is relatively low because it requires a high concentration of radiation-induced defects (i.e., a high level of absorbed dose). Examples of this method are given for alkali halides (Regulla, 1972; Miller and Endres, 1990) and phosphate glasses (Piesch et al., 1990, 1993). It is clear from the above that unlike TL, OSL is blessed with several experimental approaches in which the luminescence can be stimulated. Several of these have been mentioned already, and among the more popular are: (a) the "continuous-wave OSL" (CW-OSL) method in which the stimulation light intensity is kept constant and the OSL signal monitored continuously throughout the stimulation period, (b) the so-called "linearmodulation OSL" (LM-OSL) method in which the stimulation intensity is ramped linearly while the OSL is measured, and (c) the POSL method in which the stimulation source is pulsed and the OSL is monitored only between pulses. Each of these methods is described in depth throughout the pages of this book, especially in Chapter 2. For the present,
Introduction
3
however, we illustrate in Fig. 1.2 each of these three popular methods with experimental examples corresponding to the different stimulation modes (shown in the insets).
1.3. OSL dosimetry Aside from the different readout methods available, OSL techniques have advantages over conventional TL techniques for a number of other reasons. The most obvious advantage lies in the fact that the readout method is all optical, requiring no heating of the samples (although some additional advantages may be gained by performing the optical stimulation at slightly elevated temperatures, as will be discussed in later chapters). Apart from removing the need to provide a reliable and reproducible heating scheme, this also means that problems due to thermal quenching of the luminescence efficiency are removed. Thermal quenching is a reduction in the efficiency of luminescence as the temperature of the sample increases due to the opening up of competing, non-radiative relaxation pathways (see Chapter 2). The phenomenon has been described for two important OSL materialsmnamely quartz (Wintle, 1975) and A1203:C (Akselrod et al., 1998). Adoption of A1203 as a TL material for personal or environmental dosimetry has been handicapped by a heating-rate dependence of the TL sensitivity caused by thermal quenching of the luminescence efficiency. As the heating rate increased, so the TL peak shifted to higher temperatures for which reduced luminescence efficiency was noted. The effect is not seen at lower heating rates, and thus a heating-rate dependence for the TL sensitivity is observed (Kitis et al., 1994; Kortov et al., 1994; Akselrod et al., 1998). However, by using optical stimulation, the readout of the luminescence can be performed at temperatures lower than those for which thermal quenching occurs, and thus a significant increase in sensitivity is achieved. Quartz is an important material for retrospective dosimetry. Several of its luminescence emission bands, including those active in OSL, are known to undergo luminescence quenching (see Chapter 5; Wintle, 1975). Examination of the temperature dependence of TL, PL and OSL from this material (Spooner, 1994; McKeever et al., 1997; Bailiff, 2000; Wintle and Murray, 2000) shows that thermal quenching of the luminescence takes place at elevated temperatures such that increased OSL sensitivity is obtained if measured at lower temperatures. The all-optical nature of the OSL readout process also allows the use of "plastic" dosimetersmnamely, luminescence phosphors impregnated into a plastic matrix (e.g., polytetrafluoroethylene, PTFE). In this way, robust dosimeters may be manufactured and advantages may be gained for neutron dosimetry through the interaction of neutrons with hydrogen atoms producing knock-on protons, which then yield luminescence from the phosphor through ionisation processes (Pradhan and Bhatt, 1981). The high sensitivity of OSL also leads to advantages related to multiple readings since it is sometimes not necessary to stimulate all of the trapped charge in order to read a sufficient luminescence signal. In this way, a residual remains that can be stimulated at a later time if second, or third, etc., readout of the signal is necessary for dose verification purposes. Finally, the readout process can be made very fast by adjusting the stimulating light intensity (power) leading to advantages associated with the rapid analysis of large numbers of dosimeters.
400
Optically Stimulated Luminescence Dosimetry
~ 300
oot/ I 0
-
100
200
300 400 Time (s)
:~~'400 (b) ~300]
600
-
/
O 0
~
~ 500
~ramped
1000 Time (s)
2.5 (c) ~ 2.0
500
1500
J_ULL
,
pulsed
*~ 1.5 ,,..,
& 1.0
"1 0 0.5 1'3
0
POSL
,i
0.0
.
0 0.5
:_ ~ 1.0 Time (s)
1.5
2.0
Fig. 1.2. Experimental data illustrating three examples of OSL readout method: (a) CW-OSL, (b) LM-OSL, and (c) POSL. In (a) the sample was A1203:C (Luxel TM)irradiated with 0.3 Gy of beta rays (9~176 The CW-OSL was measured in a Rise TL/OSL-DA-15 reader using Hoya U-340 filters (7.5 mm) to discriminate between the green (525 nm) stimulation light and the OSL emission. The stimulation power used was --- 10 mW/cm 2. In (b) the sample was A1203:C (TLD-500) irradiated with 0.17 Gy of beta rays (9~176 The LM-OSL was measured under the same conditions as in (a), but the stimulation power was ramped from 0 to --- 10 mW/cm 2 in 1800 s. In (c) the sample was again A1203:C (Luxel TM) irradiated with heavy charged particles (100mGy of Fe, 500 MeV/u). The POSL was measured during 1s of stimulation using 300 ns pulsed stimulation light from the second harmonic (532 nm) of a Nd:YAG laser operating at a frequency of 4 kHz. The luminescence was detected between the pulses using a gated photon counting system. A Nd:YAG 532 nm laser line filter and Kopp 5-58 filters were placed between the sample and the PMT. Each of these three measurement methods are described in detail in Chapter 2.
Introduction
7
OSL has found use in several dosimetry applications, including personal, environmental, medical and retrospective (dating, accident) dosimetry. The distinction between these groups is, in some sense, arbitrary. For example, retrospective dosimetry can include evaluation of radiation exposure to natural materials for purposes of luminescence dating. This application could also be described as an example of environmental dosimetry in that the materials used are natural materials from the environment and the radiation is natural environmental radiation. In another example of retrospective dosimetry, however, one is concerned with the doses absorbed by locally available materials during radiation accidents for the purposes of estimating the doses received by people during the exposure event. This could be described as an example of personal dosimetry. For purposes of this book, however, we draw a distinction between these categories by describing personal dosimetry and environmental dosimetry to be those applications that use synthetic dosimeters (i.e., OSLDs) to measure the dose to either people, or to the environment. Retrospective dosimetry, on the other hand, is the dosimetry of natural or locally available materials and not the dosimetry of synthetically engineered OSLDs. This distinction is of value since the OSL procedures that one adopts are very much dictated by the type of material being used. We describe medical dosimetry separately from personal dosimetry, however, since although one uses synthetic OSLDs to obtain doses to people, the people concerned (patients) are intentionally irradiated, sometimes to high doses, with specific forms of radiation and at specific locations in the body. Such circumstances dictate the use of non-conventional OSL procedures and methods. The overall purposes of these various dosimetry applications are described below.
1.3.1. Personal dosimetry Personal dosimetry is concerned with the evaluation of deep dose, shallow dose and eye doses (quantifies Hp(10), Hp(0.07) and Hp(0.3)), respectively. Hp(10) is the dose equivalent absorbed by human tissue at a depth equivalent to 1000 mg/cm 2 (or about 1.0 cm deep below the skin surface). The interest here is in highly penetrating radiation such as gamma rays, high-energy beta particles, X-rays (> 15 keV) and neutrons. Hp(0.07) is the shallow dose equivalent absorbed by the skin at a depth of 5 10 mg/cm 2. Here the interest is in non-penetrating radiation (low-energy beta particles), X-rays (< 15 keV). Hp(0.3) is the dose to the lens of the eye at a tissue depth of 3 mm. The primary quantity of interest (NCRP, 1993, 1995) is the dose equivalent to a point in tissue (H, in Sv), related to the absorbed dose (D, in Gy) by the quality factor Q (ICRU, 1991) thus H = QD. Alternatively, one can consider the equivalent dose HT absorbed by tissue or organ T, related to the average absorbed dose by that tissue DT by the radiation weighting factor w, thus HT -- WDT. The whole body total effective dose E is then E = S'T WTHT, where WT is the tissue weighting factor and the sum is over all organ tissues. A major requirement of OSLDs in these applications is that they are approximately tissue equivalent. Thus, materials with effective atomic numbers (Ze~) near that of human tissue (Zeff = 7.6) are desired. The dose equivalent range of interest is from approximately 10 p~Sv to 1 Sv, with a required uncertainty better than approximately 10%. The expansion of the US, Asian and European space exploration programs is leading to increased exposure of people (astronauts) to space radiation. The sources of exposure for
Optically Stimulated Luminescence Dosimetry astronauts (and for electronic components) are from galactic cosmic rays (high-energy protons and heavy charged particles), solar particles (medium- to high-energy protons) and (for low-Earth orbit) trapped radiation belts (medium-energy protons and electrons). Thus, the radiation environment external to a spacecraft in low-Earth orbit consists of electrons, positrons, neutrons, protons and stable atomic nuclei up to charge Z - - 9 2 (Fig. 1.3). Energies range from a few eV for trapped electrons to 1014 MeV for galactic cosmic ray ions. Absorbed doses vary with activity (e.g., extravehicular activity, EVA) and location within the spacecraft. Typical dose rates in a space vehicle in low-Earth orbit are --~0.8 mSv per day, with Shuttle flights lasting 1 0 - 1 2 d a y s and sojourns in the International Space Station lasting several months (Benton and Benton, 2001). Because of the mixed radiation field and the dominance of high-energy, high-LET particles, the dose quantities of interest are the gray-equivalent (Gy-Eq) for short-term
Fig. 1.3. The integral LET flux spectra measured by a tissue equivalent proportional counter (JSC-TEPC) and plastic nuclear track detectors (CR-39, University of San Francisco) on the STS-57 mission at an inclination of 28.5~ and an altitude of 462 km, in June 1973 (from Benton and Benton, 2001).
Introduction
9
deterministic effects, where Gy-Eq = (RBE)DT. DT is the mean absorbed dose (in Gy) in an organ or tissue and RBE is the radiobiological effectiveness for a given radiation type. For long-term stochastic effects, the quantity of interest is the effective dose (E, in Sv), where E = ~T WTHT (NCRP, 2000). 1.3.2. Environmental dosimetry
Tissue equivalence is not an issue with dose estimation to the environment, for which the only quantity of interest is the absorbed dose D (in Gy). The primary interest in this field is the impact of "man-made" radiation on the general public. Sources of such manmade radiations include nuclear waste disposal, emissions from nuclear power and reprocessing plants, and the nuclear weapons industry. Political, community, health and environmental watchdog pressure has led to the continuous monitoring of such radiation "pollution", primarily using TL dosimeters (TLDs). This monitoring is deemed of importance despite the fact that the average whole body burden to the average population from "man-made" environmental sources is
i/ 9 B After Irradiation (Metastable)
EF
EV
9C During Optical Stimulation
9 D Back to Equilibrium
Fig. 2.1. A representation (following Br~iunlich, 1979) of the occupancy of forbidden gap states at absolute zero. (A) Before irradiation at equilibrium; EF is the Fermi level (assumed to be approximately mid-way between the top of the valence band Ev and the bottom of the conduction band Ec). All states below the Fermi level are full, as described by the occupancy function f(E) = 1, and all states above the Fermi level are empty (f(E) = 0). (B) After irradiation, some electron states above the Fermi level are now full, up to the quasi-Fermi level for trapped electrons, EFe, while several hole states below the Fermi level are now empty (with a quasi-Fermi level for trapped holes, E~). Normal distributions of forbidden gap states are assumed, one for electrons and one for holes. As optical stimulation of the irradiated sample proceeds (C), filled states are emptied and recombination with empty states occurs. The quasi-Fermi levels move towards the Fermi level, until finally all states return to their original equilibrium occupancies (D).
Optically Stimulated Luminescence Theory
17
Beginning with the left-hand figure (A), we see the Fermi-Dirac filling function at 0 K for which all electron traps (above the Fermi level) are empty and all hole traps (below the Fermi level) are full (represented by the shaded distribution). After perturbation by ionising radiation, we find a new filling function showing electrons trapped at localised states within the band gap above EF and an equal concentration of holes trapped below EF. One can define two quasi-Fermi levels, one each for electrons EFe and for holes E~. These are useful devices for describing the non-equilibrium state, which follows the perturbation in terms of equilibrium statistics by making the assumption that the trapped electron and hole populations are in thermal equilibrium over their available energy levels. During stimulated relaxation, i.e., during illumination of the irradiated sample with UV, visible or IR light, the filling function f ( E ) gradually returns to its preperturbation state. During this process, the quasi-Fermi levels gradually move back towards the equilibrium Fermi level as the trapped charge concentrations decay back to their equilibrium values. In Fig. 2.1 the stimulation is assumed to be optical and all diagrams are depicted at absolute zero temperature.
2.2. Generalised mathematical description of OSL The total concentration of occupied metastable states in the system at time t may be represented by/z(t), where: /x(t)=f
~ 'Yl
...~ '}/2
n(3"~,3"z,...3"n,t)d3"ld3"z...d3" m
(2.1)
]/m
where n(3"l, 3'2,... 3"m, t) is the concentration of occupied states 1 ---, m, described by state parameters 3"1,3"2,...3"m, and in general n( 3", t) = N ( 3")f ( 3", t). Here n(3") is the concentration of occupied states, N(3") the concentration of available states, and f(3") the occupancy of the state. (f = 1 when a state is full a n d f = 0 when it is empty.) Both n(3") and f(3") are time-dependent functions. (Note that in Fig. 2.1, the state parameter being represented is energy, i.e., 3' = E.) The state parameters 3"~, 3"2, ... 3"m dictate the stability of the metastable state under the prevailing conditions of temperature and illumination intensity, that is they govern the probability per unit time that the system will return to equilibrium, n(3"1,3"2, ... 3"m, t) is a weighting function, or distribution, expressing the concentration of occupied states possessing the parameters 3'1, 3'2, . . . 3'm" Eq. (2.1) is a time- and dose-dependent function since it increases during irradiation and decreases during stimulation. In stimulated luminescence measurements we monitor the emitted luminescence intensity during the return of the system to equilibrium. The luminescence intensity I is proportional to the rate at which the metastable states decay, such that: I(t) =
d/x(t) dt
(2.2)
Thus, the stimulated luminescence is time-dependent through the functional dependence of Idtx(t)/dtl upon n(3'1, 3'2,...3'm,t)- In order to evaluate I(t) we need a relationship
Optically Stimulated Luminescence Dosimetry
18
between Idl~(t)/dt] and n(yl, Y2, ... Ym, t). One possibility is an expression of the form: d/x(t) _ _/x t(t)P(t) dt
(2.3)
where P(t) is the probability per unit time of the decay of the metastable states/x(t). With l = 1 this is, of course, a first-order function. If each state n(yl, 72, ...Ym) has its own probability function P(71,72,... Ym) then, with l = 1 :
I(t) = d/x(t)dt -- ~
~ ]/2 " " f "
]/m
n(71, y2," "~/m't)p(71T2, , . ''Tm)dyl d72"" . 9
(2.4)
where we assume the superposition principle~i.e., there is no interaction between states. Eq. (2.4) relates to a fixed, time-independent probability P(71,3/2,-.-Ym)-For a timedependent probability:
l(t) = d/x(t)dt = f v, ~
]/mn(71' T2'"'Ym't)P('Yl' Y2'"'Tm't)d'Yl d72""d'Ym (2.5)
The form ofp depends on the stimulation method, i.e., thermal or optical stimulation of the metastable state back to equilibrium. For thermal stimulation we may write:
p ( f ) -- v g exp - ~-~
(2.6)
where v is the phonon lattice vibration frequency, K a transition probability constant and F the Helmholtz free energy barrier, which must be overcome for decay of the metastable state. The equation may be expanded using F = E - AST to yield:
p ( E , s ) = v K e x p { - ~ t e x p { - ~ - ~E} =
s e x p,,f - ~-~} E
(2.7)
The product v K e x p { A S / k } = s is usually termed the "pre-exponential factor" (dimensions s-l). AS is the entropy change associated with the transition and E is the internal energy barrier. Thus, the metastable states are characterised by m - - 2 parameters, namely Yl = E and Y2 = s. The Boltzmann term e x p { - (E/kT)} defines the probability that the energy delivered by the phonon interaction is sufficient to surmount the barrier of height E. At a fixed temperature T, it describes the isothermal decay of the metastable state back to the ground state. In terms of the energy states illustrated in Fig. 2.1 E represents the "trap depth" and p is thus the probability of thermal ionisation of the metastable electronic state and the return of the Fermi level to its equilibrium level. Our primary interest in this text is with optical stimulation and thus, for optical stimulation we have:
p(Eo)---- ~tr(Eo)
(2.8)
where @ is the optical stimulation intensity and tr (Eo) is the photoionisation cross-section for interaction of the metastable state with an incident photon, and Eo is the threshold optical stimulation energy required for charge release and a return of the system to equilibrium. Here m = 1 and Yl = Eo.
Optically Stimulated Luminescence Theory
19
In the above representations T, A and 4)are all fixed values independent of time. For optical stimulation, when the traps are emptied using a fixed wavelength A and a steady illumination intensity q~ the luminescence recorded is known as continuous wave OSL, or CW-OSL. However, a time dependence to p can be introduced by scanning the above terms with time--i.e., qb(t) or A(O. Thus, for a linear increase in the intensity of optical stimulation at a fixed wavelength:
ci9( t ) = ci9o + fl~t
(2.9)
with/3~ = d@/dt. OSL recorded is this manner is known as linear modulation OSL, or LM-OSL. Other schemes can be imagined in which the intensity is modulated in nonlinear manner ways. For example, one can imagine an exponentially increasing stimulation intensity (q/~(t)= 4)0 exp{t}), which is a scheme that can have advantages when emptying a range of traps with photoionisation cross-sections that differ by orders of magnitude. Alternatively, the stimulation may be pulsed, such that qls(t)-- 4)0 for to -< t < to + At, and qJ(t) = 0 for (to + At) > n l, N2 >> n2 and m is approximated to a large constant. The latter assumption in effect means that n l > Eo + A) the purely electronic form of the cross-section suffices (Noras, 1980). Experimentally, one can observe phonon broadening effects by observing the shift in the threshold energy Eo as a function of temperature. One can expect variations in Eo with T due to thermal expansion effects, but such large shifts can be expected if thermal broadening is a significant factor. 2.4.6. Thermal quenching A final temperature-dependent effect governing the variation of OSL with temperature is thermal quenchingmi.e., the loss of luminescence efficiency with increasing temperature. In general, the effect can be observed in several ways but is seen most often in experiments that monitor the intensity of either PL or radioluminescence (RL) from a sample as a function of temperature, under conditions of constant excitation (light excitation for PL or ionising radiation excitation for RL). In both cases, thermal quenching is manifest by a decreasing emission intensity as the temperature increases. In the context of OSL, one observes a decrease in both the peak intensity (under CW excitation) and the integrated area under the CW-OSL decay curve as the sample temperature is increased. This observation assumes that for each measurement temperature the sample has been irradiated prior to OSL measurement under identical conditions such that the only variable in the experiment is the temperature of OSL measurement. The effect is manifest also in TL when a set of TL curves is obtained at a variety of heating rates. As the TL peaks shift to higher temperatures with increasing heating rate, so the luminescence efficiency decreases and a reduced TL peak (area and peak height) is obtained. Furthermore, the shape of the peak is distorted from that which is expected from kinetic considerations (e.g., see Section 2.2) since the high-temperature side of the peak is afflicted more than the lowtemperature side. Explanation of the effect is generally centred upon one of the two modelsmnamely, the Mott-Seitz model and the Sch6n-Klasens model. The Mott-Seitz model is best understood through reference to the defect configurational coordinate diagram, such as that shown in Fig. 2.18. Again, optical excitations take place vertically from the ground state to the excited state, followed by lattice relaxation and phonon emission before deexcitation and luminescence occurs. The energy difference between the emission and the excitation energy corresponds to 2hwp (the so-called Stokes shift). The lifetime of the carrier in the excited state ~-is governed by the quantum mechanical transition selection rules. Whilst in the excited state, however, the electron can absorb an amount of phonon energy W and undergo a transition over this potential barrier to decay to the ground state non-radiatively with the emission of phonons only (Dexter et al., 1955). The temperature dependence of the excited state lifetime ~"is given by:
1 "r
--
1 To
{w}
+- ~, exp k T -
(2.62)
45
Optically Stimulated Luminescence Theory
Excited state
Ee (Q)
t~
W
r
iii
Ground state _ _ _
EQ~
%! i
I
Configurational Coordinate, Q Fig. 2.18. Configurational coordinate diagram for a defect ground state Eg and excited state Ee. If thermal energy AE is absorbed by the electron whilst in the excited state, a non-radiative relaxation to the ground state occurs.
where ro is the lifetime for radiative transitions and u is a constant (frequency factor). The luminescence decay time is thus: ~- =
To 1 + ~'ov e x p ( -
W/kT)
(2.63)
The luminescence efficiency r/ is defined as the ratio of the probability for radiative transition divided by the total transition probability. Thus: --
~" To
--
1
1 + C exp{ -
W/kT}
(2.64)
where C = ~'ov. The radiative luminescence intensity I is likewise reduced according to: I =
Io 1 + C exp{ -
W/kT}
(2.65)
with Io the unquenched intensity obtained at low temperatures. The latter expression is also expected if one considers the S c h 6 n - K l a s e n s model for thermal quenching, indicated in Fig. 2.19 (McKeever, 1985). In this view, the luminescence emission results from the recombination of charge carriers from the delocalised band with trapped carriers of the opposite sign (e.g., free electrons recombining with trapped holes at recombination centres). However, in the
46
Optically Stimulated Luminescence Dosimetry
Fig. 2.19. The Sch6n-Klasens model for thermal quenching of luminescence. Free charge carriers (say, electrons in the conduction band) recombine with trapped holes to initiate the luminescence process. However, in this model the trapped holes are thermally unstable and may be released from the hole centres at a rate equal to s exp{ - Eh/kT}. This gives rise to a decreasing recombination probability. The discussion is symmetric with respect to the sign of the charge carrier and similar descriptions can be applied to recombination of holes with trapped electrons.
Sch6n-Klasens model, the trapped carriers are considered thermally unstable such that there is a significant probability of thermal release and a concomitant reduction in the concentration of recombination sites. This, in turn, leads to quenching of the luminescence process. The net result is a reduced luminescence efficiency as given by Eq. (2.64), but with the activation energy W identified with the thermal activation energy for charge carrier release (i.e., Eh in Fig. 2.19). It should be noted, however, that PL lifetime r should remain unaffected. This process, however, will affect the intensities of RL, OSL and TL. Examples of thermal quenching for quartz are given in several publications, including RL (Wintle, 1975), OSL (McKeever et al., 1997a; Murray and Wintle, 1998) and TL (Nanjundaswamy et al., 2002). A representative plot of OSL versus sample temperature for quartz is shown in Fig. 2.20. It should be noted that when one plots luminescence intensity versus temperature (either RL, OSL or TL) there is expected to be a difference in the obtained thermal quenching curve depending upon whether one is heating the sample from low temperatures, or cooling the sample from high temperatures. This is caused by the effect of shallow traps upon the recorded intensity. At low temperatures, shallow traps are filled during the initial irradiation of the sample. As the temperature increases the trapped charges are released, increasing the free charges available for recombination. The result is an initial enhancement of the luminescence intensity, due to the emptying of the shallow traps, before the expected decrease at higher temperatures is observed due to quenching. When one starts from high temperature, however, the shallow traps are empty and, although some carriers may be trapped as the temperature drops, the effect is much less noticeable. A clear example is given in Fig. 2.21 where we see luminescence from three
47
Optically Stimulated Luminescence Theory 6x105 ,-
4x105
0 0
..2 cO
0
2xl 05
0
i
i
a
I
50
100
150
200
250
Stimulation Temperature, ~ Fig. 2.20. Integrated OSL ( 1 - 1 0 0 s) under a CW-OSL decay curve for irradiated quartz, as a function of stimulation temperature. The solid line is a fit to Eq. (2.65) with W = 0.636 eV (___0.013 eV) and C = 3.4 x 107 (_+ 0.9 X 107) (from Murray and Wintle, 1998).
examples of A1203:C containing different concentrations of shallow traps. When the concentration of these traps is high, an initial increase in the luminescence is clearly observed as the temperature rises before thermal quenching sets in. The size of the effect is reduced during cooling and the peak in the emission is correlated with the position of the TL peak from the same traps. Measurements of luminescence intensity can also be affected by the degree to which the deep traps are filled. Competition effects with the deep traps lead to significant changes in the luminescence intensity curves, depending upon the degree of filling of the deep traps. This has been convincingly demonstrated for A1203:C by Milman et al. (1998). The best way to overcome such interferences, however, is to monitor the luminescence lifetime ~- as a function of temperature (Eq. (2.63)) for which competition effects are minimised. This was shown clearly for quartz by Bailiff (2000) and for A1203:C by Akselrod et al. (1998a). Analyses of the thermal quenching data for quartz indicate a quenching activation energy of approximately 0.6 eV (Bailiff, 2000) and a classical Mott-Seitz mechanism. The main (F-centre) emission from A1203 is characterised by a quenching energy of approximately 1.08 eV (Akselrod et al. 1998a) and is also adequately described by the Mott-Seitz model.
2.5. LM-OSL 2.5.1. First- and general-order kinetics The description of OSL has so far been based entirely on CW-OSL--namely OSL stimulated using a constant intensity, constant wavelength light source. Bulur (1996) introduced an alternative technique in which the intensity of the stimulation source is ramped linearly and the OSL monitored throughout the ramp. By adopting this stimulation mode, the OSL is seen as a series of peaks, with each peak corresponding to the optical release of charge from different trap types. Thus, traps for which the photoionisation
Optically Stimulated Luminescence Dosimetry
48
"~->~' v~ 1.21'4[. . . . . . . . . . . . . . . . . . '=; oa)
.=_ E
[
1
'
0
i
0.8-
-.
~
"~",',
#1 ~
-'. "
TV
0.6-
9
V
U
u
0.4"O Z
9149 " 9
00
"o
E
(a)
#2
i1 V v
0.2"
I
*
9 I
g
vv
0.0
I
1
.~ 3000
,.U
.~ 2500 r v
D
2000
9
v v I I .......... V ~ v . _ _ ~
TL for #100
(b)
9 Phosphorescence ~ during heating , 9
700x10~ 600x103 500x103 t400x103
xi
~--
v
lsoo
,,,'-'|176176176 o
500
oe--
0
o.
350
To
Phosphorescence during cooling
~_/
300x103 "g 200x103 .~ _J
100x103
400
450
I--.
=~=~--,..=,--.~J 0 500 550
Temperature (K) Fig. 2.21. (a) Thermal quenching of luminescence from three samples of A1203:C as a function of temperature. The initial peak in the luminescence is caused by thermal release of charge from traps and subsequent phosphorescence emissionmas is evidenced by the correlation of the TL peak position with the maximum in the luminescence versus temperature curve (b) (from Akselrod et al., 1998).
cross-section is large at the particular wavelength used in the experiment are emptied first and are shown as a peak in a plot of OSL versus stimulation time. Traps with smaller photoionisation cross-sections empty more slowly and give rise to OSL peaks that appear at later times. Thus, traps with fast, slow and medium rates of de-trapping may be more easily resolved using LM-OSL compared with CW-OSL. To describe the shape of an LM-OSL curve mathematically, consider a one-trap/onecentre model in which electrons of concentration n are trapped at a localised state until stimulated into the conduction band by absorption of a photon (of wavelength hvex). The freed electron is then able to recombine at a trapped hole centre, producing an emission
Optically Stimulated Luminescence Theory
49
photon (luminescence) of wavelength hvem. For first-order kinetics (negligible retrapping) the rate of de-trapping is given by Eq. (2.30), and the corresponding luminescence (CW-OSL) intensity by Eq. (2.31), where the time-constant of the decay is ~-~ = 1/o-q~, where all the terms have their usual meaning. If, however, the intensity is linearly ramped from zero to a maximum value q~m according to: qb(t) = yt
(2.66)
then Eq. (2.30) is replaced by: dn = - o-ytn dt
(2.67)
from which we obtained a Gaussian function" n = no exp ---~-
(2.68)
The luminescence intensity (i.e., the LM-OSL intensity) is then given by: trYt2 t lose = noO'yt exp { -- --~
(2.69)
Note that for first-order kinetics the principle of superposition applies (as discussed in Section 2.2) and thus, if there are K traps of type-i, following Whitley and McKeever (2001) the equation may be rewritten as: lOSE -- yt ~K
noitri exp {TOrit2] -- - - ~
(2.70)
i--1
An experimental LM-OSL curve from a sample in which several traps are emptying simultaneously, but at different rates, can thus be described as a simple sum of first-order LM-OSL curves. Simulated example LM-OSL curves for different values of the product To-, for fixed (normalised) values of no are shown in Fig. 2.22. It should be noted that each peak starts from t -- 0, no matter what values of o- and y are used. The shape of the LMOSL curve for a single trap is that of a linearly increasing function (in proportion to the linear increase in the stimulation power) followed by a Gaussian decrease in OSL intensity as the traps deplete. The time at which the maximum is achieved is given by: /
tmax
= ~ 1
(2.71)
O-T
and the LM-OSL maximum intensity is" /max
9OSL
n~
tmax
{l}
exp-
-~
(2.72)
Thus, the ionisation cross-section at the wavelength used in the experiment can be determined from the known value of y, and the observed value of tmax. We also observe that the position of the LM-OSL peak is dependent on both the wavelength (through the wavelength dependence of or) and the linear modulation ramp rate y. Specifically,
Optically Stimulated Luminescence Dosimetry
50
2,5
.~
2
.01 units
.oo un, s
1,5
0
. ,
0,5
0
0
10
20
30
40
50
60
Time (s) Fig. 2.22. Simulated LM-OSL curves for first-order kinetics, using three different values of the product 0"% For fixed ramp rates y, the LM-OSL peaks appear at shorter times as the photoionisation cross-section or increases. Similarly, for fixed o-, the peaks appear at shorter times as the ramp rate increases. All peaks start at t = 0. For a system displaying first-order de-trapping and multiple peaks, the net LM-OSL curve is a simple addition of peaks like those illustrated.
the peak will shift to shorter times at higher ramp rates or for larger values of the crosssection. If the photoionisation cross-section has a significant temperature dependence (see Section 2.4.5), the position of the LM-OSL peak will also shift with temperature. Adopting a general-order kinetics model in which the rate of re-trapping of the released charge is significant compared to the rate of recombination (Bulur, 1996) yields: dn
dt
--
oTtn b
(2.73)
nbo-1
where b is a dimensionless positive number; b > 0, b # 1. The solution is:
O")tt2 IOSL =
no~ryt (b - 1) ~ -
]b/(1-b) + 1
(2.74)
In contrast to the first-order case, the superposition principle no longer applies if there is more than one type of trap and an experimental LM-OSL curve cannot simply be described as the sum of several non-first-order processes. The maximum of a general-order LM-OSL peak is achieved at time tmax, where:
tmax
(2.75)
(ry(b + 1)
at which the maximum intensity is:
omax (2no)(')( SL= b + l ~
b+l
)bJ, b,
(2.76)
Optically Stimulated Luminescence Theory
51
Fig. 2.23. ExperimentalLM-OSL curves from a variety of materials. (a) Quartz: 10 Gy; 280~ for 10 s pre-heat; s pre-heat;75~ measurementtemperature.(c) 160~ measurementtemperature.(b) A1203:C: 100 mGy; 180~ BeO: 100 mGy; 180~ s pre-heat; 75~ measurementtemperature. (d) NaCI: 100 mGy; 225~ s pre-heat; 25~ measurementtemperature.The curveswere obtainedusingblue lightfrom a Rise TL/OSL DA- 15 system.The inset in each case shows the CW-OSL curves obtainedunder the same conditions (from Bulur et al., 2001).
The LM-OSL technique was first applied to OSL from ZnS and SrS IR-stimulable storage phosphors by Bulur and Grksu (1997). A selection of experimental LM-OSL curves (and their corresponding CW-OSL curves) is shown in Fig. 2.23. Each curve has been obtained after stimulation of the irradiated samples with blue light, under the conditions noted in the caption. The descriptions of LM-OSL and CW-OSL curves for first-order kinetics have assumed that the luminescence intensity is directly proportional to the de-trapping rate, dn/dt. These analyses lead to the realisation that the de-trapping rate is directly proportional to the stimulation intensity. Thus, from Eq. (2.31), with p = trY, we see that [d ln(Icw_osL)/dt] oc cI9 (i.e., the slope of the ln(Icw_osL)-versus-t curve is directly proportional to the stimulation power @). Bulur et al. (2001) demonstrated this to be the case for quartz, A1203:C and BeO, but not for NaC1. For the latter a non-linear, saturating exponential relationship was found. This may be due to the inadequacy of first-order kinetics or the simple one-trap/one-centre model in describing the OSL from this material. If first-order kinetics, or the simple model, do not apply, there is no longer a direct proportionality between ICW-OSL and dn/dt and, consequently, between d ln(Icw_osL)/dt and @. Bulur et al. (2001) treated the situation empirically, however, and viewed the experimentally obtained relationship between d ln(Icw_osL)/dt and @ as a true indication of the relationship between the de-trapping
52
Optically Stimulated Luminescence Dosimetry
rate and @, and modified the LM-OSL curve accordingly. The modified LM-OSL expression is found to be adequate in describing the LM-OSL curve shape for NaC1. 2.5.2. Relationship between LM-OSL and CW-OSL If the stimulation ramp in an LM-OSL experiment is arranged so that it reaches a final stimulation power q~f in time re, such that @f is equal to the fixed stimulation power used in a CW-OSL experiment (Kuhns et al., 2000), then the observed CW-OSL decay rate will be related to the observed maximum LM-OSL by: 1 ~'d --
O")ttf
t2a• --
(2.77)
tf
Bulur (2000) describes a simple mathematical transformation that allows one to convert CW-OSL curves into LM-OSL curves. First define a variable u, thus:
u-- ~/2tP
(2.78)
or /12
t=
(2.79)
2P
where P is the total measurement period in an LM-OSL experiment, and u has the dimensions of time. Substituting Eq. (2.79) in the expression for CW-OSL (Eq. (2.31)) and multiplying by u/P yields: IOSL
{
n~176 exp -p
(2.80)
which is of the same form as the expression for LM-OSL (Eq. (2.69)). Comparing Eq. (2.69) with Eq. (2.80) we see that u maps with t, while clearly cI)/P = 3t. The transformation of u to the time t domain scales with the choice of P. Note that if P is made equal to the observation time for the CW-OSL experiment, then the scaling factor is , ~ . In Fig. 2.24 we show an example CW-OSL curve, the transformed (or "pseudo") LMOSL curve, and an experimental LM-OSL curve for comparison, for IR-stimulated luminescence from potassium feldspar. The agreement between the pseudo-LM-OSL and the actual LM-OSL for this material is clear. The transformations can also be demonstrated for second-order and general-order kinetics (Bulur, 2000). Table 2.1 lists the obtained expressions of the peak position (Umax) and the peak maximum ( I ~ ) for first-, second- and general-order kinetics for the pseudo-LM-OSL curves. 2.5.3. Wavelength dependence of LM-OSL The excitation wavelength dependence is shown in Fig. 2.25. In this figure the Whitley and McKeever (2001) simulations of the LM-OSL curves to be expected for a system with three trapping states with optical threshold energies of 1.9, 2.5 and 2.9 eV, as the stimulation energy is changed are illustrated. The data show how the curves merge into
Optically StimulatedLuminescenceTheory
53
Fig. 2.24. CW-OSL,pseudo-LM-OSL and LM-OSL curves from Na-feldspar. The CW-OSL and real LM-OSL curves were obtained using IR-stimulation. A ramp time of P = 100 s was used, both in the experiments and in the transformation calculation using Eq. (2.78) (from Bulur, 2000).
each other as the positions of their peak m a x i m a change with stimulation energy. Note that for long excitation times apparent resonances are seen at stimulation energies corresponding to the three optical threshold energies. At these long times only the slowest emptying traps contribute to the L M - O S L signal at high stimulation energies, while at low energies only those for which the stimulation energy is greater than the threshold energy contribute to the L M - O S L signal. W h e n the stimulation energy is such that several first-order L M - O S L peaks merge together to form one indistinguishable peak, the shape of the net L M - O S L signal may give the appearance of one second-order (b = 2) L M - O S L curve (Whitley and McKeever, 2001). Fitting experimental L M - O S L curves, therefore, can be misleading unless there
Table 2.1 The parameters Umax and Jtos Ltmax for the pseudo-LM-OSL curves for different kinetics (from Bulur, 2000). N is the maximum number of available trapping states and no the number of filled traps, b is the kinetic order Parameter
First-order
Second-order
"m.x
J2"N 3o'q~ no
o'@
I0max SL
{ no exp Umax
1} -
~
3no 8Umax
General-order
2 (b + 1) ~rq~ no 2no
l(2b~
b/(1-b)
(b -k- 1) Umax b-+--11
Optically Stimulated Luminescence Dosimetry
54
Fig. 2.25. Simulated LM-OSL curves as a function of stimulation light energy, for a system with three traps, of optical threshold energies 1.9, 2.5 and 2.9 eV (from Whitley and McKeever, 2001).
exists a priori information about the number of trapping states contributing to the overall LM-OSL signal. If such a priori information is not available, wavelength-dependent LMOSL curves are essential. Example fittings of experimental LM-OSL curves from A1203:C to three first-order processes are shown in Fig. 2.26. The experimental data for the different samples are well fitted to three first-order peaks and suggest photoionisation cross-sections of (3.3-3.7) • 10 - 2 ~ (1.4-1.7) • 10 -19 to (5.8-7.0) • 10 -19 c m 2. Alternatively, one could selectively bleach the different trapping states at different bleaching wavelengths before monitoring the LM-OSL curve (Singarayer and Bailey, 2002). As each trap bleaches successively one can establish the wavelength dependence of the photoionisation cross-section in a similar fashion to that for CW-OSL, as described earlier in this chapter (Section 2.3.3). 2.5.4.
Photoconductivity
As an alternative to monitoring the OSL during a linear increase in stimulation light intensity, one can monitor the current flow through the sample~namely, the PC. In this case, the equivalent expressions to the first- and general-order LM-OSL curves are: /pc - - et.tF'rrlosL
(2.81)
where e is the electronic charge,/x the free carrier mobility, Ze the free cartier lifetime, F the electric field, and IOSL the LM-OSL expression for either first- (Eq. (2.70)) or generalorder (Eq. (2.74)) kinetics. Whitley and McKeever (2001) used linearly modulated PC (LM-PC) to examine traps in A1203 that did not appear in LM-OSL data. In particular, they observed two LM-PC peaks, including a large second LM-PC peak, from samples for which simultaneously
55
Optically Stimulated Luminescence Theory Time (s) 1000 1500
5OO
40
I
I
2000
I
(a)
2500
I
~o
3000 1.2
I
~
1.0
30
0.8 5 ~, b
"~ 20
,._:
0.6 ~" . J
0.4
10 0.2 0
I
~ I
30
0
(b)
25
~
500 I
0.0
I
0"-19
10-1710-18 1
10-20
G (cm 2) Time (s) 1500 2000 I I
1000 I
,
///
o?
15
2500 I
3000 1.2 1.0 0.8
10
0.4
5
0.2
0
.
.
.
.
10-17 10-18 10-19
0
30
500
1000
"
"
0
,
.
.
.
.
,
"
(cm 2)
Time (s) 1500
2000
"
'
.
.
10-20
.
.
.
2500
,
5
0.6
0.0
3000 1.2
25
1.0
20
0.8
15
0.6~
10
0.4 ~
5
0.2
0
10-1710-18
10-19
G (cm 2)
10-20
0.0
Fig. 2.26. Deconvolution of L M - O S L curves for three samples of A1203:C. The solid line corresponds to the data while the dotted line is the best fit. The bar graphs are the photoionisation cross-sections determined from the fitting algorithm. The photoionisation cross-section axis marks are displayed on a 1/0"7 scale (from Whitley and McKeever, 2001).
56
Optically Stimulated Luminescence Dosimetry
measured LM-OSL showed only one peak (Whitley and McKeever, 2001). Possible reasons for this include the suggestion that charge released from the second trap does not recombine radiatively, or that the recombination, if radiative, results in photon emission outside the wavelength detection window for the experiment. The ratio of the LM-PC to LM-OSL yields (from Eq. (2.81)):
/pc = elxF%
(2.82)
/OSL from which we see that as long as the mobility/x and % remain constant during the detrapping process, the LM-OSL signal will peak at the same time as the LM-PC signal. However, if either ~ or % are trap limited, both parameters could change as the detrapping proceeds. Under these circumstances: d/pc dt
B/z Bt
q'eJOSL + - ~
Z~Bt I~e,U
4~,IosL
(2.83)
and we see that the LM-PC signal can peak after or before the LM-OSL signal, depending upon the time dependencies of the mobility and lifetime. Thus, simultaneously measured LM-PC and LM-OSL can reveal details about the recombination and charge carrier transport dynamics that are unavailable from measurements of LM-OSL alone.
2.6. Pulsed OSL 2.6.1. Principles of pulsed OSL The third major stimulation mode, as shown in Fig. 2.2, is pulsed OSL (POSL). To describe the principle behind the measurement of POSL, we begin by considering several stimulation pulses, of different intensities ~i (i = 1,2...) and durations (pulse widths, Ti) such that ~iTi is kept constant. The stimulation rate is proportional to the stimulation power absorbed by the sample and thus, by decreasing the pulse width in proportion to an increase in the stimulation power, the absorbed energy per pulse may be maintained fixed. Furthermore, for first-order kinetics we have (from Eq. (2.30)): An --
0
no-q), dt
(2.84)
and therefore for weak stimulation (i.e., An ~'), for the same energy input (q~T). At the end of the stimulation pulse, those centres in the excited state relax with a time constant ~-. The net effect is that the ratio of the photons emitted after the pulse to those emitted during the pulse, increases as the pulse width decreases, for constant stimulation energy. For T < < ~', most of the photons emerge after the pulse. This is shown schematically in Fig. 2.27. Here the simulated OSL curves stimulated by three different pulses, of intensifies 9 = 103, 102 and 20 energy/s, and corresponding pulse widths of T = 6.6, 66 and 300 ms are also shown. A luminescence lifetime of z = 100 ms was assumed. The vertical lines in each case represent the ends of the stimulation pulses. It is clear from inspection of the curves that the ratio of the area under the curves after the pulse to that during the pulse increases as the pulse width decreases. McKeever et al. (1996) demonstrated this experimentally. They stimulated irradiated
IOSL =
7 6
~5 102
0
2 1 0
~ 0
0.1
0.2
0.3
0.4
0.5
0.6
Time (s)
Fig. 2.27. Schematic illustrating the variation in the ratio of the light emitted during a pulse to that emitted after the pulse as the pulse width changes, for fixed stimulation energy per pulse. A luminescence lifetime of z = 100 ms was assumed, with pulse powers varying from 9 = 103 to 20 energy units/s. The pulse widths varied accordingly, from T = 6.6 to 300 ms. In each case it is assumed, for the purposes of this illustration, that the concentration of charge released per pulse is negligible compared with the total trapped charge concentration (i.e., An 90% of the light is emitted after the pulse if the pulse width is chosen to be small enough. By synchronizing the detection system to record only the light that is emitted after the stimulation pulse is off, the method provides for very effective separation of stimulation light and luminescence light, leading to the removal of noise due to the background signal from the intense stimulation pulse. The principles of the timing sequence required for the measurement of POSL are described in Chapter 7. 2.6.2. Delayed OSL (DOSL) The OSL signal after the laser pulse should decay according to the lifetime of the luminescence centres being stimulated. As noted, in A1203:C these are F-centres, with an excited state lifetime of 35 ms at room temperature. In some A1203 samples, however, a much longer lifetime is observed. The OSL is seen to decay much more slowly than one would expect due to re-trapping of the released charge in shallow traps. This is illustrated in the data of Markey et al. (1995) shown in Fig. 2.30; here the OSL decay after the stimulation pulse is fitted to two exponentials. The faster decay is due the intrinsic
600 "~"
500 j
C
~d
400
t~
oC 0 gO .C m
E ~
300 200 100
\
\\
~
0 0
0
0.0
~ 7
~
0.1
. . . . . .
I
0.2 Time (s)
I
0.3
0.4
Fig. 2.30. Decayof OSL, following a stimulation pulse from a laser, at 25~ in AleO3:C. The decay has been fitted to two exponentials. The lifetimeof the fast decay is 35 ms, corresponding to the decay of excited F-centres. The slower decay (545 ms at this temperature) is due to phosphorescence caused by the transfer of charge from deep traps to shallow traps (from Markey et al., 1995).
60
Optically Stimulated Luminescence Dosimetry
F-centre luminescence lifetime, and the other is a much slower (545 ms at 25~ highly temperature-dependent decay. By performing the experiment at different temperatures, it is revealed that the second slower component is due to trapping of charges in two shallow traps (with activation energies 0.65 and 0.77 eV, respectively). This "delayed" OSL emission has been given the acronym DOSL ("delayed OSL"; Yoder and Salasky, 1997) but the method was suggested as a technique in dosimetry as early as 1969 by Miller and colleagues (Tochilin et al., 1969; Rhyner and Miller, 1970) who studied the OSL emission from BeO. Later it was suggested for CaF2:Mn (Hanniger et al., 1982), CaSO4:Dy (Pradhan and Ayyanger, 1977; Pradhan and Bhatt, 1981) and A1203:C (Yoder and Salasky, 1997; Akselrod et al., 1998b), while Jaek et al. (1999) used the method to study deep traps in feldspars and quartz. In each case one relies upon the fact that the traps to which the charge has been optically transferred during the stimulation are unstable at room temperatures so that the charge leaks slowly out of these traps and recombines at luminescence centres. In this sense, DOSL may also be called optically stimulated phosphorescence.
2.7. Phototransferred effects
2.7.1. Procedure DOSL, as described above, is due to the phototransfer of charge from deep, stable traps to shallow, unstable traps. If the temperature at which the phototransfer takes place is low enough, the traps into which the charge is transferred are then stable and no phosphorescence is seen due to charge leakage. This is the essence of a family of techniques that use the phototransfer effect as a means of dosimetry. We discuss here the archetype of phototransferred effects, namely phototransferred TL, or PTTL. PTTL is observed in many luminescence dosimetry materials. It is defined as that TL signal originating from shallow traps following the optical transfer of charge from deep traps (donor traps) to the shallow traps (acceptor traps). PTTL can be observed experimentally in a number of ways. For example, one might irradiate at a temperature T~ at which all trapping centres are stable, and then heat to a temperature/'2 to empty the shallow (acceptor) traps, before cooling back to T~. At this point, the sample is illuminated with light of intensity q~ for a period P during which time charge is emptied from the deep (donor) traps. Some of this charge recombines at recombination centres (producing OSL, if radiative) and some charge is captured by the shallow, acceptor traps. Subsequent heating of the sample yields a TL signal (i.e., the PTTL signal) due to the thermal release of charge from the shallow trap. Alternative schemes, such as cooling to temperature T3, are possible instead of heating to 7"2 after the initial irradiation. At T3, traps that were too shallow to fill at temperature T~ are now stable. Subsequent illumination may then transfer charge from all-filled hightemperature (deep) traps into the empty, low-temperature (shallow) traps. Subsequent heating from low temperature (T3) releases charge from the shallow traps yielding a PTTL signal.
Optically Stimulated Luminescence Theory
61
Both these schemes have been used in dosimetry. Colyott et al. (1996, 1997) used the first of the schemes as a method of UV dosimetry. They irradiated A1203:C with gamma irradiation, and heated the sample to remove the main TL signal (near 200~ The sample was then illuminated with UVB irradiation, transferring charge from deep traps (stable above 600~ such that on heating a second time a PTTL signal was observed at 200~ The size of the PTTL peak was found to be proportional to the dose of UVB irradiation, and thus the technique can be used as a UVB dosimetry method. Miller et al. (1988) used the second scheme mentioned above as a method of ionising radiation dosimetry. The material used by Miller et al. was CaFz:Mn. The sample was irradiated at room temperature before being cooled to liquid nitrogen temperature. The dosimeter was then stimulated with light for a fixed period with a fixed intensity. The resulting PTTL signal from the shallow traps was then found to be proportional to the original dose of ionising radiation. Other similar results have been found for LiF (Buckman and Payne, 1976; Driscoll et al., 1983) and other common dosimetry materials. The early literature is reviewed by Jain (1983) and by McKeever (1985), while Sono and McKeever (2002) give some more recent examples. These practical applications of PTTL serve to illustrate that the PTTL intensity is proportional to both the original dose of ionising radiation (for fixed optical stimulation conditions), and the dose delivered during optical stimulation (e.g., optical energy fluence) for a fixed dose of ionising radiation. For this to be truly useful as a dosimetric tool, however, the PTTL response versus either ionising radiation dose, or stimulation energy fluence should be linear over several orders of magnitude. Not all dosimetric materials exhibit these properties and thus only a limited number of materials have proved useful in this regard. 2.7.2. M a t h e m a t i c a l d e s c r i p t i o n a n d t y p i c a l d a t a In general, PTTL is useful for examining the optically stimulated charge transfer processes that occur between trapping centres in OSL materials during illumination. As such, the models to describe PTTL are informative for understanding OSL processes. The simplest model necessary to produce PTTL is that of one deep trap, one shallow trap, and one recombination centre. (These are the minimum centres required for the effect to occur.) If n l and n2 are the shallow and deep electron trap populations, respectively, and m is the concentration of holes in recombination centres, then, at some point after irradiation and immediately at the start of the illumination period, one might have nl0 = 0 and n20 = m 0. The illumination is then assumed to excite electrons from the deep trap (the "donor trap") at a rate f -- o-q), after which they are captured by the shallow trap (the "acceptor traps"), such that we may write: dn2
dt
-- - n z f -k- nc(N2 - n2)A2
dn 1
dt
-
nc(N1 - hi)A1
(2.86)
(2.87)
Optically Stimulated Luminescence Dosimetry
62
and dm
dt
-- - n c m A m --
m
r
(2.88)
where ~"= (ncAm)-1 is the recombination lifetime and all other terms have their usual meaning. At quasi-equilibrium (dnc/dtn , ~,r_..r
•
I
I
x=0.33
-...... _.._/_:__../
fKCl:Eot
1.0 -
/
"~
"
/
x=0.5
.x-oo7
~,~%
"~" . / " " ~ ~ ' ~ _
/
x-O (KBr:Eu)
J
o') 0 E Z
Slimulation/..-~// 0.5-
~....
/ .-//l
\
N~,,
-
\-.', I -
0 40O
I
500
I
600 Wavelength
,~
. . . . . .
..
%%'%~,,
I
700
800
(nm)
Fig. 3.8.
(a) O S L stimulation and e m i s s i o n spectra from KCI:Eu (left) and KBr:Eu (right). (b) Variation in the O S L stimulation spectrum from KClxBrl -x:Eu crystals as a function of composition, i.e., as a function of x (from
Douguchi et al., 1999).
420 nm OSL signal can be produced following excitation with UV at 240 nm (Nanto et al., 1993a; Mel6ndrez et al., 1996). The OSL signal, however, is non-linear with UV dose and is only produced by exposure to high-energy UVC. 3.2.2. KBr
OSL from Eu-doped KBr has also been suggested for use in X-ray imaging phosphors and radiation dosimeters. As with KCI:Eu, the OSL emission maximum is also at 420 nm corresponding to a 4f65d-4f 7 transition of Eu 2+ ions. The OSL stimulation maximum
83
OSL Properties of Synthetic Materials
at 620 nm corresponds to absorption by neutral Br-vacancy centres (i.e., F-centres; Fig. 3.8a). A study by Douguchi et al. (1999) of the variation in the wavelength of the OSL stimulation maximum in KClxBrl-x:Eu crystals reveals a steady shift in the peak OSL stimulation efficiency as the chlorine content is increased relative to bromine, from 620 (for x -- 0) to 560 nm (for x = 1) (Fig. 3.8b). However, fading of the OSL signal was observed to be approximately 20% in 60 min at room temperature for samples with an Eu-content of 0.05 mol%, but essentially zero over the same period for an Eu-content of only 0.01 mol%. The fading rate increased, however, with an increase in ambient humidity. Although, the OSL response to X-ray dose was determined to be linear over five orders of magnitude (see Fig. 3.9), an interesting observation by Douguchi et al. (1999) was that the fading rate increased as the energy of the X-ray energy decreased, from no observed fading for 40 kVp X-rays, to about 50% fading for 20 kVp X-rays. UV-irradiated samples faded the most. This was interpreted as being due to enhanced fading at the surface connected with humidity effects.
10 4
r
10 3
.,,
h
1
..---..
J
s'
.d
.a
...__.. N9
j
J
102 f
__1
co 0
j J
f
10
f
.r
/ ,I
1
10
10 2
10 s
10 4
Irradiation time (s)
Fig. 3.9. X-ray dose dependence of several halide crystals. Key: @, KCI:Eu; [3, BaFBr:Eu; O, KBr:Eu (0.05mo1%)" O, KBr;Eu (0.01 mol%); O, KClo.33Bro.67:Eu;4-KClo.sBro.5:Eu; V, KClo.67Bro.33:Eu (from Douguchi et al., 1999).
84
Optically Stimulated Luminescence Dosimetry
3.2.3. NaC1 Nanto et al. (1993c) examined the OSL from Cu-doped NaC1 single crystals. Following 30 kVp X-irradiation and optical stimulation, three OSL emission peaks were observed at 353, 420 and 680 nm (Fig. 3.10). The peak in the stimulation spectrum was observed at 470 nm. Cu + substitutes for Na + and the 353 nm emission is believed to be due to a 3d94s-3d ~~transition in isolated Cu + ions. The stimulation band at 470 nm is attributed to X-ray-induced F-centres. Likewise the absorption band due to Cu + ions is reduced during X-irradiation, suggesting conversion to Cu 2+ ions. Thus, the mechanism proposed by Nanto et al. (1993c) for OSL emission is the stimulated release of electrons from F-centres to Cu 2+ ions, resulting in Cu+-ion conversion and emission at 353 nm. The OSL emission was found to be linear with absorbed X-ray dose over several orders of magnitude. Bailey et al. (2000) described the use of analytical-quality NaC1 in dosimetry, for the purposes of evaluating the potential of natural halite deposits for OSL dating of paleohydrological events. They examined several properties that are important in OSL dating procedures, including thermal stability (and other thermally stimulated properties such as thermal assistance and thermal quenching), and sensitivity changes during single aliquot procedures (see Chapter 6). The OSL decay curve from NaC1 is shown in Fig. 3.11 at several different temperatures. The authors interpret the decrease in the signal as a function of OSL stimulation temperature as being due to thermal quenching. Bailey et al. (2000) also noted that OSL could be induced by stimulation with visible (420-560 nm) and IR (880 nm) light, although the latter signal was very unstable and unsuited to dosimetry. OSL stimulated in the visible range, however, was found to have considerable potential for dosimetry. This range of stimulation wavelengths covers the absorption range of F-centres in this material (see Nanto et al. (1993c) and Fig. 3.10). LM-OSL and CW-OSL measurements on irradiated NaC1 were reported by Bulur et al. (2001) (see Fig. 2.23). These authors found that the optical de-trapping rate when 1.0 ~'0.8
r \
.D C
II
.ci
~0.6--
/
t
..,....
,
4--., Or) C
~0.4-C
_.J (/.)
0 0.2-02O0
1. . . . I "" 300 400 500 600 Wavelength (rim)
700
800
Fig. 3.10. Emission(full line) and stimulationspectrum(dashedline) for OSL fromX-irradiatedNaCI:Eu(from Nanto et al., 1993c).
~
100 c ~
0.8 -
190 C
0
0.6
~,
N
4
1.0 '
160 C 175C
205 C
5
0.4 ~ 0.2 _=
E t._ 0
0
0.0 ~, 0
50
100
150
200
250
T (~
,,A
o
85
OSL Properties of Synthetic Materials
2
0
10
20
30
40
50
Illumination time (s)
Fig. 3.11. OSL decay curves measured at different temperatures. The large difference in the decay observed at 100~ (top curve) and the curve obtained at 160~ (next curve down) is interpreted as being due to thermal detrapping of phototransferred charge. No increase in decay rate is indicated (suggesting no thermal assistance mechanisms). The inset shows the variation in OSL area (under the decay curve) normalised to 1 at room temperature. The data are fitted to an equation of the form given in Eq. (2.64), with W = 0.56 eV and C = 107 (from Bailey et al., 2000).
stimulated with blue light (at 470 nm, near the m a x i m u m of the stimulation spectrum; Nanto et al., 1993c) is non-linear with stimulation power. Thomsen et al. (2002) also studied the LM-OSL and C W - O S L signals from irradiated salt as a potential to use this material in retrospective accident dosimetry (see Section 6.8). 3.2.4. R b I OSL from RbI:X (with X = T1+, In +, P b 2+ o r Eu 2+) was reported by Thoms et al. (1994). Again, the OSL stimulation spectrum is dominated by absorption from F-centres, but additional stimulation features appear, depending upon the dopant used. The emission spectrum also depends upon the dopant. The important electron trapping sites are the Fcentres, Fz-type centres (impurity-perturbed F-centres) and, in Tl-doped material, T1~ centres. Important hole trapping sites are Vwand VkA-centres. Most of the study by Thorns et al. (1994) was concerned with OSL signals below room temperature. For useful X-ray storage phosphors or dosimeters, OSL signals that are stable above room temperature are necessary. The F-centres were shown to be stable up to 350 K and thus the best stimulation wavelength at room temperature is asserted to correspond to F-centre absorption, namely 780 nm in this material. Hole stabilisation above room temperature is provided by VkA-centres.
86
Optically Stimulated Luminescence Dosimetry
It is to be noted that in all of the alkali halide materials discussed here, the stimulation spectrum is in the form of a resonance peak, rather than the edge-like absorption features noted for other materials in Chapter 2. This suggests that in each of the alkali halide materials discussed here, the primary absorption process is excitation of the F-centre into an excited state that is either mixed with the conduction band states, or is accessible to the conduction band via phonon absorption. The lack of evidence of thermal assistance in the OSL process in NaC1, as observed by Bailey et al. (2000), suggests that the latter process is not the case for NaC1 at least. However, other alkali halides (e.g., LiF) are known to have an excited state in the forbidden gap but close to the conduction band edge (McKeever, 1985). In either case, ionisation of the F-centre is apparent, as indicated by photoconductivity during stimulation (e.g., for RbI; Thoms et al., 1994).
3.2.5. CaFe CaF2 doped with Mn is a popular TLD material, usually known as TLD-400 (McKeever et al., 1995). Bernhardt and Herforth suggested its use as an OSL dosimeter as early as 1974 (Bernhardt and Herforth, 1974). Specifically, these authors monitored the optically stimulated phosphorescence, or DOSL. They stimulated gamma-irradiated CaFz:Mn impregnated Teflon TM discs with broad-band light from a tungsten lamp, and monitored the subsequent phosphorescence (DOSL) from the material over a period of 15 min at room temperature. The integrated signal under the DOSL decay curve was linear with absorbed gamma dose over five orders of magnitude, from a low dose of 10 -2 Gy. Allen and McKeever (1990) studied conventional CW-OSL from irradiated CaFz:Mn. Fig. 3.12 shows the excitation spectrum for luminescence emission from un-irradiated and irradiated (259 Gy gamma irradiation) CaFz:Mn at room temperature. In the un-irradiated spectrum, the luminescence is photoluminescence (PL) and the excitation peaks correspond to spin-forbidden absorption transitions from the 6S ground state to the various 4G, 4p and 4D excited states of Mn 2+. After irradiation, an additional broad-band stimulation feature is observed peaking near 300 nm. This is the excitation band for irradiation-induced OSL. In both cases, the emission observed is that from the 4TIg(4G ) first excited state to the 6Alg(6S ) ground state of Mn 2+ ions substituting for host Ca 2+ ions. The induced OSL excitation band is believed to be caused by FA-centres and the OSL emission to be stimulated by electron de-trapping from these centres, recombining with hole centres (possibly Mn 3+ centres) to yield Mn 2+ luminescence. The excitation band bleaches with prolonged illumination at 280-300 nm and is thermally unstable. Several thermal annealing steps are observed with the lowest being at --~50~ OSL from CaF2:Mn is probably not viable for dosimetry purposes due to the thermal instability of the OSL excitation bands, and the fact that the radiation-induced OSL excitation bands overlap with the non-radiation induced PL excitation bands. Thus, there is a strong non-radiation-induced background and low-dose measurements with OSL from this material will be difficult.
OSL Properties of Synthetic Materials
1.0
.
.
.
.
l
'
'I
......
87
I
E ~0.5
Z
200
300 Wavelength (nm)
400
Fig. 3.12. Stimulation spectra of the 495 nm emission from Mn 2+ in CaF2:Mn. Curve 1: before irradiation. Curve 2: after irradiation with a gamma dose of 259 Gy at room temperature. The curves are normalised to give the same maximum intensity (at 400 nm) to facilitate comparison of the curve shapes. The emission spectrum is unaltered by the irradiation (from Allen and McKeever, 1990).
3.2.6. B a F X (X -- Br, C1, I) Understanding the OSL properties of the photostimulable X-ray phosphor BaFX:Eu (X-----Br, C1, I) has become important in recent years because of the use of these materials in digital radiography more so than in dosimetry. Fuji Corporation introduced BaFBr:Eu as a phosphor screen for use in photostimulable digital radiographic imaging, although the universal adoption of the technology has been limited due to constraints on resolution and price. Nevertheless, a high X-ray sensitivity and a linear response to dose are both important properties of such materials and it is, therefore, of interest to include the materials in this section as important OSL phosphors. When doped with europium these materials, of which BaFBr is the archetype, emit OSL at 385 nm (3.2 eV), originating from the same 5f-4d transition in Eu 2+ ions as noted earlier. The OSL intensity is directly proportional to the X-ray dose over several orders of magnitude (see Fig. 3.9). Typically, these phosphors show two stimulation bands, at approximately 600 and 500 nm. It has been suggested that these correspond to absorption by F(Br-)- and F(F-)-centres, respectively, although this is not a universally accepted viewpoint (Starick et al., 1993; Lakshmanan, 1996). The two stimulation bands are also said to be l s - 2 p transitions in F(Br-)-centres (Thoms et al., 1991; von Seggern, 1999). The mechanism for the production of OSL from this material is still under debate. Several mechanisms have been proposed, and have been reviewed by a number of authors (e.g., Lakshmanan, 1996; von Seggern, 1999; Lakshmanan et al., 2000). The first mechanism proposed was that of Takahashi and colleagues (Takahashi et al., 1984; Iwabuchi et al., 1994). Here, X- (or vacuum UV-) irradiation ionises Eu 2+ ions converting them to Eu 3+ ions, either directly or by the trapping of holes. The electrons are trapped by
88
Optically Stimulated Luminescence Dosimetry
Fig. 3.13. Energy level scheme proposed by Iwabuchi et al. (1994) to describe the OSL process in irradiated BaFBr:Eu 2+. The OSL emission process is stimulated by liberation of electrons from F(Br-)- and F(F-)-centres. The electrons recombine with holes trapped at Eu3+-centres to form excited Eu2+-centres, and result in emission of light at 385 nm (from Iwabuchi et al., 1994).
F+-centres converting these to F-centres. Illumination with light in the green-to-red region of the spectrum liberates the electrons from the F-centres, allowing recombination with the Eu 3+ ions and emission from excited Eu 2+ ions at 385 nm. The energy level scheme describing this model is shown in Fig. 3.13. A second proposed mechanism involves energy transfer from recombination of a selftrapped exciton (STE) to a Eu 2+ site (von Seggern et al., 1988; Thoms et al., 1991; von Seggern, 1999). These authors studied the spatial correlation between F(Br-)-centres and Eu 2+ ions and concluded that a significant portion of the OSL resulted from a tunnelling recombination mechanism in which electrons from F(Br-)-centres recombine with holes at Eu 2+ sites, leading to 385 nm emission from relaxation of the excited state of the Eu 2+ ions. The trapped holes are in the form of Vk-centres localised by the Eu 2+ ions. At low temperatures ( < 40 K), the recombination takes place exclusively via tunnelling between spatially correlated pairs of F(Br-)-centres and Eu2+/Vk-centre complexes. At room temperature, however, both tunnelling recombination (between spatially correlated pairs) and free electron recombination at those Eu2+/Vk-centres which are spatially uncorrelated with the F(Br-)-centres contribute to the OSL emission. For the latter process, the temperature has to be high enough so that thermal ionisation of an electron from the relaxed excited state (RES) of the F(Br-)-centre to the conduction band can take place. The recombination schemes are illustrated schematically in Fig. 3.14. Note that F(F-)centres are believed not to take part in the OSL process since it is assumed that too high a thermal activation energy is required for escape from the RES of these centres to the conduction band. Lakshmanan et al. (2000) discuss the effect of oxygen impurities on the OSL process. These impurities substitute for F ions forming 0 2- centres. OSL emission at 480 nm results when electrons from F(Br-) centres recombine with OF centres, while emission at
OSL Properties of Synthetic Materials
89
Fig. 3.14. Energy level scheme proposed by Thoms et al. (1991) to describe the OSL process in irradiated BaFBr:Eu 2+. The OSL emission process is stimulated by either of the two possible mechanisms. In the first, electrons are thermally liberated from the RES of F(Br-)-centres (activation energy 35 meV) and recombine with holes at spatially uncorrelated Eu2+/Vk-centres. The recombination energy is transferred to the Eu 2+ ions forming excited Eu 2+ ions, which relax yielding the emission of light at 385 nm (3.2 eV). In the second mechanism, the electrons tunnel from the RES of the F(Br-)-centres to spatially correlated Eu2+/Vk-centres, again resulting in emission at 385 nm (from Thoms et al., 1991).
400 nm results from the recombination of STEs at the O{- sites. At parts-per-million levels, oxygen appears to enhance the OSL sensitivity, whereas at higher concentrations the OSL from Eu 2+ may be quenched. As discussed by von Seggern (1999), an alternative mechanism to simple ionisation of Eu 2+ impurities (to form Eu 3+ ions) and trapping of electrons by F +-centres (as suggested by Takahashi et al. (1984) and Iwabuchi et al. (1994)) is the formation of F-centres during X-irradiation. Here the absorption of X-rays result in the formation of a STE that relaxes to form an F-centre and an interstitial halide atom (an H-centre). Both F(Br-)- and F(F-)centres may be formed in this way. Thus, the trapped-hole centre may be a Eu2+/H-centre complex rather than a Eu2+/Vk-centre complex (von Seggern, 1999). This straightforward mechanism, modelled on similar processes shown to occur in the alkali halides, removes the difficulty of requiting charged halide vacancies (F +-centres) to exist before irradiation. Although, primarily used in radiographic imaging, BaFBr:Eu has also been suggested for use in conventional ionising radiation dosimetry by Yamadera et al. (1995). These authors tested the OSL response from this material to Bremsstrahlung X-rays (with effective energies from 30 to 140 keV), as well as 6~ and 137Csgamma rays. Linearity from 1 IxSv to 10 mSv was found (with an upper limit of 50 mSv quoted), although the response was shown to be highly non-tissue-equivalent. An additional feature that makes this an unattractive material for dosimetry is the instability of the OSL signal after irradiation. Studies by Yamadera et al. (1995) indicate that the signal fades significantly. Two fading components were observed, with half-lives of 2.29 and 24.37 days, respectively.
90
Optically Stimulated Luminescence Dosimetry
3.3. Sulphates 3.3.1. MgS04 Hydrated salts of MgSO4, n H20, with n = 6 or 7, were studied by Le Masson et al. (2001) for potential applications in fast neutron dosimetry. The materials were prepared with Ce 3+ ions as activators. Although the UV-induced fluorescence signal revealed emission due to these activators, the OSL signal was found to be un-related to the Ce content. OSL could be induced from beta-irradiated samples using infra-red stimulation (830 nm) and BG-39 and U-340 detection filters. The IR-induced OSL signal was found to be linear with absorbed beta dose, but unfortunately exhibited significant fading (50% at room temperature over several hours). The fast neutron response (using a PuBe source) was separated from the gamma response using lead filtration and found to be significant. 3.3.2. CaS04 Calcium sulphate doped with dysprosium was one of the earliest materials to be suggested for use as an OSL dosimeter (Pradhan and Ayyanger, 1976). The method chosen, however, was DOSL due to the optically stimulated transfer of charge to shallow metastable traps. The irradiated sample was optically stimulated with light from a mercury lamp and the luminescence emission was measured after a delay of 15 s from the end of the stimulation. This relatively long delay was possible because of the long-lived phosphorescence signal, which decayed to 50% of its initial value after about 80 s. A minimum detectable limit of about 0.01 Gy (___3%) was measured by these authors. A much shorter delay time (50 ms) was used by Jaek et al. (2002) who used the DOSL signal from irradiated CaSO4:Dy to measure absorbed doses over a similar dose range (up to 30 Gy). The latter authors also used the DOSL signal to determine that the most efficient stimulation wavelengths were centred around 350 nm, for emission at 510 nm.
3.4. Sulphides 3.4.1. AS (A = Mg, Sr, Ca, Ba) The alkaline earth sulphide family of compounds (MgS, CaS, SrS and BaS) has been suggested for use in OSL dosimetry since the first suggested use of OSL as a potential dosimetric method. Antonov-Romanovskii et al. (1956) examined IR-stimulated OSL from Ce + Sm-doped and Eu + Sm-doped SrS. This early work was followed by studies by Br~iunlich et al. (1967) and Sanbom and Beard (1967) on related compounds. All compounds are stimulated with light in the infra-red region of the spectrum. Examples of stimulation spectra are shown in Fig. 3.15, for various MgS compounds, while OSL emission spectra are shown in Fig. 3.16 (from Rao et al., 1984; Mathur et al., 1986). The stimulation spectra are such that these materials can be effectively stimulated using the fundamental wavelength from a Nd:YAG laser (1.06 txm). The emission maximum near 590 nm is believed to be from 4f-transitions with Eu 2+ ions (Rao et al., 1984; Chakrabarti et al., 1988). Cerium doping also induces emission characteristics of transitions to the split
91
OSL Properties of Synthetic Materials (a)
40 D
30 m
oo z
uJ I-
z
20
10
,.
t
,I,
1000
I
11 O0
1200
WAVE LENGTH, nm
.27!
,
(b)
L z
m
,
,. I,,,.=,
z
U,.I
I.,Z .01 700
800
900
1000
11 O0
WAVELENGTH
1200
1300
1400
(nm)
Fig. 3.15. (a) OSL stimulation spectrum for irradiated MgS:Eu,Sm (from Rao et al., 1984). (b) OSL stimulation spectra from irradiated MgS:Ce,Sm. Curve 1--30 s UV exposure; curves II and III--after 300 and 600 s stimulation with 1317 nm subsequent to UV exposure (from Mathur et al., 1986).
ground s t a t e (2F5/2 a n d 2F7/2) of C e 3+ ions (i.e., an emission doublet at ---527 and --- 580 nm; Chakrabarti et al., 1988). Thus, the OSL emission from MgS is characteristic of Eu in Eu + Sm-doped phosphors, and Ce in Ce + Sm-doped phosphors. However, Chakrabarti et al. (1988) also note that the TL emission spectra from these materials is always characteristic of Sm 3+, independent of the identity of the co-dopant. The OSL and
92
Optically Stimulated Luminescence Dosimetry
4o
I
~9 3O
--
20
10
. . . .
1
600
.
.
.
.
.
~
650 Wavelength,
Fig. 3.16. 1984).
.
.
.
.
.
.
700 nm
OSL emission spectra from (A): irradiated MgS:Eu, and (B) irradiated MgS:Eu,Sm (from Rao et al.,
TL mechanisms proposed by the latter authors to account for these observations are illustrated in Fig. 3.17. Here, Sm 2+ acts as the optically sensitive OSL trap. Electrons released from Sm 2+ traps recombine with either Ce 3+ or Eu 2+, as appropriate, yielding OSL emission characteristics of these ions. These authors propose that during TL holes are released from Ce 4+ or Eu 3+ ions, leading to recombination with Sm 3+ and TL emission. A similar mechanism to the above was suggested by Nanto et al. (1999) for OSL emission from SrS:Eu,Sm. The observed emission at 600 nm was from Eu 2+ ions after stimulation with light between 800 and 1700 nm. These authors investigated the utility of this material as a UV dosimeter by monitoring the OSL emission following exposure to filtered (248 nm) sunlight.
3.5.
Oxides
3.5.1. BeO Rhyner and Miller (1970) examined the potential of BeO (discs and powder) as an OSL dosimetry material. As with many of the OSL studies from this era, however, the measurement method was actually DOSL in which the phosphorescence from the sample was monitored following a delay of approximately 2 s. More recent investigations of this material used conventional CW-OSL (Bulur and G6ksu, 1998b), or LM-OSL (Bulur et al., 2001) methods.
OSL Properties of Synthetic Materials
93
Fig. 3.17.
Models for OSL and TL emission from MgS doped with either Ce and Sm, or Eu and Sm (from Chakrabarti et al., 1988).
BeO is a material with a long history in luminescence dosimetry, being suggested initially as a TL dosimeter. Its popularity was primarily due to its near-tissue-equivalence (effective atomic number-- 7.13; McKeever et al., 1995). Several forms of the material are commercially available, although the most popular for luminescence dosimetry has been "Thermalox 995" from Brush Wellman (Brush Beryllium, USA). The crystal structure of BeO is that of wurtzite with each Be atom surrounded by four oxygen atoms in a nearly perfect tetrahedron, with each oxygen atom similarly surrounded by four TM
1000 o !
800
"-"
O) t'0
0
E 0
50 ~
~
1st m e a s u r e m e n t
50 *C 2 nd m e a s u r e m e n t
/ \_
_
[]
75 *C I st m e a s u r e m e n t
-
75 *C 2 nd m e a s u r e m e n t
-
100 ~ 2 nd m e a s u r e m e n t
600 ~'
o
~
400
0
t---
n
200 -
0
|
400
!
|
|
,!
450
|
!
|
|
500
,|
!
|
!
550
!
---
600
|
!
|
|
700
Wavelength (nm) Fig. 3.18.
Stimulation spectra for OSL from irradiated BeO, obtained at different readout temperatures, as indicated. Emission is in the range 340 ___40 nm (from Bulur and Grksu, 1998b).
94
Optically Stimulated Luminescence Dosimetry 107
(a)
-
10 6
12.8 Gy 64Gy 3.2 Gy
105
10 Gy 80o mGy
- ~
400 mGy 200 mGy 00 mGy
ffl
cO
10 4
O
cO O c"
n
103
-"
~
i
mGy 9 mGy
" ~ 1 1 . 2 mGy 5.6 mGy
102 Background
___
101
I
'
0
1
9
1
50
'
I
100
;
I
150
''
I
200
250
Time (seconds) 109
10 8 = >,
Curve Fitting
......
.,~
J,b,
'
107
~9
10 6
~
105
t--
[]
- -
_J
9
104 10 3 ' 10 -4
'''i''l
'
1 0 -3
''''"'1
10 - 2
'
''"r
'
10 -1
'''""1
'
10 ~
' ''""1
101
'
' i'""l
10 2
Absorbed Dose (Gy) Fig. 3.19. (a) OSL signals from beta (9~176 irradiated BeO, measured with a 250 s stimulation and after a 125~ s pre-heat. (b) Dose response obtained from integration of the OSL signals in (a) from 0 to 250 s (from Bulur and Gbksu, 1998b).
beryllium atoms. Common impurities include Mg, Si, Ca, B and A1 (see McKeever et al. (1995) for further discussion). Although the TL mechanism is not fully understood, Kortov et al. (1993) have suggested that Li impurities in Li-doped BeO play an important role as a hole trap, and that A12+ sites act as the recombination centres. Recombination induces luminescence emission near 330 nm. One of the main difficulties with the material as a TL dosimeter is the light sensitivity of the dosimetric TL signal (e.g. Gammage and Cheka, 1977). However, this leads to obvious potential of the material as an OSL dosimeter. Despite a very early study by Albrecht and Mandeville (1956) who examined optical stimulation of luminescence from X-irradiated BeO, the OSL properties of this material were not thoroughly investigated until the work
OSL Properties of Synthetic Materials
93
of Bulur and G6ksu (1998b), and apart from the studies of LM-OSL by Bulur et al. (2001) little has been done since then. The stimulation spectrum, for OSL emission within a detection window of 340 ___40 nm, is shown in Fig. 3.18 and consists of a broad band between 420 and 550 nm, with a maximum at 435 nm and a smaller peak near 400 nm. Strong thermal quenching of the signal is observed, with the signal intensity reducing significantly between 50 and 100~ The thermal quenching energy was determined to be between 0.48 and 0.52 eV. The CW-OSL signal is itself made up of several components, some of which are thermally unstable. Pre-heating the sample to 125~ after irradiation removes the unstable components, leaving a very stable OSL signal. Even the stable signal consists of multiple components, however, as can be observed using LM-OSL after removal of the unstable components (Bulur et al., 2001). Bulur and G6ksu (1998b) determined that the primary trap responsible for the stable OSL signal has a thermal activation energy of 1.74 eV (with a corresponding frequency factor of 6.4 x 1013 s-l). This appears to correspond to a TL signal near 340~ but a clear correlation has not been established. Surprisingly, even after pre-heating, the OSL signal was observed to fade by about 5% over approximately 1 h at room temperature, followed by a long period of stability. Using the stable OSL component, a linear dose response was obtained from less than a few mGy to over 10 Gy (as shown in Fig. 3.19). 3.5.2. Fused quartz A new OSL material based on fused quartz glass doped with Cu has been developed by the US Naval Research Laboratory (Justus et al., 1997; 1999a,b; Huston et al., 2002). The
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Fig. 3.20. Comparison of TL and OSL emission spectra from Cu-doped fused quartz glass. The TL data (dots) were obtained by integrating the TL signal from 130 to 185~ The OSL signal was stimulated with 790 nm light from a GaAs diode laser (from Justus et al., 1999a).
96
Optically Stimulated Luminescence Dosimetry
emission spectrum (Fig. 3.20) peaks at 505 nm, believed to be due to spin-forbidden 3d l ~ 3d94s transitions from Cu + ions in this matrix. The exact emission maximum varies with the chemical composition of the glass host. There is no evidence of multiple types of Cu + site (Justus et al., 1999a). The CW-OSL curve from this material is shown in Fig. 3.2 l a, and the response to absorbed 6~ gamma dose is shown in Fig. 3.2 lb. The OSL signal is associated with a TL peak at approximately 200~ The OSL mechanism is not fully
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OSL Properties of Synthetic Materials
97
u n d e r s t o o d but is b e l i e v e d to i n v o l v e t r a p p i n g o f h o l e s at C u +, f o r m i n g C u 2+. ( A l t e r n a t i v e l y , one m a y v i e w this as i o n i s a t i o n o f C u + to f o r m Cu2+.) T h e f r e e d e l e c t r o n s are t r a p p e d e l s e w h e r e at unidentified sites. S t i m u l a t i o n w i t h light ( O S L ) or heat ( T L ) r e l e a s e s the t r a p p e d electrons, w h i c h r e c o m b i n e at the C u 2+ sites f o r m i n g C u + in an e x c i t e d state. R e l a x a t i o n o f the e x c i t e d C u + ions leads to g r e e n e m i s s i o n n e a r 505 n m ( H u s t o n et al., 2002). T h e s t i m u l a t i o n efficiency p e a k s n e a r 390 n m and falls off b y about a factor o f 10 by 800 nm. D e s p i t e this, efficient s t i m u l a t i o n can be a c h i e v e d using an I R diode at 790 n m ( H u s t o n et al., 2002).
References Agersnap Larsen, N., BCtter-Jensen, L., McKeever, S.W.S., 1999. Thermally stimulated conductivity and thermoluminescence from A1203:C. Radiat. Prot. Dosim. 84, 87-90. Akselrod, M.S., Agersnap Larsen, N., Whitley, V., McKeever, S.W.S., 1998. Thermal quenching of F-centre luminescence in A1203:C. J. Appl. Phys. 84, 3364-3373. Akselrod, A., Akselrod, M.S., 2002. Correlations between OSL and the distribution of TL traps in A1203:C. Radiat. Prot. Dosim. 100, 217- 220. Akselrod, M.S., Kortov, V.S., Gorelova, E.A., 1993. Preparation and properties of o~-A1203:C. Radiat. Prot. Dosim. 47, 159-164. Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescent aniondefect oL-A1203:C single crystal detectors. Radiat. Prot. Dosim. 33, 119-122. Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-175. Albrecht, H.O., Mandeville, C.E., 1956. Storage of energy in BeO. Phys. Rev. 101, 1250-1254. Allen, P., McKeever, S.W.S., 1990. Studies of PTTL and OSL in TLD-400. Radiat. Prot. Dosim. 33, 19-22. Antonov-Romanovskii, V.V., Keirum-Marcus, I.F., Poroshina, M.S., Trapeznikova, Z.A., 1956. Conference of the Academy of Sciences of the USSR on the Peaceful Uses of Atomic Energy, Moscow, 1955, USAEC Report AEC-tr-2435 (Pt. 1), 239. Bailey, R.M., Adamiec, G., Rhodes, E.J., 2000. OSL properties of NaC1 relative to dating and dosimetry. Radiat. Meas. 32, 717-723. Bailiff, I.K., Clark, R.J., 1999. A preliminary study of the fast time-resolved luminescence in A1203:C. Radiat. Prot. Dosim. 84, 457-460. Bernhardt, R., Herforth, L., 1974. Radiation dosimetry by optically stimulated phosphorescence of CaF2:Mn. In: Newiadomski, T. (Ed.), Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, pp. 1091-1104. Bos, A.J.J., 2001. On the energy conversion in thermoluminescence dosimetry materials. Radiat. Meas. 33, 737 -744. BCtter-Jensen, L., Agersnap Larsen, N., Markey, B.G., McKeever, S.W.S., 1997. A1203:C as a sensitive OSL dosemeter for rapid assessment of environmental photon dose rates. Radiat. Meas. 27, 295-298. BCtter-Jensen, L., Banerjee, D., Jungner, H., Murray, A.S., 1999. Retrospective assessment of environmental dose rates using optically stimulated luminescence from A1203:C and quartz. Radiat. Prot. Dosim. 84, 537-542. BCtter-Jensen, L., McKeever, S.W.S., 1996. Optically stimulated luminescence dosimetry using natural and synthetic materials. Radiat. Prot. Dosim. 65, 273-280. Br/iunlich, P., Sch~ifer, D., Scharmann, A., 1967. A simple model for thermoluminescence and thermally stimulated conductivity of inorganic photoconducting phosphors and experiments pertaining to infra-red stimulated luminescence. Proceedings of 1s t International Conference on Luminescence Dosimetry, Stanford, June 1965, USAEC, 57-73. Bulur, E., BCtter-Jensen, L., Murray, A.S., 2001. LM-OSL signals from some insulators: an analysis of the dependency of the detrapping probability on stimulation light intensity. Radiat. Meas. 33, 715-719. Bulur, E., G6ksu, H.Y., 1998a. Infrared stimulated luminescence from A1203:C. Radiat. Meas. 29, 625-638.
98
Optically Stimulated Luminescence Dosimetry
Bulur, E., G6ksu, H.Y., 1998b. OSL from BeO ceramics: New observations from an old material. Radiat. Meas. 29, 639-650. Chakrabarti, K., Mathur, V.K., Rhodes, J.F., Abbundi, R.J., 1988. Stimulated luminescence in rare-earth-doped MgS. J. Appl. Phys. 64, 1363-1366. Chernov, V., Melrndrez Ao, R., Piters, T.M., Barboza-Flores, M., 2001. Thermally and optically stimulated luminescence correlated processes in X-ray irradiated KCI:Eu. Radiat. Meas. 33, 797-800. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1996. Phototransferred thermoluminescence in A1203:C. Radiat. Proc Dosim. 65, 263-266. Douguchi, Y., Nanto, H., Sato, T., Imai, A., Nasu, S., Kusano, E., Kinbara, A., 1999. Optically stimulated luminescence in Eu-doped KBr phosphor ceramics. Radiat. Prot. Dosim. 84, 143-148. Duller, G.A.T., BCtter-Jensen, L., 1993. Luminescence from potassium feldspars stimulated by infrared and green light. Radiat. Prot. Dosim. 47, 683-688. Erfurt, G., Krbetschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behavior of AlzO3:Cpotential for dosimetric applications. Radiat. Meas. 32, 735-739. Evans, B.D., Stapelbroek, M., 1978. Optical properties of F+-properties in crystalline A1203. Phys. Rev. B 18, 7089-7098. Gammage, R.B., Cheka, J.S., 1977. Further characteristics important in the operation of ceramic BeO TLD. Health Phys. 32, 189-192. Grimadova, T.I., Bessonova, T.S., Tale, I.A., Avvakumova, L.A., Bodyachevsky, S.V., 1990. On the thermoluminescence mechanism of non-doped corundum monocrystals with defect structure. Radiat. Prot. Dosim. 33, 47-50. Huston, A.L., Justus, B.L., Falkenstein, P.L., Miller, R.W., Ning, H., Altemus, R., 2002. Optically stimulated luminescent glass optical fibre dosimeter. Radiat. Prot. Dosim. 101, 23-26. Iwabuchi, Y., Mori, N., Takahashi, K., Matsuda, T., Shionoya, S., 1994. Mechanism of photostimulated luminescence process in BaFBr:Eu 2+ phosphors. Jpn. J. Appl. Phys. 33, 178-185. Jaek, I., Kerikm~ie, M., Lust, A., 2002. Optically stimulated luminescence in some thermoluminescence detectors as an indicator of absorbed radiation dose. Radiat. Prot. Dosim. 100, 459-462. Justus, B.L., Merritt, C.D., Pawlovich, K.J., Huston, A.L., Rychnovsky, S., 1999a. Optically stimulated luminescence dosimetry using doped fused quartz. Radiat. Prot. Dosim. 84, 189-192. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999b. Optically and thermally stimulated luminescence characteristics of Cu-doped fused quartz. Radiat. Prot. Dosim. 81, 5-10. Justus, B.L., Rychnovsky, S., Huston, A.L., Miller, M.A., Pawlovich, K.J., 1997. Optically stimulated luminescence radiation dosimetry using doped silica glass. Radiat. Prot. Dosim. 74, 151-154. Kitis, G., Papadopoulos, J.G., Charalambous, S., Tuyn, J.W.N., 1994. The influence of heating rate on the response and trapping parameters of c~-A1203:C. Radiat. Prot. Dosim. 55, 183-190. Kortov, V.S., Milman, I.I., Kirpa, V.I., Lesz, J., 1994. Some features of A1203:C dosimetric thermoluminescent crystals. Radiat. Prot. Dosim. 55,279-283. Kortov, V.S., Milman, I.I., Nikiforov, S.V., 1999. The effect of deep traps on the main features of thermoluminescence in dosimetric ct-Al203 crystals. Radiat. Prot. Dosim. 84, 35-38. Kortov, V.S., Milman, I.I., Slesarev, A.I., Kijko, V.S., 1993. New BeO ceramics for TL ESR dosimetry. Radiat. Prot. Dosim. 47, 267-270. Lakshmanan, A.R., 1996. Radiation-induced defects and photostimulated luminescence processes in BaFBr:Eu 2+. Phys. Stat. Sol., (a) 153, 3-27. Lakshmanan, A.R., Murase, N., Yazawa, T., Qui, J., Mitsuyu, T., Hirao, K., Tomita, A., Hoffman, A., 2000. Luminescence studies in BaFBr and BaFBr:Eu 2+. Proceedings of the ISLA-200 Conference, Baroda, February 2000, 207- 212. Le Masson, N.J.M., Bos, A.J.J., Van Eijk, C.W.E., 2001. Optically stimulated luminescence in hydrated magnesium sulfates. Radiat. Meas. 33, 693-697. Markey, B.G., Colyott, L.E., McKeever, S.W.S., 1995. Time-resolved optically stimulated luminescence from etA1203:C. Radiat. Meas. 24, 457-463. Markey, B.G., McKeever, S.W.S., Akselrod, M.S., BCtter-Jensen, L., Agersnap Larsen, N., Colyott, L.E., 1996. The temperature dependence of optically stimulated luminescence from o~-A1203:C. Radiat. Prot. Dosim. 65, 185-189.
OSL Properties of Synthetic Materials
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Mathur, V.K., Gasiot, J., Abbundi, R.J., Brown, M.D., 1986. Optically stimulated luminescence in MgS:Ce,Sm. Radiat. Prot. Dosim. 17, 333-336. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. McKeever, S.W.S., Akselrod, M.S., Colyott, L.E., Agersnap Larsen, N., Polf, J.C., Whitley, V., 1999. Characterisation of A1203 for use in thermally and optically stimulated luminescence dosimetry. Radiat. Prot. Dosim. 84, 163-168. McKeever, S.W.S., Markey, B.G., Akselrod, M.S., 1996. Pulsed optically-stimulated luminescence dosimetry using ot-A1203:C. Radiat. Prot. Dosim. 65, 267-272. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford, UK. Melrndrez, R., P@ez-Slas, R., Pashenko, L.P., Aceves, R., Piters, T.M., Barboza-Flores, M., 1996. Dosimetric properties of KCI:Eu2+ under oL, [3, F, X-ray and ultraviolet irradiation. Appl. Phys. Lett. 68, 3398-3400. Milman, I.I., Kortov, V.S., Nikiforov, S.V., 1998. An interactive process in the mechanism of the thermally stimulated luminescence of anion-defective oL-A1203 crystals. Radiat. Meas. 29, 401-410. Molnfir, G., Benabdesselam, M., Borossay, J., Lapraz, D., Iacconi, P., Akselrod, M.S., 2001a. Influence of the irradiation temperature on TL sensitivity of oL-A120:C. Radiat. Meas. 33, 619-623. Molnfir, G., Benabdesselam, M., Borossay, J., Lapraz, D., Iacconi, P., Kortov, V.S., Surdo, A.I., 200lb. Photoluminescence and thermoluminescence of titanium ions in sapphire crystals. Radiat. Meas. 33,663-667. Moscovitch, M., Tawil, R.A., Svikin, M., 1993. Light-induced fading in o~-A1203. Radiat. Prot. Dosim. 47, 251-253. Nanto, H., Murayama, K., Usuda, T., Endo, F., Hirai, Y., 1993a. Laser-stimulable transparent KCI:Eu crystals for erasable and rewritable optical memory utilizing photostimulated luminescence. J.Appl. Phys. 74, 1445-1447. Nanto, H., Murayama, K., Usuda, T., Taniguchi, S., Takeuchi, N., 1993b. Optically stimulated luminescence in KCI:Eu single crystals. Radiat. Prot. Dosim. 47, 281-284. Nanto, H., Sato, T., Kashiwagi, N., Miyazaki, M., Nasu, S., Kusano, E., Kinbara, A., Douguchi, Y., 1999. A UV dosimeter utilising photostimulated luminescence in SrS:Eu,Sm phosphor ceramics. Radiat. Prot. Dosim. 85, 305-307. Nanto, H., Usuda, T., Murayama, K., Nakamura, S., Inabe, K., Takeuchi, N., 1993c. Emission mechanism of optically stimulated luminescence in copper-doped sodium chloride single crystals. Radiat. Prot. Dosim. 47, 293-296. Pelenyov, V.E., Kortov, V.S., Milman, I.I., 2001. The interaction of deep traps in anion-defective c~-A1203. Radiat. Meas. 33, 629-631. Pogatshnik, G.J., Chen, Y., Evans, B.D., 1987. A model of lattice defects in sapphire. IEEE Trans. Nucl. Sci. NC34, 1709-1712. Poolton, N.R.J., Bulur, E., Wallinga, J., BCtter-Jensen, L., Murray, A.S., Willumsen, F., 2001. An automated system for the analysis of variable temperature radioluminescence. Nucl. Instr. Meth. B 179, 575-584. Pradhan, A.S., Ayyanger, K., 1976. Radiation dosimetry by photostimulated luminescence in CaSO4:Dy. Int. J. Appl. Radiat. Isot. 28, 534-535. Rao, R.P., de Murcia, M., Gasiot, J., 1984. Optically stimulated luminescence dosimetry. Radiat. Prot. Dosim. 6, 64-66. Rhyner, C.R., Miller, W.G., 1970. Radiation dosimetry by optically-stimulated luminescence of BeO. Health Phys. 18, 681-684. Sanborn, E.N., Beard, E.L., 1967. Sulfides of strontium, calcium, and magnesium in infra-red stimulated luminescence dosimetry. Proceedings of 1s t International Conference on Luminescence Dosimetry, Stanford, June 1965, USAEC, 183-191. von Seggern, H., 1999. Photostimulable x-ray storage phosphors: a review of present understanding. Braz. J. Phys. 29, 254-268. von Seggern, H., Voight, T., Kniipfer, W., Lange, G., 1988. Physical model of photostimulated luminescence of x-ray irradiated BaFBr:Eu 2+. J. Appl. Phys. 64, 1405-1412. Springis, M., Kulis, P., Veispals, A., Tale, I., 1995. Photo- and thermostimulated processes in o~-A1203. Radiat. Meas. 24, 453-456.
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Optically Stimulated Luminescence Dosimetry
Starick, D., Gurvich, A.M., Myagkova, M.G., Riidiger, J., Herzog, G., 1993. The influence of preparation conditions on the performance of BaFBr-Eu storage phosphors. Nucl. Tracks Radiat. Meas. 21, 39-41. Summers, G.P., 1984. Thermoluminescence in single crystal oL-A1203:C. Radiat. Prot. Dosim. 8, 69-80. Takahashi, K., Kohda, K., Miyahara, J., 1984. Mechanism of photostimulated luminescence in BaFX:Eu 2+ (X = C1, Br) phosphors. J. Lumin. 31 & 32, 266-268. Tale, I., Piters, T.M., Barbosa-Flores, M., Perez-Salas, R., Aceves, R., Springis, M., 1996. Optical properties of complex anion vacancy centres and photo-excited electronic processes in anion defective o~-A1203. Radiat. Prot. Dosim. 65, 235-238. Thoms, M., von Seggern, H., Winnacker, A., 1991. Spatial correlation and photostimulability of defect centres in the x-ray-storage phosphor BaFBr:Eu2+. Phys. Rev. B 44, 9240-9247. Thoms, M., von Seggern, H., Winnacker, A., 1994. Optical and thermal properties of electron- and hole-trapping sites in the x-ray storage phosphor RbI:X (X = T1+, In +, Pb 2+, Eu2+). J. Appl. Phys. 76, 1800-1808. Thomsen, K.J., BCtter-Jensen, L., Murray, A.S., 2002. Household and workplace chemicals as retrospective luminescence dosemeters. Radiat. Prot. Dosim. 101, 515-518. Walker, F.D., Colyott, L.E., Agersnap Larsen, N., McKeever, S.W.S., 1996. The wavelength dependence of lightinduced fading of thermoluminescence from o~-A1203:C. Radiat. Meas. 26, 711-718. Whitley, V.H., McKeever, S.W.S., 2000. Photoionisation of deep centers in A1203. J. Appl. Phys. 87, 249-256. Whitley, V.H., McKeever, S.W.S., 2001. Linearly modulated photoconductivity and linearly modulated optically stimulated luminescence measurements on A1203:C. J. Appl. Phys. 90, 6073-6083. Yamadera, A., Kim, E., Miyata, T., Nakamura, T., 1995. Development of high sensitivity X-ray and ~/-ray personal dosimeter using photostimulated luminescent detector. Appl. Radiat. Isot. 46, 467-468.
Chapter 4
Passive optically stimulated luminescence dosimetry 4.1. Personal dosimetry 4.1.1. Introduction
There are currently approximately five million personal dosimetry badges being used by radiation workers around the world (2002 data), including thermoluminescence dosimeters (TLDs), film badges, optically stimulated luminescence (OSL) dosimeters and others. Although, it may be argued that the use of OSL in personal dosimetry is not yet widespread in terms of the number of different laboratories running OSL instruments, it has to be recognised nevertheless that OSL is indeed a major player in the personal dosimetry arena since approximately 25% of the five million badges are in fact OSL dosimeters, primarily based on pulsed OSL (POSL) from A1203:C. As already described in Chapter 1, OSL has been suggested as a personal dosimetry method for several decades, with various forms of the OSL method being suggested at one point or another in the published literature. None of these early pioneering studies, however, led to a commercial-scale personal dosimetry system until the advent of POSL of A1203:C (Akselrod and McKeever, 1999), and the subsequent development of Luxel TM personal dosimeters by Landauer Inc. in the late 1990s. A possible exception to this statement is the use of radiophotoluminescence (RPL) of phosphate glasses in personal dosimetry throughout the 1980s (Perry, 1987; Piesch et al., 1986; 1990; 1993). However, as discussed elsewhere (Chapter 2), RPL and OSL are different physical phenomena and we limit the discussion here to a description of OSL personal dosimetry. Following the success of POSL in this application, newly developed commercial OSL personal dosimetry systems are now available using continuous wave-OSL (CW-OSL) as a readout mode. Descriptions of some commercial OSL personal dosimetry systems are given in the following sections. In general, the key features of OSL that make it so attractive for personal dosimetry include the ability to tune the performance across a wide dynamic dose range by being able to vary the stimulation power. This allows optimum sensitivity at both the low-dose and the high-dose ends of the dose response. Furthermore, by adjusting the stimulation power appropriate to the dose range, one has the ability to re-read the OSL signal since the high sensitivity allows dose assessment even though only a fraction of the stored charge is depleted during the measurement. Thus, a complete re-analysis is achieved and true, independent dose readings are possible. This is to be distinguished from film
102
Optically Stimulated Luminescence Dosimetry
badge dosimetry in which the developed image can be read a second time, but the film can only be developed once; thus the dose readings are not independent in this sense. With thermoluminescence (TL), of course, the entire signal is destroyed (100% depletion of the trapped charge) during one reading and re-reading is not possible. Both fast readout and fast analysis, important for processing large numbers of dosimeters, are made possible with the speed of POSL, although at the cost of expensive equipment. Multiple analyses are possible with cheaper LEDs as the stimulation source, but at a cost of slower analysis time. In addition, the all-optical nature of the OSL process removes the need for heating and allows for imaging capabilities normally only expected with film badges. 4.1.2. Landauer's Luxel TM personal dosimetry system The Luxel ~Mbadge is illustrated in Fig. 4.1 a-c. The heart of the badge, i.e., the detector itself, is shown in Fig. 4.1a and is a thin layer of A1203:C powder deposited onto a clear polyester film base. The size of the active area of the detector is 16.5 mm by 18.5 mm and the grain size of the A1203:C powder is in the range 20-90 lxm. The powder layer is protected by a thin, clear polyester tape. The detector is located inside a filter pack (Fig. 4.1b) consisting of an open window, a copper filter (250 txm thick), a tin filter (500 Ixm thick) and a 500 Ixm-thick copper grid consisting of a 5 x 5 array of 1.0 mm diameter holes. When the filter pack is folded over the detector, each of the filters sits over its own quadrant, coveting part of the A1203:C powder film. In normal analysis mode, a POSL reading is taken from those three parts of the A1203:C film that sit under the three circular filters (open, Cu and Sn). From these three independent POSL readings deep dose (Hp(10)), shallow dose (Hp(0.07)) and eye dose (Hp(0.3))--see Chapter 1 for definitions--may be evaluated through use of a suitable algorithm of the form:
D = alow -t- BIcu + Clsn
(4.1)
where Ii (i = OW, Cu, Sn) is the POSL intensity measured under the open window, copper or tin filters, respectively, and A, B and C are the coefficients unique to each of the different filters. The POSL readout procedure (Akselrod and McKeever, 1999) is described in detail in Chapter 7, while the theory behind the POSL technique is described in Chapter 2. An example POSL versus dose curve for A1203:C is illustrated in Chapter 3, Fig. 3.4. A typical POSL measurement takes 350-1000 ms per reading, and each reading depletes the POSL signal by only a fraction of the stored information available. Thus, second, third, or more readings may be performed if required, in order to achieve true second dose readings from one dosimeter (Akselrod and McKeever, 1999). 4.1.3. Landauer's InLight TM personal dosimetry system A new development in OSL personal dosimetry is the InLight TM system, also by Landauer. This is a bench-top, CW-OSL system specifically designed for personal dosimetry and uses bright LEDs (green) to stimulate the OSL. It is again based on A1203:C. The use of LEDs results in longer readout times compared with POSL, but
103
Passive Optically Stimulated Luminescence Dosimetry
Fig. 4.1. The Landauer Luxel personal dosimetry badge. (a) The A1203:C detector. (b) The filter pack, showing the open window, the Cu filter, the Sn filter and the Cu-grid filter. (c) The complete Luxel badge (courtesy of Landauer Inc., USA). TM
TM
enables dose readings for very little depletion of the signal. This in turn leads to a different paradigm for personal dosimetry badge wearers--namely that the InLight TM badges are normally distributed to individuals for a full year, with periodic readings taken twice every month during normal operation. In this way, not only are the doses corresponding to the individual wear periods evaluated, but the last reading gives the individual's total accumulated dose for the whole year. The stimulation power is such that the twicemonthly readings deplete the signal by only approximately 10% over one year. The InLight TM badge is illustrated in Fig. 4.2. Four A1203:C powder films are used as dosimeters. The operational dose range is up to 10 Gy, for use with photons > 5 keV, and
104
Optically Stimulated Luminescence Dosimetry
Fig. 4.2. The Landauer InLight personal dosimetry badge. Four A1203:C detectors, constructed in the same way as a Luxel detector, fit within the badge holder (courtesy of Landauer Inc., USA). TM
TM
with electrons > 150 keV. As with all A1203:C dosimeters, the system is insensitive to neutrons. InLight TMis designed for organisations that wish to perform their own dosimetry, whereas LuxelT~' is designed for use by radiation dosimetry service providers.
4.1.4. Beta dosimetry Akselrod et al. (1999) demonstrated the ability to measure Hp(0.07) for low-energy beta radiation using OSL from A1203:C thin dosimeters. A1203:C powder grains (20-25 txm) were crushed into the surface of an aluminium substrate resulting in a surface density of A1203:C powder between 7.5 and 9.3 mg/cm 2. Both CW-OSL and POSL measurements sources. were performed following beta irradiation from 147pm, 2~ and 9~176 Through use of appropriate Mylar thin-film filters, having an effective filter thickness of 3.8 mg/cm 2, a flat normalised energy response for Hp(0.07) was obtained from 0.224 (for 147pm) to 2.280 M e V (9~176 to within both EU and US DOELAP limits.
4.1.5. POSL imaging By using a series of radiation-absorbing filters, of different atomic number and thickness as described above, differential absorbed dose measurements can be made and information pertaining to the quality of the absorbed radiation is produced. For some dose evaluations, however, further analysis is warranted. By examining the POSL signal from that part of the detector under the copper grid (Fig. 4.1b) one can take advantage of the imaging capabilities of the OSL method to map a "radiation image" in order to detect abnormal exposure conditions. A possible abnormal exposure condition of interest might be the intentional, or accidental, exposure of the badge to a radiation source when a person is not wearing the badge. Someone placing the radiation film badge close to a radiation source in order to expose the badge (but not the person) to the radiation, would provide an example. This could be described as a "static" exposure and is to be distinguished from a "dynamic" exposure in which a person wears the film badge on his/her clothing over an
Passive Optically Stimulated Luminescence Dosimetry
105
extended period. Other potential abnormal exposure conditions include the unintended shielding of the detector by external objects (coins, paper-clips, etc.) due to the badge being worn incorrectly (say) in a person's pocket rather than being worn correctly on the outside of a person's clothing. Still other abnormal exposure conditions include contamination of the badge by radioactive contaminants, or a radiation filter being physically damaged. It is desirable, therefore, to be able to provide additional information about the conditions of the radiation exposure such that abnormal exposures may be identified. Akselrod et al. (2000) describe an analysis using Luxel TMbadges that had been exposed to a variety of radiation fields under a wide range of dynamic, static and other abnormal conditions. The approach taken by Akselrod et al. (2000) was to de-focus the stimulation laser beam during POSL readout and to image the pattern of luminescence emission from that part of the detector that was under the copper grid. Under conditions of static exposure, the image obtained (using an intensified, cooled CCD camera) of the POSL emission from the detector revealed a series of "sharp" peaks, corresponding to the penetration of the radiation field through the holes in the grid, and the absorption of the field by the copper parts of the grid (see Fig. 4.3a). On the other hand, detectors that had been moved during exposure were shown to give a broad, "blurred" image with no obvious image of the filter holes. The distinction between sharp and blurred image was assisted by performing a Fourier transform to obtain the spatial frequency of the obtained POSL image (see Fig. 4.3b). The two-dimensional discrete Fourier transform (DFT) used in the analysis by Akselrod et al. (2000) was: ~1~1
{
(nn 1
1 f(n, m) exp - j 2rr - ~ F ( n l , m l ) - NM n=O m=O
mml) } + ---M-- '
(4.2)
where N x M is the resolution of the spatial image and the transform represents the power spectrum in the frequency domain (wn, O)m), where the wn and o)m axes correspond, respectively, to the n and m axes. The low-frequency components of the DFT represent gradual variations in the POSL image, while the high-frequency components are a result of the pixel noise, sharp contrast features, etc. Thus, by selectively filtering unwanted frequencies, one can reduce the influence of unwanted noise or background signals.
Fig. 4.3. A POSL image from the Luxel A1203:Cdetector, obtained with a static 1 mGy exposure of M30 X-rays. (b) The DFT of the image pattern (from Akselrodet al., 2000). TM
Optically Stimulated Luminescence Dosimetry
106
Akselrod et al. (2000) applied a weighting function W(nl, ml), which may be viewed as a simple filter, such that its value was set to zero for all spatial frequencies above 20% of the spatial frequency range, and below 2% of the frequency range. This had the effect of eliminating the high-frequency components associated with pixel noise, etc., and the lowfrequency components associated with the dc signal in the image. Thus, only that part of the frequency spectrum corresponding to the filter pattem was retained. Inverse transformation then yields an almost-noise-free image. The image shown in Fig. 4.3a was obtained in this manner. After application of a suitable filter, numerical values may be extracted from the DFT before inverse transformation in order to calculate a parameter that represents the probability of static or dynamic irradiation. Two numerical parameters were defined to achieve this goal. The first was defined as a "shape parameter" ~:, which is a number representing the shape of the filtered DFT. The shape parameter is thus defined as: N
M
Z r __
2
nl = 0
IF(nl' m l)W(nl, m l)l
m 1= 0
Max g/1 ~ml
{[F(nl,ml)W(nl,ml)]}
(4.3)
where Max{ 9} represents the maximum of the DFT spectrum over the indicated range, and I~ represents the complex magnitude of the argument. Defined in this way, s is a general measure of the spatial frequency distribution of the image. An alternate parameter was defined as a "relative modulation" /x. From a priori knowledge of the filter pattern used in the definition of the POSL image, the ideal static image would consist of a regular array of bright POSL locations, the positions of which are dictated by the pattern of the filter used (Fig. 4. lb). This unique pattern is characterised by well-defined frequencies in the n- and m-directions. For the pattern shown in Fig. 4.3a the DFT (shown in Fig. 4.3b) is defined by eight "satellites" surrounding a central zerofrequency component. The satellites are located in the DFT at those frequencies (i.e., at those pixels) corresponding to the spatial separation of the holes in the filter in the n- and m-directions. The relative modulation, /x, is defined by extracting the intensities of the dominating four satellites along the o)n and o)m axes, thus: 4
k=l
IX-- Mean{F(nl,ml)}"
(4.4)
Here, IF(n~,m~)[ is the magnitude of the DFT at those pixels k(k-1...4) corresponding to the four satellites characterizing the filter used, and Mean{. } is the average values of those pixels in the DFT corresponding to the high-frequency range. In the determination of/~, a filter is not applied, i.e., W(n~,m l ) = 1 for all (n l, m 1). An advantage of the relative modulation method is that even if the image is partially obscured by an unwanted object (coin, etc.) the signal at the four characteristic pixels still represents only that signal from the irradiation filter, and not from the obscuring object. Thus, the pattern can still be recognised and the value of/~ can still be determined, even in this case.
Passive Optically Stimulated Luminescence Dosimetry
107
Akselrod et al. (2000) extracted values for ~: and ~ from a series of POSL images obtained under a variety of exposure conditions. They then defined a discrimination value for each parameter (~:aiscrand ~discr) to distinguish between static and dynamic exposures. Since ~discr and /[J,discr are dose-dependent, the discrimination levels were determined by plotting the sc and ~ values as a function of dose for all irradiations--i.e., for all energies and for both static and dynamic irradiations, and the dose dependencies of the discrimination levels were determined as ~discr : 623-6D-~ and ]~discr--50-12D~ The data are shown in Fig. 4.4. These functions are the loci of the sc or/x values at the lower (for ~ or upper (for ~) edges of the data for dynamic irradiations and form a clearly defined discrimination level to distinguish between static and dynamic irradiation. The procedure was found to be successful for > 90% of the cases tested when using/Xaiscr, and > 80% successful when using ~:aiscr, even when the images were partially obscured by unwanted objects (coins, paper clips, etc.).
4.2. Environmental OSL dosimetry using A1203:C 4.2.1. Measurement of the natural terrestrial background radiation The high sensitivity of A1203:C has been exploited for environmental dosimetry using TL. For example, Budzanowski et al. (1996) reported the ability to measure integrated environmental photon doses of the order of 4 IxGy using the TL signal from this material. The capability of using the OSL response of A1203:C to measure the environmental photon radiation over short periods was tested by BCtter-Jensen et al. (1997). The response of A1203:C compares well with that of a high-pressure ionisation chamber (Reuter Stokes RS-111). As seen from Table 4.1, even at low doses, the OSL uncertainties are small and the OSL measurements agree very well with those obtained with the ionisation chamber (BCtter-Jensen et al., 1997). To illustrate the OSL yield, the decay curves from A1203:C exposed to integrated natural environmental radiation doses of 0.98 and 5.10 txGy obtained over 15 and 72 h, respectively, compared with that from a 44 I~Gy 6~ gamma calibration dose are shown in Fig. 4.5 (from BCtter-Jensen et al., 1997). It has been shown that A1203:C can be completely emptied of trapped charges by exposure to daylight for some hours. This observation effectively improves the minimum detection limit of the dosimeter; by zeroing the A1203:C immediately before field measurement, any dose absorbed during travel to the site is removed. Sensitivity changes during normal measurement cycles have been shown to be negligible; this allows a simple regeneration dose measurement, without the need for any special procedures to deal with sensitivity changes (BCtter-Jensen et al., 1997). 4.2.2. Measurement of the natural space background radiation The radiation environment in space consists of a broad spectrum of low to high energy, light and heavily charged particles (HCP) (Benton and Benton, 2001). OSL is potentially a useful technique for monitoring the doses to astronauts in this environment, especially for
Optically Stimulated Luminescence Dosimetry
108 100
(a)
90 discr
80 :a. ,.0
-
50.12D
~
70
N 6o o
50
>
40
9 n~
30 20 10 I
I
I
I
1
2
3
4
Dose (mGy) 2500
(b) 2000 O O
L.
"~ 1500 E
x
s_
g_
|
9
~ discr= 623.6D -~
looo
t-
co
5OO
0
1
2
3
4
5
6
Dose (mGy)
Fig. 4.4. Discrimination levels/-/,discr (a) and ~discr (b). The levels have been selected to be the upper (in the case of/x) or the lower (in the case of ~ limits of the dynamic irradiation values, respectively, and are given by the functions /~discr = 50.12D~ and Caiscr = 623.6D-~ Thus, static irradiations yield/~ or ~ values which are above (in the case of/x) or below (in the case of ~) the discriminator levels/Xdi~cr and SCdisc,.,respectively. The datum points in the figures represent different filtrations for X-irradiation, thus: o , M30; O, M60; A, M100; 9 M150; • H150. The H150 irradiations are not possible to classify as either static or dynamic using these discrimination levels since copper has a weak attenuation coefficient at these energies (re-drawn from Akselrod et al., 2000).
long-duration portable OSL dosimeters on A1203:C, and
flights on the International Space Station, since it may be possible to use readers on board the spacecraft to enable the astronauts to read the OSL a regular basis. The only material suggested as an OSL dosimeter in space is to be useful as a dosimeter in these environments it is first necessary to
Passive Optically Stimulated Luminescence Dosimetry
109
Table 4.1 Comparison between the response of high-pressure ionisation chamber (HPIC) and A1203:C dosimeters after short-term exposure to natural environmental photon radiation. Nine A1203:C chips were used for each dose evaluation Exposure time (h)
HPIC (txGy)
A1203:C (IxGy)
15 72
1.04 ___0.05 5.13 --- 0.01
0.98 +__0.03 (n = 9) 5.10 _+ 0.02 (n = 9)
calibrate the OSL response of the material to HCP over a wide range of energies and linear energy transfer (LET). A suggestion as to the high LET response of OSL from this material may be gained from an examination of the gamma (or high-energy beta) dose response. Both the TL and OSL dose responses of A1203:C show saturation effects setting in around 50 Gy, depending on the sample. From track interaction theory (e.g., Horowitz, 2001; Horowitz et al., 2001), one can expect a significantly reduced efficiency to HCP of high LET. Early measurements of the TL efficiency to alpha particles supported this conclusion (e.g., Mukherjee and Lucas, 1993). Yasuda and Kobayashi (2001) recently investigated the possibility of using A1203:C in passive OSL dosimeters for monitoring the absorbed dose in high LET radiation fields. They exposed Landauer Luxel TM A1203 detectors to He (150 MeV/n), C (290 MeV/n), Ar (500 MeV/n) and Fe (500 MeV/n) ion beams in a heavy ion medical accelerator. As expected, a notable reduction of the OSL efficiency as a function of LET was found, except for the He ions for which an efficiency > 1 was reported (compared to 137Cs radiation). For an effective dosimetry package, the reduced efficiency of the OSL signal at high LET may be compensated by using the dosimeters in combination with plastic nuclear track detectors (e.g., Doke et al., 1995). The reasonably good linear dose response and angle dependence at high LET energies, however, make OSL from A1203:C a promising candidate for use in space dosimetry.
2oo 150
60 t~ v
O
.100 (/9 ..d (.9
"•
43.5ILtGy Co-60 calibration dose
~ ~
72 hrs Environmental radiation (5.1 pGy)
50-
|
0
20
,
,
40
,
Time (s)
,
60
,
,,
|
80
,
100
Fig. 4.5. OSL decay curves from A1203:C dosimeters exposed over 15 and 72 h to the natural environmental background radiation representing evaluated integrated doses of 0.98 and 5.10 lxGy, respectively, compared to that from a 44 I~Gy 6~ gamma calibration dose (from BCtter-Jensen et al., 1997).
110
Optically Stimulated Luminescence Dosimetry
4.3. UV dosimetry Until recently, atmospheric ozone has absorbed all but approximately 1% of wavelengths below 320 nm. Recent evidence of ozone depletion in the stratosphere has generated interest in the biological impact on plants and animals resulting from increased exposure to these wavelengths (e.g., Lubin and Jensen, 1995; Driscoll, 1996). In addition, increases in erythema (sunburn), skin cancers, eye disorders and DNA damage have shown to increase as the UVB level increases. This has created a need for ultraviolet dosimetry of wavelengths shorter than 320 nm, particularly at southern latitudes where ozone depletion appears to be most significant (e.g., Quintern et al., 1994). UV dosimetry using TL has been suggested in the past (e.g., Chakrabarti et al., 1990; Yeh and Su, 1996; Trinkler et al., 2000) and offers the advantage of being able to place the dosimeters in situ, without requiring any special monitoring or logistical considerations (e.g., portable field power source for any electronics which other UV measuring systems may require). Colyott et al. (1997) designed a UVB field dosimeter that measures absorbed ultraviolet dose, for the wavelength band centred at 307 nm, based upon the phototransferred TL properties of thin layer A1203:C detectors. More recently, Colyott et al. (1999) used the phototransferred OSL (PTOSL) signal from A1203:C to measure the integrated UVB dose. OSL from A1203:C involves the optical stimulation of charge carriers trapped at the so-called main dosimetric trap, which is thermally stable up to 465 K (see Chapter 3). However, deeper traps, thermally stable up to 900 and 1200 K, also exist and the stimulation efficiency has been shown to be wavelength- dependent for both deep traps and the main dosimetric traps (BCtter-Jensen and McKeever, 1996). Since the wavelength dependence is different for each type of trap, careful selection of the optical stimulation wavelength allows each type of trap to be probed selectively. In general, PTOSL involves the production of OSL by phototransfer of charge to empty traps from deeper, filled traps. Thus, by filling the deeper traps (as well as the dosimetry traps) of A1203:C with a pre-dose of irradiation and subsequently preheating the sample to remove any trapped charge from the dosimetry traps, charge can be transferred from the deep traps to the dosimetry traps by exposing the sample to UV wavelengths. When subsequently stimulated with blue-green wavelengths (470-530 nm), a PTOSL signal is observed from the sample as the transferred charge is released from the dosimetry traps. The PTOSL signal is proportional to the light exposure, as well as to the initial pre-dose of irradiation. The PTOSL UVB dosimeter designed by Colyott et al. (1999) is shown schematically in Fig. 4.6. The dosimeter container that includes an optical interference detection filter centred at 307 nm, is made of Teflon, is watertight and can thus be used both in air and water. The UVB dosimeter normalised PTOSL response as a function of natural sunlight exposure is shown in Fig. 4.7. The data show a nearly linear relationship over at least three orders of magnitude in UVB exposure. As a result, the dosimeter has a nearly linear response from some minutes to several weeks total sunlight exposure. Trinkler et al. (2000) showed that aluminium nitride, A1N:Y203, is an ultra-sensitive TL and OSL dosimeter material with a sensitivity one order of magnitude higher than that of A1203:C. Trinkler et al. (2000) investigated the blue-light stimulated OSL properties of this material after UVB exposure and they concluded that even
Passive Optically Stimulated Luminescence Dosimetry
111
Fig. 4.6. Schematic diagram of the UVB dosimeter. Dimensions are: cap/base diameter, 25.4 mm; window diameter, 12.7 mm; height 19.1 mm (from Colyott et al., 1999).
though significant fading of the OSL signal was observed, the very high OSL sensitivity of this material makes it attractive as a UVB dosimeter when very short exposure times can be applied. This would be practical when using portable readout instrumentation in the field.
Fig. 4.7. UVB dosimeter normalised PTOSL response as a function of natural sunlight exposure (from Colyott et al., 1999).
112
Optically Stimulated Luminescence Dosimetry
4.4. OSL and RL remote optical fibre dosimetry in medical applications 4.4.1. Real-time (RT) in vivo monitoring of doses during radiotherapy Therapeutic radiation oncology treatments, including teletherapy and brachytherapy as well as related procedures such as the treatment of restinosis, require the delivery of highly localised doses of radiation to patient target organs. The efficacy of the radiation treatment, however, requires knowledge of the absorbed dose at the organ of interest to within ___5%, with a higher risk of local recurrence or a higher risk of complications resulting from incorrect exposure. Furthermore, since it is inevitable that healthy organs and tissue will also be exposed during treatment, over-exposure carries with it a concomitant risk of secondary cancers. It is necessary that all possible measures be taken to reduce the toxicity effects of undesired exposure to as low as possible. This requires both the accurate calibration of radiotherapy sources and the accurate assessment of dose at critical locations on or within the body. Thus, determination of the spatial distribution of dose to tissue from the source is an essential aspect of effective health care and treatment. As a result of international regulations (International Commission on Radiological Protection, 2000) there is a growing demand to improve methods for in vivo measurements of the absorbed doses to patients. So far, patient monitoring has been performed using one of the several available detector systems; radiochromic dye films, plastic scintillators, TL dosimeters, diode detectors, or MOSFET detectors. However, each of these systems has significant disadvantages for general-purpose external dosimetry. Apart from the scintillators, the readout system is not coupled directly to the detectors, and this in turn requires a separate post-irradiation evaluation of the dosimeters. The result is that no RT dose or dose rate information is provided. Ideally, a RT in vivo dosimeter is needed to measure absorbed doses during exposure, mainly to provide feedback of important information to the physician during treatment. OSL is an obvious method for RT in vivo measurements of absorbed doses because the stimulation of a dosimeter can be made simultaneously with the detection of luminescence, e.g., via light fibres using remotely placed light sources. Since the stimulation wavelength is different from that of the emitted luminescence, CW-OSL measurements can be carried out using only a single optical fibre in connection with a suitable detection filter placed in front of a photomultiplier tube cathode. Furthermore, in addition to absorbed dose information as provided by OSL, the prompt radioluminescence (RL) signal generated by the therapy radiation source directly reflects the dose rate at any time during the treatment. Thus, in addition to a small-size sensor, the main advantages of an optical fibre dosimeter over the currently available radiation detectors used in clinical applications are the capabilities of measuring both the RT dose rate and the absorbed dose. When the sensor size is similar to the field size, the sensor will provide information on the average dose distribution, rather than the dose at the centre of the radiation field. 4.4.2. Optical fibre dosimeters Roy et al. (1997) designed a remote OSL dosimeter system based on rare earth doped alkaline earth sulphides (e.g., MgS) as the dosimeter material coupled to the end of an optical fibre and stimulated with an infrared laser diode. This dosimeter system yielded
Passive Optically Stimulated Luminescence Dosimetry
113
a dose response from 40 lxGy to 10 Gy. Justus et al. (1999a,b) developed Cu+-doped quartz rods, which they used as remote fibre OSL dosimeters under stimulation with 790 nm light from a GaAs laser. The OSL characteristics of Cu+-doped quartz are significantly different from those of natural quartz, which makes it possible to use lowenergy light for stimulation. Huston et al. (2001) constructed fibre dosimeters by drawing a 20 mm diameter by 1 m long Cu+-doped glass rod into a 1 km length of 400 txm diameter. This fibre was used to construct a four-channel fibre optic dosimeter system for monitoring the dose delivered to patients undergoing cancer radiotherapy. The radiation-sensitive portion of the optical fibre dosimeter consists of a 2 mm long, 0.4 mm diameter piece of Cu-doped glass that is fusion spliced to one end of a 1 mm long, 0.4 mm diameter optical fibre. A black Teflon jacket surrounds most of the fibre assembly to prevent external light from entering the fibre. The end of the fibre is coated with aluminium to reflect both the stimulation and the signal light to improve the efficiency of the dosimeter. The system, which is shown in Fig. 4.8, has been tested successfully over a range from 10 mGy to 10 Gy. Ranchoux et al. (2002) reported a fibre-based remote OSL system based on A1203:C single crystals, Cu+-doped silicate fibres and alkaline earth sulphide (MgS). A titaniumsapphire tuneable laser was used to stimulate the Cu+-doped silicate fibre (860 nm) and MgS (980 nm) whereas the A1203:C was stimulated using 514.5 nm light from an argon laser. Polf et al. (2002) examined A1203:C optical fibres for their potential use as RT luminescence dosimeters for use in radiotherapy. The OSL and RL responses of the A1203:C fibre probes were measured and it was found that both RL and OSL signals increased linearly with dose rate and the absorbed dose, respectively, within the actual To/From PC
PMT's 7-
Laser
Filter
To/From DosimeterProbe Fig. 4.8. Schematicdiagram of a remote multiple optical fibre dosimeter system (from Huston et al., 2001).
114
Optically Stimulated Luminescence Dosimetry
Fig. 4.10. OSL as function of dose measured using the dual-fibre dosimeter. See text for explanation (from Polf, 2002).
Passive Optically Stimulated Luminescence Dosimetry
Optically Stimulated Luminescence Dosimetry
116 2000
I
I
I
,...-., (fJ Q.
.o.
tO Q.
/
60Co ' 137 Cs
/
1000
.f"
0
._I
.,-.. cO
./
, , o/
.J
/
(a) L
/=
o
/
0
./~
n,"
90Sr
10-
0o~
002
O
/
./
/,
/"
/"
/
O"
oJ e
0z
J o
Dose rate [Gy/min]
(b)
q
~
Dose [Gy]
Fig. 4.12. (a) RL versus gamma dose, and (b) OSL versus beta dose rate for the single optical fibre system (from Andersen et al., 2002).
OSL response to 0.5 Gy Deviation from mean value for 6 MV photons Fig. 4.13. Reproducibility measurements using the single optical fibre system in clinical beams. The OSL responses are normalised to that obtained using 6 MV photons. As seen, the precision is < 0.5 % (from Andersen et al., 2002).
Passive Optically Stimulated Luminescence Dosimetry
117
range in radiotherapy ( 0 . 5 - 8 5 Gy). Polf et al. (2002) designed a dual-fibre system using the green light from a 40 m W Nd:YAG laser to stimulate the OSL signal. RL was experimentally generated using a 100 mCi 9~176 source. The OSL response as a function of dose was linear in the dose range from 50 mGy to 10 Gy and the OSL signal increased for doses up to 100 Gy. Fig. 4.9 shows a schematic diagram of the dual-fibre system. Fig. 4.10 shows the OSL response as a function of absorbed dose. Fig. 4.10a compares the RT OSL signal with the standard OSL signal from the same fibre. The latter was obtained by irradiating to the doses indicated and reading the corresponding OSL. Fig. 10b shows the same data but with the RT signal corrected for depletion during the RT measurement. The under-correction at higher doses is due to a dose-dependent increase in the OSL depletion rate over this range. Andersen et al. (2002) and Aznar et al. (2002) used a single-fibre system combined with a 2 0 m W Nd:YAG (532 nm) laser as a remote optical fibre dosimetry system for radiotherapy (see Fig. 4.11). To produce OSL, the laser beam is focused through a wavelength-discriminating beam-splitter positioned at a 45 ~ angle relative to the incident beam, and via the light fibre into the dosimeter. The OSL is transmitted from the dosimeter in the same fibre and reflected through 90 ~ onto the photocathode of a miniature PM tube. A narrow UV band transmission filter (e.g., Hoya U-340) mounted in front of the photocathode rejects the scattered green light from the laser. Fig. 4.12a and b shows the RL response versus 137Cs gamma dose rate and the OSL response versus 9~176 beta dose, respectively, obtained with the one-fibre system. Fig. 4.13 shows that a reproducibility of less than 0.5% could be obtained when exposing the A1203:C fibre to clinical beams of 6 and 18 MV photons, and 20 MV electrons.
References Akselrod, A., Akselrod, M.S., Agersnap Larsen, N., Banerjee, D., BCtter-Jensen, L., Christensen, P., Lucas, A.C., McKeever, S.W.S., Yoder, C., 1999. Optically stimulated luminescence response of A1203 to beta irradiation. Radiat. Prot. Dosim. 85, 125-128. Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-176. Akselrod, M.S., Agersnap Larsen, N., McKeever, S.W.S., 2000. A procedure for the distinction between static and dynamic radiation exposures of personal dosimetry badges using pulsed optically stimulated luminescence. Radiat. Meas. 32, 215-225. Andersen, C.E., Aznar, M.C., BCtter-Jensen, L., Baeck, S.A.J., Mattson, S., Medin, J., 2002. Development of optical fibre luminescence techniques for RT in-vivo dosimetry in radiotherapy. Presented at the IAEA symposium on standards and codes of practice in medical radiation dosimetry, Vienna, November 25-28, 2002 (IAEA-CN-96-118). Aznar, M.C., Polf, J.C., Akselrod, M.S., Andersen, C.E., Baeck, S.A.J., BCtter-Jensen,L., Mattson, S., McKeever, S.W.S., Medin, J., 2002. Real-time optical fibre dosimetry in radiotherapy. Presented at the American Association of Physics in Medicine (AAPM) 44th annual meeting in Montreal, July 14-18, 2002. Benton, E.R., Benton, E.V., 2001. Space radiation dosimetry in low-earth orbit and beyond. Nucl. Instr. Meth. B. 184, 255- 294. BCtter-Jensen, L., McKeever, S.W.S., 1996. Optically stimulated luminescence dosimetry using natural and synthetic materials. Radiat. Prot. Dosim. 65, 273-280. BCtter-Jensen, L., Agersnap Larsen, N., Markey, B.G., McKeever, S.W.S., 1997. A1203:C as a sensitive OSL dosemeter for rapid assessment of environmental photon dose rates. Radiat. Meas. 27, 295-298.
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Optically Stimulated Luminescence Dosimetry
Budzanowski, M., Bilksi, P., BCtter-Jensen, L., Delgado, A., Olko, P., Sfiez-Vergara, J.C., Waligorski, M.P.R., 1996. Comparison of LiF:Mg,Cu,P (MCP-N, GR-200A) and (et-A1203:C TL detectors in short-term measurements of natural radiation. Radiat. Prot. Dosim. 66, 157-160. Chakrabarti, K., Mathur, V.K., Abbundi, R.J., Hill, M.D., 1990. UV induced trapping in powder and sintered CaSO4:Tm and CaSO4:Dy. Radiat. Prot. Dosim. 33, 35-38. Colyott, L.E., Akselrod, M.S., McKeever, S.W.S., 1997. An integrating ultraviolet-B dosemeter using phototransferred thermoluminescence from ct-A1203:C. Radiat. Prot. Dosim. 72, 87-94. Colyott, L.E., McKeever, S.W.S., Akselrod, M.S., 1999. An integrating UVB dosemeter system. Radiat. Prot. Dosim. 85, 309- 312. Doke, T., Hayashi, T., Nagaoka, S., Ogura, K., Takeuchi, R., 1995. Estimation of dose equivalent in STS-47 by combination of TLDs and CR-39. Radiat. Meas. 24, 75-82. Driscoll, C.M.H., 1996. Solar UVB measurements. Radiat. Prot. Dosim. 64, 179-188. Horowitz, Y.S., 2001. Theory of thermoluminescence gamma dose response: the unified interaction model. Nucl. Instr. Meth. B 184, 68-84. Horowitz, Y.S., Satinger, D., Gamboa-deBuen, I., Buenfil, A.E., Ruiz-Trejo, C., 2001. The extended track interaction model: supralinearity and saturation He-ion TL fluence response in sensitized TLD-100. Radiat. Prot. Dosim. 84, 29-34. Huston, A.L., Justus, B.L., Falkenstein, P.L., Miller, R.W., Ning, H., Altemus, R., 2001. Remote optical fibre dosimetry. Nucl. Instr. Meth. B 184, 55-77. International Commission on Radiological Protection, 2000. Prevention of accidental exposures to patients undergoing radiation therapy. ICRP Publication 86, Annals of the ICRP 30, 1-70. Justus, B.L., Pawlovich, K.J., Merritt, C.D., Huston, A.L., 1999a. Optically and thermally stimulated luminescence characteristics of Cu+-doped fused quartz. Radiat. Prot. Dosim. 81, 5-10. Justus, B.L., Merritt, C.D., Pawlovich, K.J., Huston, A.L., Rychnovsky, S., 1999b. Optically stimulated luminescence dosimetry using doped fused quartz. Radiat. Prot. Dosim. 84, 189-192. Lubin, D., Jensen, E.H., 1995. Effects of clouds and stratospheric ozone depletion on ultraviolet radiation trends. Nature 377, 710-713. Mukherjee, B., Lucas, A.C., 1993. Light conversion efficiency of aluminium oxide dosimeters irradiated with 241A1TIalpha particles. Radiat. Prot. Dosim. 47, 177-179. Perry, J.A., 1987. RPL Dosimetry: Radiophotoluminescence in Health Physics. Adam Hilger, Bristol. Piesch, E., Burgkhardt, B., Fischer, M., Rrber, H.G., Ugi, S., 1986. Properties of radiophotoluminescence glass dosimeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293-297. Piesch, E., Burgkhardt, B., Vilgis, M., 1990. Photoluminescence dosimetry: progress and present state of the art. Radiat. Prot. Dosim. 33, 215-226. Piesch, E., Burgkhardt, B., Vilgis, M., 1993. Progress in phosphate glass dosimetry: experiences and monitoring with a modern dosimetry system. Radiat. Prot. Dosim. 47, 409-413. Polf, J.C., 2002. A study of optically stimulated luminescence in A1203 fibres for the development of a real-time fibre optic dosimetry system. PhD. Thesis, Oklahoma State University, Stillwater. Polf, J.C., McKeever, S.W.S., Akselrod, M.S., Holmstrom, S., 2002. A real-time, fibre optic dosimetry system using A1203 fibres. Radiat. Prot. Dosim. 100, 301-304. Quintern, L.E., Puskeppeleit, M., Rainer, P., Weber, S., E1 Naggar, S., Eschweiler, U., Horneck, G., 1994. Continuous dosimetry of the biologically harmful UV-radiation in Antarctica with biofilm technique. Photochem. Photobiol. B. 22, 59-66. Ranchoux, G., Magne, S., Bouvet, J.P., Ferdinand, P., 2002. Fibre remote optoelectronic gamma dosimetry based on optically stimulated luminescence of A1203:C. Radiat. Prot. Dosim. 100, 255-260. Roy, O., Magne, S., Gaucher, J.C., Albert, L., Dusseau, L., Bessiere, J.C., Ferdinand, P., 1997. All optical fibre sensor based on optically stimulated luminescence for radiation detection. Presented at the 12th International Conference on Optical Fiber Sensors OFS'97. Oct. 28-31, 1997, Williamsburg, Virginia, USA. Trinkler, L., BCtter-Jensen, L., Christensen, P., Berzina, B., 2000. Studies of aluminium nitride ceramics for application in UV dosimetry. Radiat. Prot. Dosim. 92, 299-306. Yasuda, H., Kobayashi, I., 2001. Optically stimulated luminescence from A1203:C irradiated with relativistic heavy ions. Radiat. Prot. Dosim. 95, 339-343. Yeh, S.-M., Su, C.-S., 1996. UV induced thermoluminescence in rare earth oxide doped phosphors: possible use for UV dosimetry. Radiat. Prot. Dosim. 65, 359-362.
Chapter 5
OSL properties of natural materials 5.1. Quartz 5.1.1. Crystal structure and point defects Quartz is the most common mineral in our environment. It is found in granite, hydrothermal veins and volcanic rocks, as well as in sedimentary deposits derived from such solid materials. If undisturbed, these sediments contain a record of past environmental change, both natural and anthropogenic, in terms of organic remains or artefacts. These sediments are also made into building materials, such as bricks, and pottery. Thus, the potential use of a dose reconstruction technique based on quartz grains is enormous, whether as a dating tool in archaeology and Quaternary geology, or in nuclear accident dosimetry. Studies of the luminescence behaviour of synthetic and amorphous quartz are also important, since they play a role in the manufacture of electronic devices. Quartz has a simple molecular structure, SiO2, in which the Si and O atoms are linked as shown in Fig. 5.1. When formed as a result of processes beneath the Earth's surface, quartz contains particular impurities that act to create electron traps and provide recombination centres. Most quartz is in the structural form known as alpha-quartz. On heating, the quartz can be restructured as it passes through two phase transitions, at 575 and 870~ to form beta-quartz and tridymite, respectively. The alpha-beta structural change at 575~ is displacive only and is fully reversible when cooling takes place slowly. Since the alpha-beta phase transition involves an increase in the size of the crystal lattice, it might be expected to have a significant effect on the optically stimulated luminescence (OSL) behaviour. The defect structures of natural minerals are complex. Many defects within the silicate minerals are based on the SiO4 tetrahedral structure common among them. The archetype of these minerals, quartz, is one of the most well studied natural minerals in terms of its luminescence properties, primarily because of its utility in luminescence dosimetry, particularly dating. One can define two basic types of defect within quartz--extrinsic defects related to impurities, and intrinsic defects related to structural imperfections such as missing oxygen or silicon atoms. The 40% ionic and 60% covalent nature of the S i - O bond in SiO2 leads to a rather rigid structure and misplaced lattice ions tend to associate with the impurities and, thus, combinations of impurities and structural defects exist forming complex structures. Such defects may well play important roles in the thermoluminescence (TL) and OSL properties of this material, and related materials. Among the most ubiquitous of impurities in quartz is aluminium, in the form of A13+ ions. These substitute with ease for the host tetravalent Si 4+ ions, with the charge
120
Optically Stimulated Luminescence Dosimetry
silanol groups
0
0
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0
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.,, i 0 Si I 0I
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Fig. 5.1. Schematicof quartz lattice showing common defects (from G6tze et al., 2001).
imbalance provided by monovalent impurities (M +) such as H +, or alkali ions Na + and Li +, forming neutral centres of the type (A104/M+) ~ (Nassau and Prescott, 1975; Weil, 1975; Cohen and Makar, 1982; McKeever, 1984; Martini et al., 1995). Charge compensation by H leads to the formation of OH-bonds that can be observed in infrared spectroscopy (Brown and Kahan, 1975; Subramaniam et al., 1984). Irradiation of quartz at low temperatures (e.g., liquid nitrogen temperature (LNT)) results in the capture of a hole by these defects, leading to the formation of (A104/M+) + defects. However, subsequent warming to room temperature or, alternatively, irradiation at room temperature, leads to the formation of (A104) ~ in which the charge compensation is provided by a trapped hole. This defect gives rise to the well-known smoky colouration of smoky quartz and of heavily irradiated quartz (Nassau and Prescott, 1975; McKeever, 1984; McKeever et al., 1985). Radiation-induced conductivity measurements (Hughes, 1975; Martini et al., 1986) indicate that the alkali ions, or the H + ions, diffuse away from the A13+ impurity along the c-axis channels during irradiation (at room temperature) or during the post-irradiation heating (if irradiated at low temperature). Other common impurities in quartz include Ge and Ti, both of which form electron-trapping centres. During the process of warming irradiated quartz, or during irradiation at room temperature, electron traps of the (Ge4+/M+) ~ are formed when alkali ions diffuse away from the A1 site to become trapped at (Ge4+) - centres. Similarly, (Ti3+/H+) - electron traps may be created (McKeever et al., 1985). Additional important defects within SiO2 include the so-called E1-centres, of which there are several. The E~ centre consists of an unpaired electron trapped at one of the nonequivalent Si sites next to an oxygen vacancy (Isoya et al., 1981). The E~-centre also consists of an oxygen vacancy, but with an associated proton, and with the electron located
OSL Properties of Natural Materials
121
on the other non-equivalent Si site. A third related defect, the E~-centre, is identified as an oxygen vacancy with a hydride ion bonded to one of the Si atoms, and with an electron shared between the two Si atoms (Isoya et al., 1981). Correlating specific defects with specific TL peaks in quartz has proved to be extremely difficult. In a study of synthetic quartz McKeever et al. (1985) studied the annealing curves of electron spin resonance (ESR) signals and correlated the 110~ TL peak with the electron trap (GeO4)-. Few other correlations of this type exist in the literature. From the point of view of the origin of the luminescence bands from quartz, identification of the exact defect causing the observed emissions is difficult. This is primarily due to the broad-band nature of the emissions, and the slight variations that appear in the wavelengths for maximum emission from one quartz sample to another, perhaps due to different degrees of overlap and strengths of the various emissions. As a result, one can never be sure that the observed emission reported for one quartz sample matches the reported emission observed from another. McKeever (1984) gives a detailed review of the early (pre-1984) luminescence literature on quartz, and Krbetschek et al. (1997) give a comprehensive discussion of the literature up to 1996. The luminescence bands of importance for radiation dosimetry lie in the visible-UVA part of the spectrum, specifically from approximately 360 to about 420 nm (UVA-violet), 460 to 480 nm (blue) and between 610 and 630 nm (orange) (Krbetschek et al., 1997). In any one sample, the observed emissions in these regions may be composite bands. Of major importance for TL and OSL dosimetry are the UVA-violet emissions. In OSL measurements at room temperature, these have been reported at approximately 365 nm (Huntley et al., 1991). In TL at 110~ the emission appears nearer 380 nm, while higher temperature TL peaks show emissions at 410 nm (for TL at 200-220~ and 430 nm (for TL at 305-325~ Franklin et al. (1995) suggest that the traps giving rise to each of these signals form a "family" of centres, with each using the same recombination centre. The emission wavelength depends upon the temperature at which the signal appears and is thus highly temperature-dependent, implying significant phonon coupling with the host lattice. Not surprisingly, therefore, this emission is seen to undergo strong thermal quenching, with an apparent activation energy near 0.6-0.65 eV (Wintle, 1975, 1997; Nanjundaswamy et al., 2002). The source of this emission is uncertain, even after many experimental studies have been devoted to identifying the defects causing the various emissions in quartz. Yang and McKeever (1990) attempted to identify the point defects associated with the so-called "pre-dose effect" (Zimmerman, 1971) in this material using electron spin resonance and concluded that the 380 nm emission was due to the recombination of electrons with holes trapped at H304 centres. These centres consist of three protons occupying a Si 4+ site, with a trapped hole for charge neutrality. The precursors to the H304 centres are believed to be H404 centres (Nuttall and Weil, 1980) in which a hydrogen ion is replaced by a hole during irradiation. Others relate the 380 nm emission to electron recombination with holes trapped at (A104) ~ centres (Halperin and Sucov, 1993; Martini et al., 1995), or at (A104/M +) + defects (Luff and Townsend, 1990; Kalceff and Phillips, 1995). However, as discussed earlier, the latter defect is only stable below room temperature and cannot be the cause of the above-room-temperature TL signals of interest in dosimetry. (This highlights the difficulty of identifying specific
122
Optically Stimulated Luminescence Dosimetry
emissions in quartz with particular defects, since many defects give rise to emissions in the same wavelength region.) The blue emission (from 460 to 480 nm) has most frequently been identified with the (A104) ~ centre (Nassau and Prescott, 1975; McKeever, 1984; McKeever et al., 1985; Yang and McKeever, 1990; Rink et al., 1993; Woda et al., 2002) although Hashimoto et al. (1987) reported an anti-correlation of the blue TL emission with A1 content, and others have connected the blue emission to intrinsic features, such as self-trapped-exciton relaxation (Luff and Townsend, 1990; Kalceff and Phillips, 1995). The orange emission (from 610 to 630 nm) has been correlated with non-bridging oxygen hole centres (Kalceff and Phillips, 1995) and U-centres (Luff and Townsend, 1990). The lack of certainty regarding the identification of the defects involved in the luminescence processes in quartz has led to a corresponding uncertainty in describing the TL and OSL emission mechanisms. While some identifications are reasonably well placed, e.g., the identification of the 110~ TL with the electron centre (GeO4) ~ none are certain. Of particular interest in luminescence dating and dosimetry studies has been the pre-dose effect first described by Zimmerman (1971). The effect is an observed increase in sensitivity of the 110~ TL peak following irradiation (the predose) and heating ("activation"). Zimmerman (1971) described a phenomenological model for the process, but others since then have attempted to identify specific defects and defect interactions that may cause the effect. McKeever et al. (1985) and Yang and McKeever (1990) noted that the l l0~ TL peak was emitted at both 380 and 470 nm, but that only the 380 nm emission was involved in the pre-dose process; the 4 7 0 n m emission did not sensitise. This was confirmed in later emission spectra studies by Akber et al. (1988). From measurements of IR absorption spectra and ESR, Yang and McKeever (1990) suggested that the activation of the pre-dose effect took place via the thermally stimulated movement of hydrogen ions in the lattice from hydrogen ion traps, created during the pre-dose irradiation, to the H304 centre precursors. A similar mechanism has been suggested recently by Itoh et al. (2001; 2002) who prefer to identify the 380 nm emission with (A104) ~ centres and relate the activation process to the movement of alkali ions and their trapping at unidentified defects (X) to form (XM) ~ electron traps. A suggested impurity for the unknown defect is Ge, and thus the l l0~ peak would then be a (Ge4+M+) ~ centre. As shown by McKeever et al. (1985) however, such defects are stable up to 300~ and cannot be the origin of the 110~ TL peak; nevertheless, there are several attractive features of the models of Itoh et al. (2001, 2002) that deserve further study. In addition to those centres that are directly related to the production of luminescence emission, many other defects exist that affect the TL and OSL emission only indirectly. For example, if recombination occurs without the emission of a photon (non-radiative recombination), then such centres act as competitors to the luminescence centres and their concentration can dictate the strength of the luminescence emission observed. Such defects can be observed by non-luminescence techniques such as ESR and optical absorption, but their existence can only be inferred from luminescence measurements. It is with such a background that OSL studies of quartz need to be introduced.
OSL Properties of Natural Materials
123
5.1.2. Decay curve shapes obtained under continuous stimulation--CW-OSL 5.1.2.1. Stimulation sources Under steady optical stimulation, the OSL from sedimentary quartz is observed to decrease continuously, though not as a single exponential (Fig. 5.2). This type of OSL has been termed continuous wave-OSL (CW-OSL) (see Chapter 2). The non-exponential decay was first noted by Huntley et al. (1985) in their study using 514.5 nm light from an argon-ion laser with the sample held at room temperature. Blue-pass filters were placed in front of the photomultiplier tube in order to reject the stimulation wavelength; the Coming 7-59 and 5-58 filter combination passed emission that peaked around 410 nm. More recently, stimulation has been performed increasingly with cheaper light sources, such as filtered halogen lamps (BCtter-Jensen and Duller, 1992) and blue light emitting diodes (LEDs) (BCtter-Jensen et al., 1999a,b). The use of stimulation wavelengths from 420 to 550 nm, and at 470 nm, respectively, has resulted in the need for a shorter wavelength detection filter (e.g., 7.5 mm of Hoya U-340, which transmits from 250 to 390 nm, peaking at 340 nm in the near ultra-violet or a 1 mm Corning BG-39 and two 2.5 mm Hoya U-340 filters). Bailey et al. (1997) reported the OSL decay rates to be similar for both an argonion laser and a filtered halogen lamp when similar power is delivered to the sample. Typical power levels at the samples have been reported as --- 50 mW/cm 2 for an argon-ion laser (Huntley et al., 1985; Huntley et al., 1996), ---16mW/cm 2 for the filtered halogen lamp on a Rise reader (BCtter-Jensen, 1997) and --~35 mW/cm 2 for 49 blue LEDs on a Rise reader (BCtter-Jensen et al., 2000). More detailed considerations of the experimental constraints on the measurement of OSL from quartz can be found in Chapter 7. 5.1.2.2. Effect of the llO~ TL trap Analysis of the OSL decay curve from quartz provides information on the optical stability of the various OSL signal components. This information is fundamental to their suitability for dating. Once a signal has been selected, its other properties, such as thermal
Fig. 5.2. Natural OSL decay curves for sedimentary quartz obtained for stimulation with a filtered halogen lamp when the sample is held at elevated temperatures. Prior to stimulation, each aliquot was held for 100 s at that temperature to minimise any thermally stimulated signal (from Murray and Wintle, 1998).
124
Optically Stimulated Luminescence Dosimetry
stability can be investigated. For OSL from quartz it was recognised soon after the early studies that the signal measured at temperatures below 100~ was affected by the presence of the 110~ TL trap. Fig. 5.2 shows OSL decay curves obtained for naturally irradiated (58 Gy) quartz, stimulated at temperatures ranging from 25 to 175~ The curves have been normalised using a 0.1 s OSL measurement at 25~ prior to the measurement at elevated temperature. The non-exponential behaviour is clearly seen in the log-linear plot. McKeever et al. (1997a) obtained a set of decay curves for thermally annealed (600~ for 20 s) and laboratory-irradiated (31 Gy) quartz aliquots that had been pre-heated (125~ for 20 s) before being measured at the given stimulation temperatures (Fig. 5.3). These data are normalised to the initial signal value and plotted on a linear scale. Spooner (1994a) presented his OSL decay curves, measured at temperatures from 20 to 253~ on a linear-log plot (Fig. 5.4). For all three data sets (Figs. 5.2, 5.3 and 5.4), an obvious change in decay rate can be seen between stimulation at 75 and 100~ as would be expected if charge was entering the 110~ trap and was then being optically stimulated from there at a slower rate. Studies of the phototransferred thermoluminescence (PTTL) observed at 110~ at the end of room temperature stimulation confirm such charge trapping (Smith and Rhodes, 1994; Wintle and Murray, 1997). In addition, Bailey (1997) and Wintle and Murray (1997) also reported the optical stimulation of charge from the 110~ TL peak. Bailey (2000a) modelled the effect of charge phototransfer during OSL measurement, with particular regard to methods of equivalent dose (De) determination. He compared simulations at 1.0 I
i
I
I
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10
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125
OSL Properties of Natural Materials
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20 and 160~ for his model, in which parameters were chosen as being similar to those obtained for real samples. The effect of re-trapping was observed for the simulated decay curve at 20~ There was also a small effect ( < 6%) on the De estimate obtained for increasing stimulation time, as the phototransferred component became more important. However, at these stimulation times, the slow component of the OSL signal will have a far greater effect (see Sections 5.1.2.4 and 5.1.11). Alexander et al. (1997) also modelled the phototransfer process and concluded that the data obtained by Wintle and Murray (1997) were consistent with a significantly lower optical stimulation rate from the l l0~ (acceptor) trap compared to that from the deeper OSL (source) traps. 5.1.2.3. Dependence on power
Simple models (Chapter 2) predict a proportional change in the decay rate with stimulation power. Several studies have been carried out to investigate the effect of changing the stimulation power delivered to a quartz sample on the shape of the decay curve. The results are most easily compared when the OSL is plotted as a function of energy delivered to the sample (mJ) and the OSL is given in counts per unit energy (cts/mJ). For example, Duller and B~tter-Jensen (1996) showed that the OSL decay curves
126
Optically Stimulated Luminescence Dosimetry
obtained for a sedimentary quartz using the Ris0 filtered halogen lamp were almost identical over a 100-fold change in illumination intensity (0.11-12 mW/cm2). The decay curves in Fig. 5.3 were obtained using a stimulation power of---16 mW/cm 2, whereas similar data, already shown in Fig. 2.13, were obtained with the power decreased by a factor of 0.0125 to --~ 2 mW/cm 2. For the latter, the signal intensity (as shown by the noise level of the measurements; compare Fig. 2.13 and Fig. 5.3) and the decay rate was much less (much shallower decay curves). An additional feature can be observed when the lower stimulation power is applied at 50 and 100~ a peak in the OSL is seen at around 3 s stimulation time. This is a further evidence of re-trapping in the 110~ TL trap and was predicted from the accompanying computer model (McKeever et al., 1997a). See Chapter 2 for a full discussion of these effects. The effects of changing illumination intensity have also been carried out using short exposures to laser light for both the fast component (Spooner, 1994a) and the slow component observed after the fast and medium components had been removed by light exposure (Bailey, 2000b). For definitions of "fast" and "slow" components, see Section 5.1.2.4. For the fast component, linearity was observed over the power range 0.28238 mW/cm 2 and for the slow component, linearity was observed over the range 0 - 1 2 0 mW/cm 2. In both studies, the linearity of the response was taken as an evidence for electron un-trapping which was the result of a single-photon absorption process. Using blue (470 nm) LEDs, Bulur et al. (2001) measured the OSL decay curves using 11 stimulation powers. They calculated the de-trapping probabilities (bleaching rates) from the initial slopes of the ln(OSL) versus t plots, and Fig. 5.5 shows the linear relationship between the de-trapping probability and the stimulation power. This is fundamental for the linear modulation OSL (LM-OSL) experiments that will be discussed in Section 5.1.3.1. 5.1.2.4.
Three components
From the discussion in Section 5.1.2.2, it can be inferred that in order to observe the behaviour of the deep OSL trap(s), it is necessary to stimulate at temperatures that keep the
Fig. 5.5. De-trappingprobabilitiesplotted as a function of stimulationpowerfor quartz and two otherphosphors with data obtained from CW-OSL measurements (from Bulur et al., 2001a).
OSL Properties of Natural Materials
127
l l0~ TL trap empty. Several temperatures have been used, e.g., 220~ (Smith and Rhodes, 1994), 160~ (Bailey et al., 1997) and 125~ (Wintle and Murray, 1997). Using laser stimulation at 220~ for a naturally irradiated quartz, Smith and Rhodes (1994) identified three exponential components, designated fast, medium and slow, according to their rate of decay under optical excitation. More recently, Bailey et al. (1997) used a filtered halogen lamp and stimulation at 160~ to investigate these three signals. Bailey et al. (1997) carried out an empirical analysis by identifying the slow component, and then subtracting it from the remaining data; this procedure was repeated for the medium component. The results for one sample, stimulated at 160~ using --~ 12 mW/cm 2 of 420-560 nm light from a filtered halogen lamp, is shown in Fig. 5.6a,b. The slow component shows negligible decay over the first 70 s of stimulation (Fig. 5.6a), but can be seen as the sole component from 70 s onwards (inset in Fig. 5.6b). Fig. 5.6b shows that the experimental data are well fitted by the sum of three exponential decays. For the three samples studied, the major contribution to the total area under the decay curve was from the fast component, with the slow component being the next important. For the sample in Fig. 5.6, the fractions are 0.75, 0.09 and 0.16, respectively. However, as seen in Fig. 5.6a, the contribution of the slow component to the initial part of the signal (e.g., in the first 1 s) is < 1%. Also, for the sample shown, the medium component only contributes 3% towards the initial 1 s OSL signal. The fast and medium components are removed by heating to 400~ leaving the slow component, which will be discussed in Section 5.1.10.
5.1.2.5. Effect of stimulation wavelength Ditlefsen and Huntley (1994) measured both the initial slope and the initial intensity as a function of wavelength for a sample of Australian sedimentary quartz. They used a detection filter combination made up of two Coming 7-37, a Coming 7-60 and a Schott BG-39 filters to provide a 30 nm wide transmission band at 360 nm. This enabled them to stimulate using seven laser lines with wavelengths up to 458 nm, whilst observing the major emission at 365 nm. They demonstrated that the initial slope of a decay curve, So, is proportional to the square of the initial OSL intensity (I2) when the intensity is expressed as photon counts per mJ/cm 2 and plotted as a function of the energy received by the sample (in mJ/cm2). They plotted the square root of So versus the initial intensity for six wavelengths, Fig. 5.7. The linearity of response was taken as evidence of the initial OSL signal being derived from a single electron trap. Taking this relationship further, Huntley et al. (1996) showed that the ratio So/Io is related directly to the excitation cross-section. A sensible value of around 10 - 1 7 cm 2 was calculated for the initial OSL, but it was also noted that the decay curve deviated from being a simple exponential as the signal depleted, and other traps with different cross-sections became significant contributors to the OSL signal. In a related experiment, Alexander et al. (1997) measured the initial slope of the l l0~ PTTL decay curve as a function of wavelength. They showed that for green stimulation there was one source trap, but, as the wavelength was reduced, additional traps contributed to the PTTL. In a study of some New Zealand quartz, Duller and BCtter-Jensen (1996) varied the stimulation wavelength (with 25 nm bandwidth) from 425 to 575 nm. Having first demonstrated that it was possible to correct for variations in power by plotting the OSL as a function of illumination energy (rather than time), Duller and BCtter-Jensen (1996) made
128
Optically Stimulated Luminescence Dosimetry
Fig. 5.6. (a) OSL decay curves obtained for naturally irradiated sedimentary quartz held at 160~ during stimulation for 70 s from a filtered halogen lamp. The signal has been described by the sum of three exponential components, termed fast, medium and slow. (b) The observed data are compared with the sum of three exponential components, with the inset showing the decay out to a stimulation time of 870 s (from Bailey et al., 1997).
such plots at seven stimulation wavelengths (Fig. 5.8). It can be seen that more energy is required to reduce the signal as the stimulation wavelength increases. The efficiency of stimulation was monitored by calculating the amount of energy required to reduce the OSL by 50%. When this value is used as a normalisation factor and the resulting curves are plotted as a function of the product of the energy and the efficiency, the form of the OSL
129
OSL Properties of Natural Materials I
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Fig. 5.7. Square root of the initial rate of decrease in luminescence ~/(-S0) plotted against initial luminescence intensity (I0) for stimulation using several different monoenergetic stimulation sources (redrawn from Ditlefsen and Huntley, 1994). 6000 Stimulation w a v e l e n g t h 5000
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130
Optically Stimulated Luminescence Dosimetry
decay curves is identical in the range 425-575 nm. This suggests that OSL studies should be equivalent, whether stimulation is provided by a 514.5 nm laser line or broad-band wavelengths, such as 420-560 nm from the filtered halogen lamp on the Rise reader (BCtter-Jensen and Duller, 1992). 5.1.2.6. Effect of stimulation temperature OSL decay curves obtained using high-stimulation intensities show a strong thermal dependence. Two main effects can be seen in Figs. 5.2 and 5.4. Although, there is a small increase in the initial signal intensity as the temperature is raised from 25 to 50~ the dominant effect is the significant decrease in intensity of the initial signal as the temperature is raised further. Also, when the signals are normalised using the initial signal intensity (obtained at elevated temperatures) as in Fig. 5.3, it can be seen that the decay curves become steeper with increasing temperature. Similar behaviour has been reported by others (e.g., Smith and Rhodes, 1994; Spooner, 1994a). The decrease in intensity of the fast component as the temperature is raised is the result of the reduced efficiency of the luminescence centres due to thermal quenching (see Sections 5.1.6.2 and 5.1.9.1; see also Chapter 2). Smith and Rhodes (1994) reported that both the fast and medium components showed thermal quenching. Bailey (1997) used the 110~ TL peak response to a test dose to measure the sensitivity during measurement of the decay curve and found that no optical de-sensitisation occurred as the slow component was measured, whereas it was observed during the decay of the fast and medium components. This observation led Bailey (2000b) to suggest that the slow component used different recombination centres from those of the other components and the 110~ TL peak. This apparently conflicts with the data of Murray and Wintle (1998), which demonstrated that both the initial signal (0.4 s, 5.2 mJ/cm 2) and that at the end of their 100 s decay curve were affected by thermal quenching, unless the latter was from the medium component. The increase in initial intensity and the initial decay rate with increasing temperature have been explained in terms of thermal assistance (Spooner, 1994a; Murray and Wintle, 1998). This results in an increased rate of trap emptying. The activation energy for thermal assistance has been obtained as a function of wavelength (Spooner, 1994a; Huntley et al., 1996) and will be discussed in Section 5.1.9.2. 5.1.3. Linear modulation OSL--LM-OSL 5.1.3.1. LM-OSL at 160~ with 470 nm stimulation Another way of optically de-trapping electrons is by linearly increasing the stimulation light intensity from zero to a maximum value (Bulur, 1996), instead of stimulating with a constant illumination intensity. The resulting luminescence is termed LM-OSL (see Chapter 2). The LM-OSL from a single trap initially increases and then decays after reaching a maximum. The time at which the OSL reaches the maximum value depends on the ramp rate and the photoionisation cross-section. The latter is dependent on both the wavelength and temperature (see Section 5.1.11). If more than one trap contributes to the CW-OSL decay curve, as found by Bailey et al. (1997), then more than one peak will be seen in the LM-OSL plots. Singarayer and Bailey
OSL Properties of Natural Materials
131
(2002) measured the fast and medium components of the CW-OSL for a sedimentary quartz sample, and converted them to LM-OSL curves using a mathematical transformation (Bulur, 2000). These pseudo-LM-OSL data sets were derived after the grains had been exposed to light of different wavelengths. This enabled Singarayer and Bailey to determine the photoionisation cross-section for each component as a function of wavelength (see Section 5.1.11) Some LM-OSL curves for quartz are shown in Fig. 5.9. Fig. 5.9a shows the LM-OSL measured at 160~ using blue LEDs (470 nm); the power was raised from zero to a maximum of 50 mW/cm 2 in 50 s (Bulur et al., 2001). This sample of quartz had been irradiated and heated many times (in order to stabilise the sensitivity) before being given a 10 Gy dose. A pre-heat for 10 s at 280~ was applied before measurement of the LM-OSL.
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132
Optically Stimulated Luminescence Dosimetry
Fig. 5.9 (continued)
Three components were determined by curve fitting (Bulur et al., 2001). The inset in Fig. 5.9a shows the OSL decay curve for an equivalent aliquot obtained for full power in CW mode at room temperature. The advantage of using LM-OSL is that it enables separation of signals from traps with different photoionisation cross-sections, i.e., different decay rates under CW stimulation. The ramp rate can be chosen to obtain the best separation of the peaks. Fig. 5.9b shows the LM-OSL measurements made immediately after irradiation (Bulur et al., 2000). The stimulation power was increased from zero to 25 mW/cm 2 over a period of 3600 s. In one case, the sample was held at 25~ and in the other at 125~ In each experiment, there is a sharp peak appearing shortly after the power is switched on. BCtter-Jensen et al. (1999a) deduced that this peak corresponded to the fast component found under constant stimulation (Bailey et al., 1997). The second peak is missing from the sample measured at 125~ Bulur et al. (2000) take this as evidence for it being related to the trap responsible for the 110~ TL peak, as previously suggested by BCtter-Jensen et al. (1999a). Besides linear plots, the data from Fig. 5.9a can be displayed on log-log plots, Fig. 5.9c, as linear-log plots, Fig. 5.9d and as log-linear plots, Fig. 5.9e. In Fig. 5.9a,c, it is just
133
OSL Properties of Natural Materials
possible to see the slow c o m p o n e n t starting to appear at the end of the 50 s stimulation time. However, ramping the OSL over a longer time (3600 s) and using a 25 Gy dose and a stimulation temperature of 160~ Bulur et al. (2000) obtained the data set in Fig. 5.9f. Under these conditions, the m a x i m u m for the slow component was observed. However, it was not totally emptied, as can be seen by the results of repeated ramped stimulation of the same sample. Poolton et al. (2000) presented results for a n u m b e r of annealed quartz samples in their comparison with E P R measurements. For a sample that had been given a 3 kGy g a m m a irradiation after being annealed at 400~ L M - O S L curves obtained using blue LEDs ( 4 7 0 n m ) are shown as a l o g - l o g plot in Fig. 5.10a and a l i n e a r - l o g plot in Fig. 5.10b. These were obtained using a slower ramp rate, going up to 35 m W / c m 2 in 7200 s with stimulation at 160~ For this sample, fast and slow components can be identified as the OSL that is released between 10 and 100 s and between 400 and 7200 s, respectively (Poolton et al., 2000). It is interesting to note that although the intensity of the fast component appears in these plots, the integrated photon count is
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OSL Properties of Natural Materials
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in fact small compared with that of the slow component. This conclusion was also high reached by Bailey et al. (1997) in their study of CW-OSL measurements of quartz. 5.1.3.2. LM-OSL at different temperatures with 526 nm stimulation Kuhns et al. (2000) used green LEDs (526 nm) ramped from 0 to 29 mW/cm 2 in 2000 s to obtain the LM-OSL curves from a variety of quartz samples--from sediments, natural rock crystals and synthetic single crystals. The OSL was recorded through a 1 mm Schott BG-39 and two Hoya U-340 filters. Fig. 5.11a-c shows the results for samples given a 93.6 Gy dose and pre-heated to 162~ at l~ prior to ramped stimulation with the sample held at 127~ Fig. 5.12a-c shows the CW-OSL for identically treated samples obtained using the green LEDs to give 20 mW/cm 2 at the samples, which were also held at 127~ In addition, Fig. 5.11d shows the LM-OSL obtained at room temperature without pre-heating for the rock crystal samples. Considerable differences in both absolute intensity and form can be seen when comparing the behaviour of either LM-OSL or CW-OSL for different types of quartz. Interesting comparisons can be made between the LM-OSL at room temperature (Fig. 5.11d) and that obtained at 127~ (Fig. 5.11b) for the rock crystals. Firstly, a nonzero initial luminescence signal can be seen for the room temperature measurement that was made immediately after irradiation. This shows the presence of phosphorescence. Secondly, three peaks are observed in the LM-OSL curves at room temperature (maxima occur at t - - 1 0 0 , 400 and 1500s), whereas only two peaks (at 40 and 1400 s) are observed at 127~ The middle component seen in Fig. 5.11d corresponds to the charge being optically released from the trap equivalent to the 100~ TL peak, previously reported by BCtter-Jensen et al. (1999a). The fast component appears at different ramp times for the two temperatures, 100 s at RT and 4 0 s at 127~ Kuhns et al. (2000) related this behaviour to the temperature dependence of the photoionisation cross-section, i.e., the thermal assistance of charge release seen for the initial part of the CW-OSL (Spooner, 1994a; Murray and Wintle, 1998). 5.1.3.3. LM-OSL from single grains using 532 nm Bulur et al. (2002) observed the LM-OSL from single grains when stimulated at 160~ using a green (532 nm) diode-pumped solid-state laser, with the power being raised from 0 to 10 mW in 10 s. Three different types of behaviour were recorded for the 81 grains measured. One type contained only the easy-to-bleach component, as identified in the bulk sample measurements; another grain showed only a hard-to-bleach component; while the third type showed both the components.
Fig. 5.11. LM-OSLas a function of diode power ramp time on a lin-lin plot. Quartz was given 93.6 Gy and stimulation was with green LEDs (526 nm) ramped from zero to 20 mW/cm2 in 2000 s. (a)-(c) were measuredat 127~ after pre-heating to 162~ at l~ and (d) was measured at RT with no pre-heat. (a) Sedimentaryquartz, (b) and (d) natural rock crystal quartz, (c) synthetic single crystals (from Kuhns et al., 2000).
Optically Stimulated Luminescence Dosimetry
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5.1.4. Pulsed OSL
Optical stimulation can also be provided in discrete pulses. The signal can be observed during the pulse to provide pulsed OSL (POSL) (see Section 2.6.1; McKeever, 2001) This is also known as time resolved luminescence (TRL) (Bailiff, 2000; Chithambo
OSL Properties of Natural Materials
137
and Galloway, 2000a). The pulse width is chosen to be less than the intrinsic lifetime of the luminescence centre. It is also possible to measure a delayed OSL signal (DOSL; see Section 2.6.2). This is also known as optically stimulated afterglow (OSA) (Jaek et al., 1999).
5.1.4.1. Time resolved luminescence Time resolved luminescence (TRL) is observed when the stimulation is pulsed, using a pulsed, tunable laser (Bailiff, 2000) or using pulsed LEDs (Chithambo and Galloway, 2000a-c; Galloway, 2002). Chithambo and Galloway (2000a) designed a control circuit that was capable of generating pulses from 3 to 30 Ixs using 16 LEDs (emitting at 525 nm) to provide a light intensity of 1 mW/cm 2 at the sample. Using this system they obtained the time-resolved spectrum shown in Fig. 5.13a (Chithambo and Galloway, 2000a); this quartz had been annealed (500~ for 2 min), irradiated (150 Gy) and then pre-heated (220~ for 5 min) before being measured at room temperature. This time-resolved spectrum has a dynamic range of 64 txs. The luminescence was observed using a conventional photomultiplier tube (EMI9635QA) with a Schott BG-39 and a Schott UG11 filter combination. Fig. 5.13a shows the data obtained for two runs, one with a sample present and the other without. The first part of the background signal, obtained whilst the diodes are switched on, has a component derived from the scattered light from the diodes. After the diodes are switched off, the background signal is derived from the photomultiplier noise. The OSL is well above these background signals. When the diode is switched on, the luminescence can be seen to increase. After the diodes are switched off, the luminescence (L(t)) decays exponentially. The lifetime, ~', was obtained by fitting a single exponential function. L(t) = L(tl)exp{ - ( t - tl)/'r } where tl is the stimulation pulse width, and t is the time since the start of the stimulation pulse (in Fig. 5.13a this was 11.2 txs). Using this equation, Chithambo and Galloway (2000a) obtained luminescence lifetimes in the range of 3 0 - 4 0 txs. Bailiff (2000) used a laser pulsed at 10 Hz for 5 ns using 470 nm wavelength photons. He used detection filters that passed in the near ultraviolet, with FWHM transmission from 280 to 380nm in front of a fast linear-focussed photomultiplier tube (EMI9831QB). Bailiff (2000) obtained similar lifetimes (33 +_ 0.3 txs) for seven samples of natural quartz extracts from sediments and ceramics. In each case, the decay was fitted by a single exponential (Fig. 5.13b). Similar measurements on synthetic quartz gave a slightly longer lifetime (40 _+ 0.6 txs). However, varying the stimulation wavelength from 450 to 650 nm did not cause any change in the lifetime, but only in the yield. Both Bailiff (2000) and Chithambo and Galloway (2000c) made pulsed measurements whilst holding the quartz samples at a range of temperatures from 20 to 200~ and observed that the lifetime of the time-resolved spectra decreased with increasing stimulation temperature (Fig. 5.14a,b). Bailiff (2000) noted that the integrated intensity decreased concomitantly. On the basis of the comparison of the thermal response for both the radioluminescence (RL) (RL observed at 360 nm) (see Section 5.1.6.3) and the lifetime, Bailiff (2000) concludes that the behaviour is due to the changing competition between radiative and non-radiative transitions within the luminescence centre as the temperature is increased, i.e., thermal quenching (see Section 5.1.9.1). This view is
Optically Stimulated Luminescence Dosimetry
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supported by Chithambo (2002) and Nanjundaswamy et al. (2002) who conclude that these experiments confirm the relationship of Eq. (2.64), thus supporting the Mott-Seitz mechanism for thermal quenching (see Section 2.4.6). Chithambo and Galloway (2000b) reported small changes in values of ~-(from 29 to 39 Ixs) as a function of cumulative stimulation time for quartz, given a 150 Gy dose after heating to 500~ for 2 min or 220~ for 5 min; however, Galloway (2002) subsequently suggested that this may have been an instrumental artefact. In a further study on the same material, Galloway (2002) studied the effect of high-temperature annealing on the lifetime measured at 20~ For quartz annealed for 7 min, the lifetime was constant (41.5 txs) for temperatures up to 500~ but showed a decrease when the 7 min annealing runs were
OSL Properties of Natural Materials 50
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raised in temperature up to 100~ (Fig. 5.15). The extent of reduction in lifetime could be modified by applying short pre-heats (60 and 300 s) after delivery of the beta dose. The reduction in lifetime was interpreted in terms of a model with two luminescence centres for the OSL process, rather than one, each with a different lifetime (41.5 and 31.5 lXS,if there is no pre-heat prior to measurement). In Galloway' s model, the change in observed lifetime relates to holes being moved from the high-lifetime centre to the lowlifetime centre. This was seen as analogous to the transfer of holes from non-radiative centres to luminescence centres in models of TL (Zimmerman, 1971) and OSL sensitisation with thermal treatment (BCtter-Jensen et al., 1995). The temperature
Optically Stimulated Luminescence Dosimetry
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dependence of this process was characterised by an energy W = 1.4 + 0.4 eV, taken to be the energy above the valence band of the high-lifetime centre. Galloway (2002) repeated previous measurements (Chithambo and Galloway, 2000c) of the influence of stimulation temperature on lifetime, whilst simultaneously recording the OSL intensity. For un-annealed quartz, the decrease in lifetime and the decrease in OSL intensity occurred with the same activation energy (0.74 _+ 0.06 eV and 0.73 +_ 0.14 eV, respectively) when the data were fitted with an equation of the form used for thermal quenching. These values relate to the behaviour of the highlifetime centre, and an energy of 0.79 _+ 0.15 eV was deduced for the short-lifetime centre. By opening and closing the beta source, it is possible to obtain pulsed RL curves whilst heating the sample (Poolton et al., 2001). Data for such a measurement on quartz are shown in Fig. 7.18a (see Section 7.10), where thermal quenching is clearly seen.
5.1.4.2. Delayed optically stimulated luminescence or optically stimulated afterglow Delayed optically stimulated luminescence (DOSL) was observed for quartz by Jack et al. (1999) who termed it optically stimulated afterglow (OSA). OSA is considered to be due to the recombination of electrons released from shallow traps as the sample is held at room temperature (Jack et al., 1999). These traps were filled by electrons put into the conduction band by the optical stimulation. OSA measurements were made for stimulation wavelengths between 400 and 250 nm (Hiitt et al., 2001) and demonstrated a continuous increase in luminescence signal in response to shorter wavelength stimulation (see Fig. 5.21 of Section 5.1.5.4). For OSA measurements, it is necessary either to store the sample after irradiation--Hiitt et al. (2001) used 3 weeks--or to pre-heat to empty the shallow traps filled by the laboratory irradiation.
OSL Properties of Natural Materials
141
5.1.5. Excitation spectra Several different experimental approaches have been taken to observe the response of the OSL signal to exposure to light of various wavelengths. They can be divided into those in which the OSL is measured using the same stimulation wavelength, and those in which stimulation is made at different wavelengths.
5.1.5.1. Bleaching response spectrum The first spectral information concerning the initial fast component of the OSL came from detailed studies of the bleaching of the OSL signal (Spooner, 1994a). The OSL signal was measured using short exposures to the 514.5 nm argon-ion laser line and observed at room temperature with 2 mm of Schott BG-39 and 5.1 mm of Coming 7-51 filter glass in front of the photomultiplier tube. Bleaching was achieved with narrow-band (10 nm FWHM) interference filters in front of an 800 W halogen lamp. The resulting decay curves for wavelengths from 400 to 900 nm are given in Fig. 5.16. It can be seen that there is negligible ( < 5%) bleaching for wavelengths greater than 690 nm. Thus, no signal would be expected when quartz is stimulated at room temperature using infra-red LEDs (880 nm). Based on these findings, the lack of an infra-red stimulated 100
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Optically Stimulated Luminescence Dosimetry
142
luminescence (IRSL) signal at room temperature is regularly used as a test for purity of quartz extracted from sediments. From Fig. 5.16 it can also be seen that although bleaching can be achieved by exposure to wavelengths around 600 nm, the bleaching is more efficient at shorter wavelengths. The decay in the OSL signal for exposure to 514.6 nm can be seen close to the centre of the data set. Spooner (1994a) reported that this decay was equivalent to the OSL decay curve obtained using the 514.5 nm argon-ion laser line as a stimulation source in dating applications. For wavelengths around 400 nm, the stimulation is 10 times faster than 514.5 nm, with 50% depletion being reached after --- 10 mJ/cm 2 for the former, compared with 200 mJ/cm 2 for the latter. Thus, it can be inferred that it would be advantageous, both for more rapid measurement and for an increased signal to noise ratio, to use shorter stimulation wavelengths. As a result, BCtter-Jensen et al. (1999a) selected blue LEDs with peak emission intensity at 470 nm. The filtered halogen light source developed for quartz stimulation by BCtter-Jensen and Duller (1992) is a broad-band source, with wavelengths from 420 to 550 nm. Matching their measurements of the thermal assistance energy obtained using this source to the wavelength-dependent data of Spooner (1994a), Murray and Wintle (1998) concluded that the effective stimulation wavelength for the filtered lamp was 468 nm. The similarity of the OSL decay curves for quartz using these two light sources was shown by BCtter-Jensen
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Fig. 5.17. Bleaching of the (a) fast and (b) medium components at 20~ using LED sources with data from pseudo-LM-OSL curves (from Singarayer and Bailey, 2002).
OSL Properties of Natural Materials
143
(1997) for powers of 6 and 28 mW/cm 2, respectively. Using Fig. 5.16, the 50% depletion level is reached with 40 mJ/cm 2. More recently, Singarayer and Bailey (2002) measured the decrease in the blue (470 nm) stimulated OSL after room temperature exposure to 430, 470, 500, 525 and 590 nm light from LED sources. Fig. 5.17 shows the exponential nature of the decays for the fast and medium components that had been separated using pseudo-LM-OSL plots (see Section 5.1.3.1). A significant advantage of obtaining the bleaching spectrum for the OSL is that it can be compared directly with the bleaching of the TL peaks in quartz. TL bleaching characteristics had been reported by Spooner et al. (1988). They noted that by the time the 325~ TL peak had been removed, the other TL peaks (at 370 and 480~ had only been depleted by 20%, as shown in Fig. 5.18a, in which the OSL signal reduction has been added (Spooner, 1994a). From these results, Spooner (1994a)concluded that bleaching of the 325~ TL peak and the prompt emission of OSL are both related to single-photonabsorption photo-ionisation direct to the conduction band. Huntley et al. (1996) disagree with the claimed relationship, on the basis of the relative reduction of the two signals caused by a single laser (488 nm) exposure of 80 mJ/cm 2. However, they expected complete removal of all the TL in the vicinity of 325~ Wintle and Murray (1997) observed the decay of a small peak centred at 325~ found on the shoulder of a higher peak that did not bleach (as also seen in the data of Huntley et al. (1996)). Wintle and Murray (1997) observed this signal and the OSL during exposure to blue/green light from a filtered halogen lamp. Both signals decayed proportionally (Fig. 5.18b) and were effectively erased by an exposure of 10 s, corresponding to 130 mJ/cm 2. Thus, the correlation between the two signals is confirmed. 5.1.5.2. Excitation spectra after bleaching by 514 + 25 nm light Duller and BCtter-Jensen (1996) used a scanning monochromator to investigate the OSL as a function of stimulation wavelength from 400 to 560 nm, the range of the instrument (BCtter-Jensen et al., 1994a). The sample of sedimentary quartz had received a dose of 10.8 Gy and a pre-heat (220~ for 5 min)after being heated to 450~ Using a scan speed of 2.5 nm/s, they observed the luminescence using two Hoya 2 mm Hoya U-340 filters. The excitation spectra were obtained after a set of samples had been bleached for various lengths of time by 514 + 25 nm light from the same monochromator (0.4 mW/ cm2). The data thus obtained (Fig. 5.19) were then re-plotted as a set of almost identical bleaching curves. Duller and BCtter-Jensen (1996) concluded that the similarity of the decay curves in the range 450-610 nm and over two orders of magnitude of signal reduction indicated that the same trap (or traps) were being probed by all these wavelengths. 5.1.5.3. Continuous scanning of stimulation wavelengths BCtter-Jensen et al. (1994b) used the same light source and scanning monochromator to obtain continuous OSL output, as the stimulation wavelengths were changed from 400 to 750 nm. The signal was measured using two Hoya U-340 filters. The OSL output from an irradiated (8 Gy) sedimentary quartz that had previously been heated to 850~ is shown in Fig. 2.6 as a function of stimulation energy. BCtter-Jensen et al. (1994b) concluded that the
144
Optically Stimulated Luminescence Dosimetry 108
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OSL Integral, counts Fig. 5.18. (a) The bleach response spectrum for OSL with data taken from Fig. 5.16 and Spooner (1994a). Also shown are TL data from Spooner et al. (1988) showing the energies required to completely bleach the 325~ TL peak and reduce the 370 and 480~ TL peaks to 80% of their initial intensity (from Spooner, 1994a). (b) Reduction of OSL and a small TL peak at about 325~ as a function of green light stimulation up to 100 s. The label beside each point gives the exposure time in seconds (from Wintle and Murray, 1997).
OSL Properties of Natural Materials
14000 I 12000 "-" -~
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145
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Fig. 5.19. Stimulationspectra obtained after exposure to 514 _+ 25 nm light (obtained using a narrow band-pass monochromator in front of a halogen lamp) for periods of time ranging from 0 to 3200 s, as indicated in legend. The spectra were obtained using a resolution of 25 nm and have not been corrected for variations in stimulation power as a function of wavelength (from Duller and BCtter-Jensen, 1996).
deviation from true exponential behaviour for photon energies beyond 2.5 eV may be related to the presence of an absorption band in this region. A similar deviation can be seen in Spooner's data (Fig. 5.18a).
5.1.5.4. Excitation using interference filters and xenon lamp Kuhn et al. (2000) used a set of interference filters, in combination with a heat absorbing and an edge cut filter, to stimulate OSL from quartz. They observed the OSL between 300 and 390 nm, obtained using MUG-2 and Hoya U-340 filters. Data were obtained for sedimentary quartz samples from 13 locations around the world and a typical OSL stimulation spectrum in given in Fig. 5.20. It shows a break in response at ---2.5 eV similar to that reported in the previous section. However, Kuhn et al. (2000) put forward a different model to that of BCtter-Jensen et al. (1994b), with two types of traps providing the source of the electrons. They also recommend that stimulation in this region ( 5 0 0 - 5 2 0 nm) should be avoided. It should be noted that this includes the 514.5 nm line from an argon-ion laser and the wavelengths from the blue plus green ( 4 2 0 - 5 5 0 n m ) light OSL attachment for the Rise TL/OSL system (Markey et al., 1997). Kuhn et al. (2000) suggest that the energy at which this behavioural break occurs may be dose (or dose rate) dependent. If so, this would lead to problems with age determination procedures. Htitt et al. (2001) obtained a stimulation spectrum from 950 to 420 nm for OSL from both natural and synthetic quartz (doped with Cu) using a xenon lamp and a monochromator (Hfitt et al., 1988). They measured the OSL with a filter (UFS) that has
Optically Stimulated Luminescence Dosimetry
146
photon energy in eV 2.8 2.6 2.4
3.0 I
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Fig. 5.20. OSL excitation spectrum for a naturally irradiated sedimentary quartz from Burkina Faso obtained using interference filters. Integrated OSL is plotted as a function of the central wavelength for each filter (from Kuhn et al., 2000).
a transmission window centred on 330 nm, but with its half width from 300 to 380 nm (Fig. 5.21). Using a novel approach, they extended their investigation of the stimulation of electrons from the OSL trap by observing the delayed luminescence (OSA) (see Section 5.1.4.2) occurring at least 0.05 s after optical stimulation is ended.
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Wavelength, nm Fig. 5.21. Stimulation spectra for OSL (1) and OSA (2) of natural quartz doped with Cu; signals observed with near-UV filter (redrawn from Hiitt et al., 2001).
OSL Properties of Natural Materials
147
5.1.5.5. Excitation using laser lines from 458 to 645 nm In an early study, Godfrey-Smith et al. (1988) reported OSL measurements of the equivalent dose for quartz made using krypton laser lines from 413 to 799 nm. However, quantification of photon output was only later reported by Ditlefsen and Huntley (1994) who used selected wavelengths from argon-ion, H e - N e , krypton and diode lasers to stimulate OSL from quartz; the OSL was observed with a filter combination (two Corning 7-37, one Coming 7-60 and one 2 mm Schott BG-39) that transmitted at 360 +_ 30 nm. Fig. 5.22 shows the log of the initial intensity (corrected for the different incident energies) as a function of stimulation photon energy. In a later paper, Huntley et al. (1996) show the initial intensity in terms of the number of 360 nm photons observed per incident photon as a function of stimulation energy. The data formed a smooth curve, and indicated a production rate approaching 1 in 10 l~ for the highest stimulation energy (2.71 eV, 458 nm) (see Fig. 5.54 in Section 5.1.11). 5.1.5.6. Stimulation in the infra-red 7 8 0 - 9 2 0 nm As discussed in Section 5.1.5.1, wavelengths greater than 690 nm appear to cause negligible reduction in the 514.5 nm OSL signal stimulated at room temperature (Spooner, 1994a). However, Godfrey-Smith and Cada (1996) reported a stimulation peak at 840 nm for two sedimentary quartz samples included in their study of IR stimulated spectra from feldspars. In contrast, for sedimentary quartz, doped with Cu to give a higher sensitivity, Hiitt et al. (1999) found a monotonic decline in the OSL intensity from 600 to 1000 nm.
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148
Optically Stimulated Luminescence Dosimetry
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Fig. 5.23. The thermal dependence, over temperaturesranging from 100 to 500 K, of the OSL stimulatedby a variety of narrow wavebands obtained using a filtered light source. Background count rates were subtracted and the data were corrected for depletion by the cumulative short exposures used to obtain each data set (redrawn from Spooner, 1994a).
Spooner (1994a) showed that a weak OSL signal could be stimulated by wavelengths greater than 700 nm (Fig. 5.23). Seven narrow wavebands (10 nm FWHM) in the range from 480 to 860 nm were used to stimulate an Australian sedimentary quartz at temperatures from - 170 to 200~ OSL, stimulated by the 860 nm light, can be seen when stimulation is carried out at temperatures above 50~ As shown in Fig. 5.23, the IR stimulated signal obtained using the same detection filters is about four orders of magnitude less than for stimulation at 515 nm. It must be noted that this factor is specific to the measurement system, since the data were not normalised for incident beam energy. Use of a 10 times more powerful stimulation source, e.g., a 1 W solid-state infra-red laser diode giving about 400 mW/cm 2 of 830 nm IR at the sample, has been found to remove the fast component without affecting the medium component when the exposure is performed at 160~ (Singarayer and Bailey, 2002). Bailey (1998) reported that IR stimulation at 875 nm only gave rise to IRSL when stimulation was carried out at temperatures greater than 200~ When this was observed, the OSL signal (stimulated with blue/green light) decayed at the same rate following IR exposure. This suggests that both the IRSL and the fast component of the OSL are derived from the same trap. As will be shown in Section 5.1.9.2 (Fig. 5.49), the thermal activation energy determined for the IRSL output fits the thermal activation energy versus wavelength curve determined for the visible wavelengths (Spooner, 1994a). Thus, it must be concluded that pure quartz has an IRSL signal and that merely determining the presence or absence of IRSL is not an adequate test for feldspar contamination. A more appropriate test is whether the OSL signal can be reduced by IR exposure (Duller, pers. comm.).
149
OSL Properties of Natural Materials 5.1.6. Emission spectra
Spectral characteristics of luminescence emitted from quartz are of vital importance for the correct choice of optical filters to optimise detection of weak signals, as are often found in retrospective dosimetry. Additionally, in the case of OSL, it is important to minimise wavelengths from the stimulation source. In TL, it is the blackbody radiation that needs to be rejected, as temperatures are raised. Table 7.1 (in Chapter 7) and Table B 1 in the appendix of Aitken (1998) provide useful summaries of the most commonly used band-pass filters for OSL measurements. The latter gives characteristics such as peak wavelength and F W H M for particular filter thicknesses. For other studies, e.g., of kinetic behaviour, a narrower bandwidth can be selected using a long-pass filter together with an interference filter. Some combinations are discussed below. 5.1.6.1. OSL emission spectra Using a spectrometer based on a micro-channel plate detector, OSL emission spectra of quartz from Australian dune sands have been obtained using excitation at 647 nm from a krypton laser (Huntley et al., 1991). Fig. 5.24 shows the single emission band that is centred on 365 nm when the measurements were made at room temperature, confirming earlier studies using colour glass filters (Huntley et al., 1989). The results in Fig. 5.24 have provided the base for optical filter selection in routine measurements, e.g., Hoya U-340.
200
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Fig. 5.24. Emissionspectra of several samples of sedimentary quartz from south-east South Australia aged from 0 to 800 ka, obtained for stimulation using the 647 nm line from a krypton laser (from Huntley et al., 1991).
150
Optically Stimulated Luminescence Dosimetry
No experiments have been performed at elevated temperatures to confirm or deny the claim by Franklin et al. (1995) that the luminescence centres used are the same as those used by the lower TL peaks, as discussed in Section 5.1.6.2. 5.1.6.2. TL emission spectra There have been many studies of TL spectra, on quartz crystals (McKeever, 1984), quartz of different geological origins (Rink et al., 1993) and quartz extracted from sediments (Scholefield et al., 1994; Scholefield and Prescott, 1999). An extensive review of emission spectra has been provided by Krbetschek et al. (1997). Fig. 5.25 shows the main TL emission bands as a function of temperature in the range 50-400~ The colour definitions used in the following sections (violet, blue, orange) are those from the German Industry Norm (DIN 5031) as reproduced by Aitken (1998). It should be noted that spectra for natural samples are obtained using sub-samples made up of a large number of grains. Several studies suggest that individual grains are dominated by either blue or "red" emission (e.g., Hashimoto et al., 1986; Huntley et al., 1988a,b). As the result of a recent study involving thermal annealing of natural quartz, Hashimoto et al. (1996) have suggested that the blue luminescence may originate from grains of a-quartz, whereas the red emission may relate to grains that have been heated beyond the phase transition at 870~ Thus, it is not clear to what extent any spectra will have been influenced by mixing of such grains as would result from sediment-forming processes. 5.1.6.2.1. 3 6 0 - 4 2 0 n m (near UV to violet). Emission in the near UV to violet (360-420 nm) is characteristic of the low-temperature TL peaks (100-210~ that are present subsequent to laboratory irradiation. In Fig. 5.25, an apparent shift in peak emission to lower energies with increasing temperatures is seen. The UV-violet data in Fig. 5.25 are based on spectral studies made by Franklin et al. (1995) on sedimentary quartz. This sample gave a strong emission at 470 nm for the higher temperature TL peak. In Fig. 5.26, four TL peaks (at 100, 180, 220 and 305~ are marked by crosses. Franklin et al. (1995) concluded that they form a family of traps that use the same recombination
Fig. 5.25. Main TL emission bands of quartz from 50 to 400~ obtained from a literature survey (from Krbetschek et al., 1997).
OSL Properties of Natural Materials
151
Wovetength ( n m )
500 400 ~---~)/~~~
400
300
,
....
_
.-- 300
"~ 200
1(30
X
9 2.5
1
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3.0 3.5 P h o t o n e n e r g y {eV)
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Fig. 5.26. Contour map of three-dimensional TL spectrum for unbleached natural quartz from Australia that has been given an additional 60 Gy dose. The crosses mark approximate peak locations, and the OSL emission at 22~ from Fig. 5.24 (from Franklin et al., 1995).
centre and suggested that the emission peak shifts to lower energies as the temperature is increased. The 365 nm OSL emission at 22~ (Huntley et al., 1991) is also shown in Fig. 5.26. The near UV to violet luminescence has a particular feature that can be seen during TL and OSL measurements made from just above room temperature to 300~ There is a decrease in luminescence efficiency with increasing temperature due to the increased probability of non-radiative transitions. This is known as thermal quenching (Chapter 2, and Chen and McKeever, 1997). It was first observed in luminescence studies on quartz by Wintle (1975) and more recently has been documented for the three peaks at 180, 220 and 305~ by Franklin et al. (1995) and for both the 325~ TL peak and OSL by Spooner (1994a). Spooner' s data are shown in Fig. 5.27a. The TL data were obtained by varying the heating rate from 10~ to as low as 0.002~ For each TL data point, the 325~ signal was obtained by the subtraction of glow curves from identical aliquots bleached with light from which the ultraviolet and blue components had been removed. The resulting data are shown in Fig. 5.27b. From Fig. 5.27a it can be seen that thermal quenching affects the luminescence output from about 30 to 250~ This figure demonstrates the equivalence of the luminescence from the "325~ '' TL peak and the integrated fast component of the OSL. Nanjundaswamy et al. (2002) investigated thermal quenching of the UV emission (340 + 30 nm) related to the 110~ TL peak. Heating rates from 0.01 to 9~ were used and the TL peak areas were plotted as a function of peak temperature. The resulting graph agreed with the predicted thermal quenching behaviour, based on previously determined physical constants (McKeever et al., 1997a). It is interesting to note that Nanjundaswamy et al. (2002) corrected their data for the shift in emission wavelength with temperature,
152
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Cumulative time at temperature (hr) Fig. 5.38. (a) Sensitisation of the OSL from a sample previously heated to 500~ and given a dose of 51 Gy before storage times of up to 22 h at 160 ( 9 ), 180 (u 200 (11), 220 ( . ) , 240 (A), 260 ( 9 ) and 280 (0) ~ (b) Sensitisation of the 110~ TL peak as a function of time held at various temperatures: 160 ( 9 ), 180 ( 9 200 (I?), 220 (V), 240 (11), 260 (D) and 280 ( , ) ~ (redrawn from Wintle and Murray, 1999).
peak as a sensitivity monitor; this was used by Murray and Roberts (1998) in a singlealiquot regenerative protocol. Vartanian et al. (2000) investigated the changes in OSL sensitivity for quartz from archaeological material and synthetic hydrothermal quartz. For the latter, different concentrations of aluminium and lithium were added as the crystals were grown, and the concentrations of these elements and alkali ions (e.g., Na and K) were measured. A correlation was found between these elemental concentrations and luminescence sensitivity. Vartanian et al. (2000) suggested that the sensitivity change caused by
OSL Properties o f Natural Materials
169
pre-heating in the range of 200-250~ is irreversible and is due to defect migration. They suggested the selection of low-temperature pre-heats in order to avoid sensitisation, unless the change can be monitored (as in the case of the SAR procedure).
5.1.8.3. Thermal stability 5.1.8.3.1. Isothermal decay. Isothermal decay of the OSL can be used to monitor the thermal emptying of the trap that gives rise to the OSL signal. However, these measurements are only correct either when no sensitivity change occurs during the thermal treatment (e.g., the sample has been fully sensitised by thermal annealing; Smith et al., 1990) or when any sensitivity change is monitored (e.g., using the 110~ response to a test dose; Murray and Wintle, 1999a) or using data taken after the initial sensitisation has ceased (Spooner and Questiaux, 2000). The latter two studies were undertaken on naturally irradiated quartz in order to avoid measuring OSL from traps that are only filled by laboratory irradiation and to avoid sensitivity changes resulting from the laboratory irradiation. Li et al. (1999) also showed exponential decay for storage at 260~ provided a storage time of 30 s was exceeded, allowing the major change in sensitivity to have taken place. Murray and Wintle (1999a) used single aliquots to determine the decay curves of the initial OSL signal for storage at temperatures between 160 and 280~ Fig. 5.39 shows the sensitisation-corrected decay curves of the natural OSL of a sedimentary quartz (---30 ka with D e "~ 51 Gy). Storage times of up to 25 h were used. Ninety-nine percent of the signal can be represented as a single exponential decay, implying that a single trap is being emptied by these thermal treatments. Using the corrected data set (c.f. earlier measurements by Wintle and Murray (1998)), a trap depth of 1.59 ___0.05 eV and a preexponential factor given as lOgl0 s -- 12.9 + 0.5 were obtained. Spooner and Questiaux (2000) reported a trap depth of 1.59 eV and lOgl0 s -- 12.5, giving a calculated lifetime of 21 x 106 years at 20~ This makes it suitable for dating samples up to 1 million years old. The exponential decays shown by Smith et al. (1990), Murray and Wintle (1999a) and Spooner and Questiaux (2000) support their conclusions that the major part (> 95%) of the initial OSL signal from a naturally irradiated sample is derived from a single trap. This is in disagreement with the conclusions of Huntley et al. (1996) who did not observe exponential decays and thus considered several traps with different photoionisation crosssections to be involved. Their isothermal decay data were also obtained on natural quartz, but no allowance for sensitivity changes was made. Murray and Wintle (1999a) also made measurements on the same material that had been optically bleached and given 51 Gy and also heated to 500~ and given 51 Gy. The sensitivity-corrected data were again plotted, but in these cases, two components were seen in the thermal decay. The components contributed 61 and 38% for the bleached and 69 and 29% for the heated samples. The values of E for the dominant component (also that which decayed more slowly) were higher than that for the natural, namely 1.73 ___0.09 and 1.69 ___0.04 eV, respectively. The cause of this behaviour for laboratory-irradiated samples was not explained. Instead a best estimate of 1.66 ___0.03 eV was used for the component that depleted most slowly in the laboratory experiments and assumed to be that which remained in nature. The lifetime at 20~ proposed for this component was ---1.1 • 108 years. For the other component, seen only in laboratory-irradiated
Optically Stimulated Luminescence Dosimetry
170
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Cumulative time at temperature, hr Fig. 5.39. Isothermal decay curves for OSL signal from natural quartz. The OSL signal was corrected for sensitisation using the 110~ TL peak response to a test dose given after each measurement. Results are shown on three time scales (from Murray and Wintle, 1999a).
samples, the equivalent lifetime was --- 380 years. These results suggest that a pre-heat is necessary to remove the unstable charge. Based on the trap depth of 1.14 + 0.14 eV and pre-exponential factor given as logl0s = 9.5 + 1.5, pre-heats of 16 h at 160~ or 5 min at 220~ or 10 s at 280~ would be sufficient to remove it. Using these trap depth data, and the sensitisation data, Murray and Wintle (1999b) conclude that for their 30 ka Australian quartz sample, it would not be possible to select a pre-heat time and temperature that would provide complete sensitisation of natural and laboratory-irradiated aliquots, without causing some loss of charge from the OSL trap.
5.1.8.3.2. Pulse annealing. Another approach in obtaining and displaying the data on thermal stability is to plot the OSL signal remaining after a fixed time at various temperatures. Rhodes (1988) used 5 min heat treatments at a range of temperatures up to
171
OSL Properties of Natural Materials
240~ on a naturally irradiated sedimentary quartz and on the same material that had been given an additional laboratory irradiation. However, interpretation of such plots in terms of thermal erosion of OSL traps is complicated by the sensitivity changes brought about by the thermal treatment. Wintle and Murray (1998) carried out a similar experiment but used the response of the 110~ TL peak to a test dose (Fig. 5.40b) to correct for such sensitivity changes. In their experiment, a single aliquot was heated for 10 s at progressively higher temperatures. The uncorrected data in Fig. 5.40a suggest that the laboratory-irradiated sample has a large thermally transferred OSL signal. However, after correction, this effect is much reduced (Fig. 5.40c).
J O9 ,e--
10
1!
(a)
4
~
do ..-, o
0
5x10 3
i 100 200 300 Temperature, ~
if) L..
LI..
e0
I- o
oO -o~
1.25
-
O "O
>"
-~9 "~d E s,_ E o L.o Zu_
1.00 0.75
-
0.50
'
I
'
I
'
1.5 O ,~
0
10 4 "
5
9 nI Z I
~ == iT o
~
L..
o O
5x10 3
~ ~
, [
100 200 300 Temperature, ~
0 100
200
300
400
500
Preheat Temperature, ~
Fig. 5.40. (a) Initial OSL obtained at 125~ after pre-heating for 10 s at temperatures from 160 to 500~ for natural sample (O) and for sample that had been bleached at 125~ for 200 s and irradiated with 56 Gy ( 9 ); (b) 110~ TL peak from 0.1 Gy dose given after each OSL measurement in (a); (c) OSL data from (a) corrected for thermal activation using data from (b) (from Wintle and Murray, 1998). (The inset shows the ratio of the two data sets.)
Optically Stimulated Luminescence Dosimetry
172
1.2
-
r_..!
(a)
~
...
ooo. ~
...,.v,~oO o ~
.
o
'~ ~ \
0.8
9 0.5~
1.0~
9 2.0~
0oc,,
O N
E
0.4
k-\\
o Z
"
0.0
I
I
(b)
1.6
~,,
c.
m
...J
1.2
O
"o N .
.~' I
0.8
o Z
9 1.0~ O 2.0~ 9 3.0~
~ \
_.**L,~Lo~ ~ b :
i
m
E
V.
-"
o.i
0.4
- I i'll'
\
''It
0.0 100
200
300 Temperature (~
400
500
Fig. 5.41. Pulse annealing curves using different heating rates; (a) natural quartz, and (b) quartz annealed at 500~ and given 50 Gy (from Li and Chen, 2001).
Li and Chen (2001) made OSL measurements on both natural and laboratory-irradiated quartz after heating at rates from 0.5 to 3~ to progressively higher temperatures from 50 to 450~ in 10~ steps (Fig. 5.41). They did not maintain the temperature, but cooled the sample immediately, a process termed "pulse annealing". No sensitivity measurement was made, but they used the data in Fig. 5.41 to obtain the percentage reduction in OSL signal per ~ (Fig. 5.42). Positive data points indicate the decay of the OSL signal and relate to thermal untrapping, whereas negative data points indicate enhancement of the signal and relate to sensitivity increase. When these data are obtained at more than one heating rate, it is possible to obtain the thermal activation energy and frequency factor for the process (Li et al., 1997). Li and Chen (2001) obtained a trap depth of 1.75 + 0.03 eV for thermal depletion and 1.38 eV for the sensitisation process that peaked at 250~ for the laboratoryirradiated sample, and confirmed the latter value with isothermal decay experiments. The lifetime at 20~ associated with this process, was about 30 ka and is thus likely to be the
173
OSL Properties of Natural Materials
0.3 (a) Natural
~o
0.2
=
O
0.5~
.3 L
--O--9 1.0~ -.2.0~ --'-.-V . . . . 3.0~
]l~t~ ~,.~ , ~ t~ I Y~,'~ i I \
o.1
~
0.0
0 -0.1 0.4 oo
o~
0.3
,-
0.2
tO o = "o .
(b) Annealed 9
o 9
1.0 ~
2.0 ~ o
3.0 C/s
I~
1 I
_.d
I
I
_
.._!
0
0.1 0.0 -0.1
200
300
400
500
Temperature (~ Fig. 5.42. Percentagereduction in OSL signal plotted as a function of pulse anneal temperature; data for (a) natural and (b) annealed quartz from Fig. 5.41 (from Li and Chen, 2001).
cause of the natural sensitisation that occurred for sedimentary samples deposited several tens of thousands of years ago. 5.1.8.4. Irradiation at elevated temperatures Since shallow traps remain empty during environmental irradiation, but continue to fill during laboratory irradiation, it is possible that the electron-trapping probability for the OSL traps may be different under the two conditions. In particular, some effect would be seen if the shallow traps associated with the 110~ TL peak saturated during the laboratory irradiation. This possibility was investigated by Wallinga et al. (2002) by performing laboratory irradiation at temperatures from 35 to 260~ on a sample of sedimentary quartz that had been repeatedly heated to 500~ to stabilise the sensitivity. A 20% monotonic decrease in the electron-trapping probability was found for temperatures from 35 to 185~ Although OSL traps fill more slowly at higher irradiation temperatures, there is no step change; this suggests that shallow trap saturation does not affect the OSL signal from the deeper trap. The observed decrease in OSL with irradiation temperature may be related to
174
Optically Stimulated Luminescence Dosimetry
a reduction in the capture cross-section for the OSL traps as the temperature is increased; this will have no effect on equivalent dose determination when irradiation is made at ambient temperature. Bailey (2002) suggested that irradiation at elevated temperature may also provide an additional SAR procedure. He proposed the use of the RL obtained at the end of the regeneration irradiation as a monitor of the luminescence efficiency. For the natural data point, it would be necessary to give a small dose to enable the appropriate RL to be measured. This approach was tested using his computer model for quartz (Bailey, 2002), but has not been applied to real samples.
5.1.8.5. Thermal transfer A number of dating studies involving quartz samples from glaciated areas have reported an increase in OSL signal following application of a pre-heat (Rhodes and Pownall, 1994; Rhodes and Bailey, 1997). The pre-heat causes charge to be transferred from thermally stable light-insensitive traps in the region of 300~ to the OSL trap via the conduction band. For material taken from streams in glaciated areas of the Himalayas, Rhodes and Pownall (1994) found D e values in the range 2 4 - 30 Gy, and these values were only halved by an 8 h exposure to daylight before D e measurement. However, in these studies, multiple aliquot dating procedures were used, allowing no account to be taken of any sensitivity change and not permitting D e t o be determined as a function of temperature. A more comprehensive study on a variety of glacial sediments and an aeolian sand was reported by Rhodes (2000). The SAR protocol was applied in order to account for sensitivity change, though very little was found for these samples for pre-heats up to 280~ as monitored by both the OSL response to the test dose in the SAR procedure and the 110~ TL peak. Pre-heats of 10s were used, with the temperature range of 160-360~ being covered in 40~ steps. All the samples were optically bleached with 200 s of 4 2 0 560 nm light at 125~ using a stimulation power of 12 mW/cm 2, in order to make comparison between the responses of samples from the Himalayas and from West Greenland. The results are shown in Fig. 5.43, together with the result for a sample of 24 ka dune sand from Alabama. The latter gave a negligible value of D e ( < 0.1 Gy) until a pre-heat of 280~ was exceeded. (Note that this pre-heat would not be used in a dating run since all the OSL charges would have been thermally erased (see Section 5.1.8.3)). The glacigenic quartz gave non-zero values of D e , with the Himalayan samples (L6 and L8) giving increasing De values (up to 20 Gy) as the pre-heat temperature is increased. In further studies on one of these samples (L6), the TL was measured as the sample was heated to the pre-heat temperature, ranging from 200 to 280~ in 20~ steps. The OSL was then measured at 160~ The pre-heating was measured many times, until the TL and OSL signal reached negligible levels. Plots of the initial OSL versus the TL from the preceding pre-heat were made (Fig. 5.44). The relationship was taken to imply the charge transfer via the conduction band (Rhodes, 2000). Charge transfer was also reported for quartz from aeolian deposits (e.g., Rhodes, 1988); however, in these early studies, no sensitivity monitoring was carried out. More recently, studies of young dune sands by Bailey et al. (2001) have shown negligible thermal transfer for temperatures below 260~ (Fig. 5.45), with a modem sample giving D e values of 0.03 _+ 0.02 Gy (corresponding to an age of 20 _ 10 years). For pre-heats above 260~ D e values up to 2 Gy were obtained. Likewise, Murray and Clemmensen (2001) report no
OSL Properties of Natural Materials
a) L6 & L6 *
4O 3O
5
20 o
g~
3
~
2
10 ,
0
100
200
,
,
300
400
b) HM46
o
|
0
100
(.9
|
.
300
400
d) A122~
0-2t
c) G R 6
015
.,
10
o%'t 0.1
(.9 4
v
cl
n
200
10s Pre-heat Temperature (~
10s Pre-heat Temperature (~ 15
175
o
v
5
~; o { "
.
.
.
200
100
.
300
400
a
. 0
.
100
.
.
200
0 300
0
10S Pre-heat Temperature (~ 4O
e)L6*
3O
~'~~
~ []
,
0
100
-
p.
200
[]
[] .
300
10s Pre-heat Temperature (~
100
9
9 ~ 200
-=,
.
300
400
10s Pre-heat Temperature (~
20 10
,
203010
400
f) L8*
[] [] t
D .
.
400
[]
.
0
100
200
[]
=
i
300
400
10s Pre-heat Temperature (~
Fig. 5.43. De as a function of 10 s pre-heat temperature for glaciofluvial samples from the Himalayas (L6, L8 and HM46) and from West Greenland (GR6) and for a dune sand from Alabama (A122). All samples were exposed to laboratory light before measurement (from Rhodes, 2000).
increase in D e values with pre-heat temperature for their aeolian dune sands, including a very young sample for which they obtained a D e of 0.082 _ 0.009 Gy over the range of 160-280~ (corresponding to an age of 100 _+ 30 years). The data for this and another sample from the same area of Denmark are shown in Fig. 5.46. Vartanian et al. (2000) also concluded that thermal transfer was not the explanation for the increase in OSL that resulted from pre-heating samples of archaeological ceramic. Besides charge being transferred from deep traps, around 300~ it is also necessary to consider whether charge may be transferred from shallower traps to the main OSL trap. In this case, the charge from shallower, but optically stable traps may be transferred during burial. Indeed, one of the reasons suggested for pre-heating prior to OSL measurement was to ensure that the amount of charge transferred was equal for both natural and laboratory samples. These traps were thought to relate to the TL peaks, sometimes observed at ---230~ Once again, using SAR it is possible both to carry out D e measurements over a range of temperatures and to be sure that different sensitivity changes for natural and laboratory-irradiated material does not cause error. In the literature there are a number of
Optically Stimulated Luminescence Dosimetry
176
.o
t
(a)
~" 0.1 "~ 0.08
L6 -Initial n o r m a l i z e d O S L vs PH TL 240~ 220~
o.o6t
~ J 200 ~ o-- ~o 0.04 .~_ o "a "6 0.02
--
260~
0
0
2000
4000
6000
8000
10000
Total TL during PH ramping (counts) (b)
L6 - I n i t i a l n o r m a l i z e d O S L vs PH TL
.s A 0.12 " 1 ~
, , o~
o.1 4 , "
240~
260~
~ 0.08 0 =~ 0.06
.~ 8 0.04 = "6 0.02
--
0
|
0
10000
20000
30000
40000
Total TL during PH ramping (counts) Fig. 5.44. Initial OSL signal as a function of TL measured during pre-heat. Values obtained from repeated heating to the given temperature (a) for lower temperature pre-heats and (b) data for higher temperature pre-heats (from Rhodes, 2000).
2.5
Sample AB2-1
2.0
.-,9
1.5
1.0 $
0.5
0
140
*
§
1~0
1~0
260
2;20
2a,0
t
§§
260
280
300
320
Pre-heat temperature (~ Fig. 5.45. Plot of De as a function ofpre-heat temperature fora young dune sand withDe = 0.41 --- 0.13 Gy (for 160-260~ and giving an age of 310 ___90 years (from Bailey et al., 2001).
177
OSL Properties of Natural Materials
0.20 (.9
(a)
De (160-300~ 0.15 - =0.082+0.009 Gy
O
s
t" I1) t~
0.10 -
xlI
,
m
0.05
O" UJ
0.00
100+30 years !
!
!
!
150
200
250
300
(b)
(.9 O n
--
t
w
w
i
t@ t~ >
m
{7" UJ I
I
I
i
150
200
250
300
Pre-heat Temperature, ~ >,
60 -
(c)
@ if) 0
121
40 -
1-i1) >
.m
ID"
20 -
D e (200 to 3 0 0 ~
LLI
= 4 4 . 8 + 1.4 Gy i
150
I
200
i
250
|
3OO
Pre-heat Temperature, ~ Fig. 5.46. Plots of D e as a function of pre-heat temperature for (a) 0.082 and (b) 4.42 Gy (from Murray and Clemmensen, 2001), and (c) for 44.8 Gy (from Murray and Olley, 1999).
plots of D e as a function of pre-heat temperature, see for example Figs. 5.45 and 5.46. None of these show deviation from a plateau in a way that would imply the presence of interference from a shallower trap. 5.1.9. Raised temperature OSL
5.1.9.1. Thermal quenching As mentioned in Section 5.1.6.2.1, the luminescence emission at 3 6 0 - 4 2 0 nm is quenched as the room temperature is raised. It was first noted by Wintle (1975) who observed a discrepancy in measurements of the trap depth of the 325~ peak in quartz.
178
Optically Stimulated Luminescence Dosimetry
The isothermal decay method and Hoogenstraaten's method of trap depth determination gave values of 1.7 _ 0.1 and 1.69 - 0.03 eV, respectively, whereas the initial rise method gave 1.05 ___ 0.03 eV. The latter predicted a very low value of the mean life at 20~ as only ---200 years. This was clearly inappropriate for such a deep trap with independent evidence of its stability from dates obtained for archaeological samples. This difference in trap depth energy, 0.64 eV, can be explained by the luminescence centre having a luminescence efficiency r/that is temperature-dependent, such that r / = K expW/~, where W -- 0.64 eV. Thermal quenching should not affect the isothermal TL decay measurements, as long as no sensitivity changes occurred. However, thermal quenching would have a substantial impact on data derived with the luminescence measured at different temperatures. This was demonstrated in studies of the 325~ TL peak by Spooner (1994a) (see Section 5.1.6.2.1). Duller et al. (1995) measured the OSL from sedimentary quartz obtained on stimulation at temperatures ranging from 20 to 450~ They reported thermal quenching with W -0.63 eV and K -- 2.8 x 107. In a more detailed study, Huntley et al. (1996) measured the luminescence output as a function of temperature from 20 to 220~ They observed the OSL using filters centred at 356 nm, after subtraction of the TL signal that is observed above 150~ as the sample is heated. Several stimulation sources were used, as shown in Fig. 5.47 using lines from several laser sources, namely 454, 488, 514.5, 633 and 674 nm.
10-10tO
2.54 ev
.t.-,
0
'=ca- 10 .!,.,.,
2.73 ev
-11-
2.41 ev
tor
i,.,,. to
10-12-
r
0 0 cO
~r.- 10-13-
10-14-
1.96 ev
1.84 ev
6
5'0
16o
Temperature (~C)
260
2so
Fig. 5.47. Temperature dependence of the luminescence during excitation with selected lines from argon, He-Ne, and diode lasers (redrawn from Huntley et al., 1996).
179
OSL Properties of Natural Materials
Thermal quenching gives rise to the decreases in OSL observed for stimulation temperatures above 140~ Huntley et al. (1996) reported determining values for W and K similar to those presented by Wintle (1975). Murray and Wintle (1998) used their OSL decay curves obtained at elevated temperatures (50-175~ to demonstrate thermal quenching. They plotted both the initial OSL (first 0.4 s of their decay curve) (fast component) and the total integrated signal (100 s) as a function of temperature (Fig. 5.48). Both data sets show thermal quenching, though the initial OSL signal also shows the effect of thermal assistance (see Section 5.1.9.2). The similarity in thermal quenching behaviour implies that the slow component uses the same luminescence centres as the fast component. Using the integrated OSL signal, they fitted the equation for thermal quenching, with the parameters being W -0.61 __+0.02 eV and K -- 2.0(___1.2)• 107. Omitting the data points at 25 and 50~ the data were fitted again; the data used in this analysis are shown in Fig. 2.20. Wintle and Murray (2000) gave the values as W = 0 . 6 3 6 _ 0.013 eV and K - 3.40(___0.9)x 107. McKeever et al. (1997a) also carried out OSL measurements as a function of temperature and determined the parameters to be W - 0.60 eV and K -- 7.9 • 106. As part of a timeresolved luminescence study, Chithambo (2002) reported thermal quenching for quartz that had been heated above the phase-change temperatures. He calculated the thermal quenching energy to be W - 0.65 +-0.10eV, when using green (525 nm) LEDs for stimulation. This was in agreement with the value of 0.63 _ 0.07 obtained by Chithambo and Galloway (2001) when using blue (470 nm) LEDS. 5.1.9.2.
Thermal assistance
OSL can be stimulated at temperatures below or above room temperature, the range being limited by the apparatus used. Most studies have been in the temperature range from room temperature up to about 200~ (e.g., Murray and Wintle, 1998). At higher temperatures, particularly above 280~ the OSL trap is significantly thermally emptied (see Section 5.1.8.3). More importantly, the OSL signal is drastically reduced by thermal
0.8
8
o.6
d0
0.4
9
0.2 0.0 0
100
200
300
400
500
Stimulation Temperature, ~
Fig. 5.48. InitialOSL (first 0.4 s of decay curve, 9 and integrated signal (0-100 s, 9), normalisedto unity for values at 25~ plotted against stimulation temperature (from Murray and Wintle, 1998).
180
Optically Stimulated Luminescence Dosimetry
quenching, as discussed in Section 5.1.9.1. This can be seen for the OSL decay curves in Fig. 5.4 that have been normalised by a short OSL exposure at 20~ prior to the elevated temperature measurements. Besides the effect of the 110~ TL trap (discussed in Sections 5.1.2.2 and 5.1.2.6), the initial decay of the OSL is also more rapid at higher temperatures. This can also be seen in the normalised data set of McKeever et al. (1997a) reproduced in Fig. 5.3. This change in initial decay rate is the result of thermal dependence of the process of photoeviction (Spooner, 1994a). Spooner (1994a) used a cryostat to examine the temperature dependence from about 100 to 473 K ( - 173 to 200~ Over this temperature region, he measured the OSL signal (in the UV) using several different stimulation wavelengths (Fig. 5.23). Using these data he was able to determine the thermal activation energy (Eth) for each wavelength used, and the dependence of thermal assistance energy on stimulation photon energy is given in Fig. 5.49. A similar analysis was reported by Huntley et al. (1996). These results are important for understanding the optical stimulation mechanism (see Section 5.1.11). Besides using monochromatic light in such experiments, it is also possible to obtain the thermal activation energy for a mixed blue and green light source, as used in dating. Using the initial part of the OSL signal, and considering the thermal quenching, Murray and Wintle (1998) obtained a thermal activation energy (Eth) of about 0.045 eV. This value is consistent with an effective optical stimulation energy of 2.65 eV (468 nm), obtained by projecting the data given in Fig. 5.49, or using the calculation of Huntley et al. (1996). Chithambo (2002) reported activation energies for thermal assistance to be 0.06 +-- 0.01 eV for several samples of thermally annealed quartz. 5.1.10. The slow component
As mentioned in Section 5.1.2.4, there is a component of the OSL signal that remains after the initial part of the OSL decay curve has been removed by light exposure. This slow 0.40
.
.
.
.
.
.
.
'"
"
I
"
I
9
~" 0.35 >"
0.30
~) 0 . 2 5 -
0.20 "N
~"~
0.15
"~ 0.10
0.05 1~ i-0.00
1.6
' 1.7
.........
' 1.8
"
Optical
= 1.9
....... ' 2.0
Stimulation
"
' 2.1
photon
"
' 2.2 energy
"
' 2.3
"
' 2.4
"
~ 2.5
"
2.6
(eV)
Fig. 5.49. Thermalassistanceenergyas a function of stimulationenergy obtainedfrom data in Fig. 5.23 (from Spooner, 1994a).
181
OSL Properties of Natural Materials
component underlies the main signal and usually contributes only a few percent to the initial part of the main decay curve. It is best observed using a stimulation temperature that keeps the 110~ trap empty, e.g., 160~ (Bailey, 2000a,b). After 100 s of exposure to the blue and green ( 4 2 0 - 5 6 0 nm, 16 m W / c m a) light from the filtered halogen lamp in a Rise TL/OSL reader, the remaining OSL signal is the slow component. It is usually measured with stimulation taking place at 160~ or even 250~ (Bailey, 2000b; Singarayer et al.,
2000). As shown in Sections 5.1.3.1 and 5.1.3.2, the slow component is also seen when the power to the light source, usually blue (Bulur et al., 2000; Poolton et al., 2000) or green (Kuhns et al., 2000) LEDs, is increased linearly with time during stimulation. Chithambo and Galloway (2001) investigated the time-resolved luminescence of the slow component in their quartz sample. They found the lifetime to be strongly dependent upon stimulation temperature, with values remaining constant at 36 ___ 2 txs from 20 to 125~ but decreasing to only 8 txs by 200~ 5.1.10.1. Thermal stability A significant difference between the slow component and the rapidly bleachable components (fast and medium together) relates to the thermal stability. The rapidly bleachable component relates to a trap that empties at around 325~ thus it is removed by heating to 400~ However, irradiated quartz that has been heated to 500~ shows a slowly-decaying OSL component (Fig. 5.50). Further information on the thermal stability can be found by taking irradiated samples to progressively higher temperatures in an attempt to thermally erode the slow component. Bailey (2000b) used one aliquot of a sample to measure the effect of incremental temperature increases. These showed an increase in the slow component of the OSL as the temperature was raised from 400 to 500~ and then a decrease as the temperature is raised further to 700~ (Fig. 5.51).
100000
~,
0
10000
1000
. . . . . . . . t (s) 100
............................
I 0('~)
1000()
1 hr
,
,
|
10
100
1000
10hr
1 ........ |
10000
,1 ......
100000
Illumination time (s)
Fig. 5.50. Slowcomponent OSL for sample of modem sand given a 50 Gy beta dose and then heated to 500~ The main measurement was made with a 514.4 nm laser line; the inset measurement was made using blue and green light from an incandescent lamp. Both stimulations were with the sample at 160~ (from Bailey, 2000b).
Optically Stimulated Luminescence Dosimetry
182
25 OSLR measurement procedure
~ / ~
I100sos.,250~
/
~
o
20 .~
\,,'
15 ~,
leat to T, hold 10sI 1 0 0 s1 6O0S~L ,
10 " |
9O.....'
200
I
~
I
300
400
O.. ~ ~
I
500
'
I
J
600
I
700
Activation temperature, T (~ Fig. 5.51. Effect of pre-heat temperature on the initial level of slow component OSL, measured using sequence shown in inset. Thermal activation characteristics of the 110~ TL peak as measured using the response to a 0.5 Gy test dose (from Bailey, 2000b).
The 110~ TL peak response was monitored during this experiment, but its thermal response is clearly not the same as that of the slow component. Singarayer et al. (2000) used a multiple aliquot measurement procedure for other samples. They found similar behaviour (Fig. 5.52), but greater signal at 300~ a behaviour that they attribute to "recuperation"--transfer of charge during pre-heating (Bailey, 2000b). The mechanism giving rise to the peak in the slow component OSL between 550 and 600~ is not known.
1.4 1.2
#
TQG
---
SAQ1
-"
OJ2 SL205
0.8 0.6
"
0.4
"
0.2
-
,
!
!
|
"~
300
400
500
600
700
Pre-heat Temperature (~ Fig. 5.52. Effect of pre-heat temperature on the initial level of slow component OSL measured using separate aliquots on different samples obtained by pulse annealing (from Singarayer et al., 2000).
OSL Properties of Natural Materials
183
Interpretation is complicated by the likely opposing mechanisms of sensitisation and thermal erosion. However, Singarayer et al. (2000) used these data to choose a pre-heat temperature of 450-500~ for thermal removal of the rapidly bleachable component. 5.1.10.2.
Growth curve
Another significant difference between the slow component and the rapidly bleachable component is the far higher saturation level found for the slow component. The higher saturation level is demonstrated by the data of Singarayer et al. (2000) shown in Fig. 5.53, where the response for the slow component obtained with a single-aliquot additive-dose
25000
(a)
0
20000 0
G~ 15000
0
9
10000
.=
5000 ,
D e = 377_+23Gy D o = 2790Gy
r" -500
u
!
|
5OO
1500
Added dose (Gy) 3.5
~(b)
v
2.5 2 9 1.5 9
1 0.5
De = = 278_+37Gy D o = 88_+3Gy w
0
I
200
|
400
600
Dose (Gy) Growth curves for a sample of quartz. (a) Single-aliquot additive dose growth curve for the slow component OSL, and (b) single-aliquot regenerative dose (SAR) growth curve for initial part of the fast component of the OSL. Both are fitted with saturating exponential curves (from Singarayer et al., 2000).
Fig. 5.53.
184
Optically Stimulated Luminescence Dosimetry
procedure is compared with that using the SAR procedure for the rapidly bleaching component. The natural signal for the latter is effectively indistinguishable from the measured OSL saturation level. Both data sets were fitted to a saturating exponential curve, I = I 0 ( 1 - exp(-D/Do)). The values of Do obtained for this sample were 88 ___ 3 Gy for the fast component and 2790 Gy for the slow component. Although these results look promising for the use of slow component, Singarayer et al. (2000) also reported more complex behaviour with added dose for a modem sample. For these samples, the OSL signal was found to decrease with repeated pre-heat/stimulation cycles. This behaviour violates one of the basic assumptions of the additive dose measurement protocol. However, Singarayer et al. (2000) devised a measurement procedure that appears to permit correction for this decay, which has the added complication that the extent of decrease is dose-dependent.
5.1.10.3. Optical bleaching An additional problem with the use of the slow component for dating sedimentary quartz is the implied extra time that would be required to zero the signal at deposition. For experiments using white light from a solar simulator, Singarayer et al. (2000) showed the bleaching of the slow component to be sample dependent, with times between 17 h and 1 week needed to reduce signals to a negligible level. This would limit the applicability of this signal for dating sediments.
5.1.10.4. TRL Chithambo and Galloway (2001) used pulsed blue (470 nm) LEDs to observe the TRL from the slow component of their quartz sample, which had been given a laboratory irradiation and then bleached for 150 s with the diodes in order to remove the fast and medium components. Both thermal quenching and thermal assistance were observed (see Section 5.1.9 for equivalent data for the fast component). First, the thermal quenching was observed directly from the luminescence intensity and the parameters were similar to those for the fast component, W = 0.68 ___0.11 eV and K -- 2 x 107. Then the luminescence lifetimes were measured as a function of temperature and were found to decrease from 36 to 7.8 Ixs as the temperature was increased from 100 to 200~ and the values of W and K derived from these data were consistent with those for the direct measurements.
5.1.11. Photoionisation cross-section One of the most fundamental parameters that relates to OSL is the photoionisation cross-section, o-. It is a function of stimulation wavelength and thus can be obtained experimentally only for near-monochromatic stimulation (see Chapter 2 for discussions of the various procedures to measure o-; Bailey, 2002). Huntley et al. (1996) derived an equation that equated the excitation cross-section, o-, to the ratio So/Io where So is the initial slope of the CW-OSL decay curve and I0 is the initial luminescence intensity. These values were obtained for an Australian sedimentary quartz using seven laser stimulation lines, with energies from 1.92 to 2.71 eV (646-458 nm, respectively). The ratio So/Io is plotted as a function of stimulation energy in Fig. 5.54 (~). The values for the vertical axis range from 10 . 2 0 to 10 -17 cm 2 for energies of 1.92 and 2.71 eV, respectively. Huntley et al. (1996) point out that their value of 2.71 eV is
185
OSL Properties of Natural Materials 10-10 r
o o r
10-17
Q.
E 10 -11
tO .,i,..,
(9 "O
o
.J~
O c-
10-18
i_
(9 Q.
N
10 -12
E
r"
0
0
0 tO
10-19 % ~0
0
x= 10-la __o
1.8
210
212 214 216
10 -2o 2.8
Photon energy (eV) Fig. 5.54. Initial luminescence intensity ( 9 ) Io and relative slope, So/Io (G) as a function of incident photon energy (redrawn from Huntley et al., 1996).
close to the value calculated as an approximation for the cross-section in studies of the photoelectric effect. By combining simple equations to describe both LM-OSL and CW-OSL, Kuhns et al. (2000) calculated the photoionisation cross-section for the three components they found in the stimulation of an aeolian quartz by green LEDs (526 nm). The fitting of three first order components to both LM-OSL and CW-OSL data sets is shown in Fig. 5.55. For the fast component (shown as component 1 in each figure), o - w a s calculated to be 1.48 X 10 -18 cm 2. This is the value for 526 nm (2.36 eV) and it can be found to be similar to the value of 3 x 10-18 cm 2 from Fig. 5.54. Larsen et al. (2000) observed the LM-OSL from a glaciofluvial quartz that had been irradiated, illuminated and heated to 550~ in the laboratory. This treatment was repeated until the OSL sensitivity and the CW-OSL curve shape were constant. The sample was then given a dose of 25 Gy and a pre-heat of 10 s at 220~ The LM-OSL was observed at 160~ for stimulation with blue LEDs (470 nm). The most rapidly bleached component was determined to have a photoionisation cross-section of 9.0 x 10-17 cm 2. This value is similar to the value of 2 • 10 -17 cm 2 from Fig. 5.54 for stimulation with a photon of energy 2.64 eV. Using pseudo-LM-OSL plots (Section 5.1.3.1) to separate the fast and medium components, Singarayer and Bailey (2002) observed the depletion of the blue (470 nm) stimulated luminescence when samples were exposed to 430, 470, 500, 525 and 590 nm (Fig. 5.17). From these plots, they calculated the photoionisation cross-section for each component as a function of bleaching wavelength (Fig. 5.56a) . The ratio of the values
186
Optically Stimulated Luminescence Dosimetry 1800 (a)
~-. 1600 "~
1400 1200 1000 800
~"
600
O
400
200 0 200
400
600
800
1000
Time (s)
100000
(b)
Sum C
10000
0 0
1000 0
10
20
30
40
50
Time (s) Fig. 5.55. (a) LM-OSL, and (b) CW-OSL data for a dune sample, showing how each signal has been split into three first-order components (from Kuhns et al., 2000).
changes radically with photon energy (Fig. 5.56b). This finding led Singarayer and Bailey (2002) to propose selective removal of the fast component by IR stimulation. For their sample, IR exposure times of ---8000 s (830 nm, 1 W) with the sample held at 160~ resulted in the complete removal of the fast component, whilst the medium component remained untouched. 5.1.12. Modelling processes giving rise to OSL in quartz McKeever et al. (1997a) developed a model for OSL production (discussed in Section 2.4.4), and included a discussion of its applicability to quartz. This model was used by McKeever et al. (1997b) to model OSL sensitivity changes during single-aliquot procedures. The large volume of experimental data published on quartz OSL in the following years, led Bailey (2001) to formulate a model specifically for this material. To encompass the width of information on OSL and TL, Bailey (2001) proposed that there
187
OSL Properties of Natural Materials 1E-16
(a)
1E-17 ~lff
A IN
1E-18
l
1E-19
/ i~!
1E-20
-'
~1~ "'" ~ l q . . . . . . . ' ~ ,
f
,," ,~"
E
t~
"
,
%
.' ," '
9Fast 9Medium
lil9
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. . . .
I
2 E ~9
30
E b
20 10
"O G)
. . . .
2.5
',
. . . .
3
3.5
99. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.I
.................................
il 2.2
2.4
2.6
2.8
3
3.2
3.4
P h o t o n E n e r g y (eV) Fig. 5.56. (a) Photoionisation cross-section as a function of stimulation energy for the fast and medium components of quartz when exposed to light at room temperature; (b) ratio of data from (a) also as a function of stimulation energy (from Singarayer and Bailey, 2002) 9
should be five electron trapping centres and four recombination centres. The electron traps were those for the 110~ TL peak, the 230~ TL peak, the fast and medium components of the OSL and an additional deep trap. Electrons in the latter and the 230~ TL trap were not able to be optically stimulated. Two of the recombination centres are thermally unstable and non-radiative. In contrast, the third recombination centre is thermally stable and radiative. The fourth centre is thermally stable, but non-radiative. Experimental data were used to constrain the model parameters and include behaviour such as thermal quenching. Subsequently, Bailey (2002) incorporated three optically active electron trapping centres that give rise to the slow components observed experimentally (Bailey, 2000b; Singarayer and Bailey, 2002). The new model also includes experimental values for the optical photoionisation cross-section (o-) (Section 5.1.11) and the thermal assistance energy (Eth) (Section 5.1.9.2). Banerjee et al. (2002) have taken Bailey's (2001) model and used it to test the SAR protocol. They used a modelled pre-heat of 220~ for 10 s, OSL measurement at 125~ a test dose of 0.5 Gy with a cut heat to 170~ and four regeneration doses to construct the sensitivity-corrected OSL growth curve. They recovered a dose of 2.04 Gy, 2% higher than the stimulated environmental dose of 2.00 Gy. This level of accuracy was achieved despite a 58% increase in the observed OSL sensitivity during the four regeneration dose cycles. The model was also used to investigate the dependence of the OSL growth curve on dose rate. Below the saturation level (attained by ---500 Gy), the
188
Optically Stimulated Luminescence Dosimetry
sensitivity-corrected OSL signal was independent of dose rates that were varied by two orders of magnitude.
5.1.13. Summary Quartz is the most extensively studied mineral, owing to it being found in materials that require dating (such as pottery and sediments) and those that could act as dosimeters in the case of a nuclear accident. Its OSL characteristics make it suitable for dating sedimentary material that is up to 150,000 years old, with equivalent doses of around 100 Gy. At the same time, it has been used to obtain doses of about 10 mGy for recently fired materials. Both applications have been achieved using the SAR measurement procedure developed on fundamental experimental studies, as summarised by Wintle and Murray (2000) and given in more detail in Chapter 6.
5.2. Feldspars 5.2.1. Crystal structure Feldspars are aluminosilicates made up of A104 and SiO4 tetrahedral units, with the oxygen atoms being shared between adjacent tetrahedra. This structure allows chargecompensating cations (e.g., K +, Na + and Ca 2+) to be accommodated within the tetrahedral framework and gives rise to a range of feldspars with different chemical compositions. In addition, there are many elements than can substitute for Si or A1, as well as for K, Na and Ca, and these affect the luminescence emission. For the purposes of luminescence dating, naturally formed feldspars are described in terms of being either plagioclase or alkali depending upon their chemical composition. These two types have different density ranges and a degree of separation can be achieved when a mixture of grains is introduced into a heavy liquid made up to an appropriate density (2.58 g/cm3). The lighter potassium-rich (alkali) feldspars, such as orthoclase (KA1Si308), will float and thus be separated from the plagioclase feldspars that form a series with varying amounts of sodium and calcium. The end members of the plagioclase series are sodium-rich albite (NaA1Si308) and calcium-rich anorthite (CaA12Si208). This classification of feldspars is based on chemical composition, but feldspars can also be classified according to their structure. The degree of ordering of the A1 and Si atoms in the crystal lattice depends upon their mode of formation. Feldspars formed at high temperatures tend to have a disordered structure, with sanidine being an example. Sanidine is a highpotassium content feldspar found in volcanic rocks that cool rapidly from temperatures in excess of 1000~ Slower cooling, e.g., of a granite body, will result in a more ordered structure, resulting in other high-potassium minerals such as orthoclase or microcline. Similarly, sodium-rich feldspar will form a range of albite structures depending upon the cooling rates. In addition, cooling slowly will cause mixed crystals to be formed, with alternating zones of potassium-rich and sodium-rich feldspars; these are known as perthites. The zoning usually occurs at a scale that is too small to affect dosimetry. However, perthitic
OSL Properties of Natural Materials
189
feldspars may be expected to have mixed luminescence properties. Also, plagioclase feldspars may be found with inter-grown anorthite and albite layers. Further information on classification of feldspars can be found in the literature (e.g., Deer et al., 1992). Duller (1997) and Krbetschek et al. (1997) discuss luminescence properties related to structure. 5.2.2. Decay curve shape obtained under continuous stimulationmCW-OSL and CW-IRSL
5.2.2.1. Stimulation sources Photostimulation of feldspars can be achieved using both visible and infra-red wavelengths (see Section 5.2.5). The first OSL signals from feldspars were obtained with 514 nm light from a laser (Huntley et al., 1985), and the first IRSL signals were obtained using selected wavelengths from a xenon lamp (Htitt et al., 1988). Routinely, IRSL stimulation is now achieved using IR LEDs. Numerous continuous wave (CW) IRSL decay curves have been published and none are found to be exponential. Bailiff and Poolton (1991) fitted their IRSL decay curves with power functions of the form (1 + Bt)-1 or (1 + Bt) -2, where B is a constant that depends upon the initial charge population. These result in decay curves that do not approach zero in the same way as OSL from quartz (Section 5.1.2). Bailiff and Barnett (1994) used a more general function of the form (1 + Bt) -P, where 1 < P < 2. These could be used down to 10% of the initial signal intensity. For the development of single-aliquot dating techniques for feldspars (see Section 6.11.2.1), it is necessary to characterise the first few percent of the decay. Galloway (2000) measured the decay in IRSL brought about by 10 successive IR stimulations; after 10 measurements the signal was reduced to no less than 85% of the initial value. When expressed as a function of total IR stimulation time, the data were fitted by a straight line; the rate of this initial decay was shown to depend upon the temperature and duration of the previously applied pre-heat. The decay curve shape obtained with visible stimulation is also not exponential; the shape of the CW-OSL can be used to distinguish between quartz and feldspar grains when they are stimulated at 532 nm in single-grain OSL systems (Section 7.7.3).
5.2.2.2. Effect of stimulation temperature 5.2.2.2.1. Initial part of signal. Using the initial part of the signal, it is possible to follow the IRSL signal as a function of temperature. Fig. 5.57a,b shows the luminescence from repeated 0.1 s IR stimulation of potassium-rich feldspar samples when they are heated from room temperature up to 500~ The TL signal is recorded between the IRSL measurements, and the net IRSL can thus be obtained as the difference between the two data sets (Duller and BCtter-Jensen, 1993). This type of measurement, called thermooptical luminescence (TOL), by Duller and Winfle (1991) was first proposed by Htitt et al. (1988) who used the data to support a mechanism for IRSL production. The data in Fig. 5.57 were obtained for violet emission (340-440 nm), but later studies by Rieser et al. (1997) using other K feldspars showed different behaviours for different emission wavelengths (e.g., 410 nm versus 560 nm).
190
Optically Stimulated Luminescence Dosimetry
x 103 400
(a)
300tO
ot"- 2000 (/)
.c_ E := 100_.1
0~,-
50
150
250
350
450
Temperature (~
X 10 3
700
Boot
(b)
-~g500t o~ 4001 ,--=~ 300] g
~: 200 ._J
100 ~
50
i
150
|
2~i0
|
350
450
Temperature(~ Fig. 5.57. TOL measurements of TL and IRSL (for 0.1 s every 10~ whenheating at 10~ for sedimentaryKfeldspar (a) naturally irradiated, (b) given an additional dose of 18 Gy (from Duller and BCtter-Jensen, 1993).
These properties have practical implications for dating. The rise in IRSL with temperature up to about 220~ could be used to enhance the magnitude of the signal during dating procedures. However, it should be noted that the emission spectra show small peak shifts when IRSL measurements are performed at elevated temperatures (Duller and BCtter-Jensen, 1997; see Section 5.2.6.1). More importantly, Poolton et al. (2002b) showed
191
OSL Properties of Natural Materials
both theoretically and experimentally that using an elevated measurement temperature is inappropriate. They determined D e for a sample of K-feldspar expected to have received a dose of 0.5 Gy since its documented deposition 300 years ago. Values of D e obtained with the SAR protocol (see Section 6.11.2.1.2) are shown in Fig. 5.58. The increasing values of D e with stimulation temperature were speculated to be caused by thermally induced accessing of recombination centres that had not been bleached at deposition. Poolton et al. (2002b) concluded that the most accurate evaluation of dose would result from IRSL measurements made at the temperature at which bleaching had occurred in nature (e.g., 10~ for the sample from the Netherlands in Fig. 5.58). This is in contrast to the widespread use of 50~ as the stimulation temperature in single-aliquot procedures to ensure fixed temperature stimulation in sequences that use pre-heating after irradiation. The varying dependence of IRSL on temperature for different feldspars has been proposed as a method of distinguishing between different feldspar types, e.g., microcline and orthoclase (Krbetschek et al., 1997). Fig. 5.59 shows measurements of blue and yellow emission as a function of temperature for a sample of microcline (Duller, 1997). The rapid drop of the 560 nm (yellow/green) IRSL near 100~ was not due to the emptying of the electron traps, as can be shown by pulse annealing experiments (Duller, pers. comm.). Duller and BCtter-Jensen (1993) carried out TOL measurements with IR (875A80 nm) and blue/green light (420-550 nm) stimulation of a potassium-rich feldspar extract (Figs. 5.57 and 5.60). For the natural samples, both the IRSL and OSL signals increase with temperature up to about 250~ (Fig. 5.57a and Fig. 5.60a) and the authors suggested that both processes involve a thermally assisted transition. BCtter-Jensen (2000) plotted the two signals against each other and found that they increased in proportion between 50 and 200~ (Fig. 5.61). The OSL signal for the laboratory-irradiated grains increases very little with temperature (Fig. 5.60b), suggesting that the OSL signal is affected by the presence or absence of charge in the low-temperature traps. This is in direct contrast to the IRSL signal
"
l l l l l l , l t l l ' l l l ' l l ' l l ' l l
~
"
v
i
a0 fID >
--
9
LU
9
..
+§
-
Expected Dose Level
ii.
0
; . ..1
, I , I I , l l l l , , , , I , J , , l l , , ,
0
50
100
150
200
250
Temperature (~
Fig. 5.58. Equivalentdose for a sedimentaryK-feldspar measured as a function of the stimulation temperature for IRSL used in the SAR protocol. The expected dose level is based on the historical age of the specimen (300 ___20 yr) and the measured dose rate (from Poolton et al., 2002b).
Optically Stimulated Luminescence Dosimetry
192 (a)
25000-
Blue Emission
CO
o
9
20000-
~
_o.OD-D E
15000
I~
0
o (-
10000
.o~
E -'s
~
/
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TL IRSL
~_ "o
'
/
0 O) (D
.~
..... [ ] ......
\~
O.
5000 t].
...1
.....
I
I
100
i
200
i
300
400
-
-
Temperature (~ (b) 4000 -
Yellow Emission I~'q..
"r"-" 0 O.
3000
0 C (D 0 (D C
:
o
.
.
.
.
.
.
[ ] ......
TL IRSL
b
(0 0 v
:9
-
2000
0
1000
E __I
100
200
300
400
Temperature (~
Fig. 5.59. TOL measurements of TL and IRSL (for 0.1 s every 10~ when heating at 4~ for a microcline sample given a 2.2 kGy dose 2 years previously (a) obtained using blue luminescence and (b) using yellow emission (from Duller, 1997).
for laboratory-irradiated grains (Fig. 5.57b), which shows the same increase with temperature as seen following natural irradiation (Fig. 5.57a). Since the temperature dependence of the OSL signal is affected by the degree of shallow trap filling, whereas the IRSL signal is not, this indicates that the OSL process involves transport via the conduction band, as opposed to a localised transition model for IRSL. The temperature dependencies in each case are, therefore, dominated by different phenomena, viz., hopping transportation for IRSL (Poolton et al., 2002a) and trapping/de-trapping in shallow traps
193
OSL Properties of Natural Materials
xlO 3 140
(a)
1207u~ t,--
lOO-
::3 0
.9..o 80o
t.--
m
6060-
.~_ E
4040
ffl
._1
20; .~
OT' 50
150
250
350
4~i0
Temperature (~
x 103 400
(b)
300t.0 o o r
200-
escence
0
E _J
100-
0
50
150
250
350
450
Temperature (~
Fig. 5.60. TOL measurements of TL and OSL (for 0.1 s every 10~ when heating at 10~ for sedimentary K-feldspar with optical stimulation at 420-550 nm: (a) naturally irradiated, (b) given an additional dose of 18 Gy (from Duller and BCtter-Jensen, 1993).
for OSL. Temperature dependencies of this type were discussed in Section 2.4.5 and illustrated in Fig. 2.14. This behaviour also contrasts with that obtained for quartz w h e n stimulated with blue/green light ( 4 2 0 - 5 6 0 nm) (Duller et al., 1995). For quartz, the OSL decreases
Optically Stimulated Luminescence Dosimetry
194 70
60
.~
50 o x
40
o _J
30
v
O
~ e 250~ 200~
S 150~ f O l 0~ 50oc
20 10
|
50
|
1O0
,
|
|
|
150
200
250
300
350
IRSL (cps x 103) Fig. 5.61. OSL versus IRSL for the natural signals from 50 to 250~ using data from Figs. 5.57 and 5.60 (from BCtter-Jensen, 2000).
monotonically as it is heated from room temperature to 250~ the consequence of thermal quenching (see Section 5.1.9.1). Although, the data of Duller et al. (1995) were obtained for quartz and feldspar grains that had similar OSL signal intensities at 50~ the changes in OSL output as the samples are heated could be used to provide information on the relative proportions of quartz and feldspar present in the mixture. The lack of an OSL signal from quartz when measured at stimulation temperatures above 250~ implies that OSL measured above 250~ is likely to be dominated by the signal from feldspars. 5.2.2.2.2. Decay curve shape. McKeever et al. (1997a) measured both IRSL and OSL decay curves for feldspars for stimulation carried out over a range of temperatures. When plotting normalised IRSL decay curves, it can be seen that the initial part of the curve became steeper with increasing stimulation temperature (Fig. 5.62a). Similar data were presented by Poolton et al. (2002b). For the equivalent OSL measurements McKeever et al. (1997a) found that there was no change in the decay curve shape (Fig. 5.62b) and concluded that unlike IRSL, OSL production in feldspars does not involve a process of thermal activation. However, the absolute signal increases with temperature, as can be seen for the integrated OSL signals (inset to Fig. 5.62b). This increase corresponds to the increase seen in the TOL measurements using blue/green light stimulation (Fig. 5.60).
5.2.3. Linear modulation IRSL Linear modulation IRSL (LM-IRSL) of irradiated feldspar grains from a heated sediment was first observed by Bulur (1996) and this study was followed by the presentation of LM-IRSL data for both potassium- and sodium-rich feldspars (Bulur and
195
OSL Properties of Natural Materials 1.0
I
I
I
I
(a) 0.8
..J
co O 0.6
50~
"O N
100~
E ,.- 0.4 o z 0.2
o.o
I
I
I
I
20
40
60
80
100
Time (s) 1.0
0.8 u~
e-
'
&
.E 0.6 -
22s~
&y 2,0000 \'k\\,,,,,.,,.. /
O3 "10 I9
.N_ 0.4
" ~ , ~ /
30
'
~. I-,," 1 - o;- ,oo ,;o'
50 oc
...,..\%-L~__/
...i
o
'
o Temperature ( C)
l"rs~
-
7s~
t~
E t_ O
z
0.2
0.0
.... 0
I
i
I
I
5
10
15
20
Time (s) Fig. 5.62. (a) IRSL decay curves obtained for IR (880 nm) stimulation at temperatures from 50 to 200~ for a feldspar sample given 83.3 Gy and then pre-heated at 220~ for 10 s. (b) OSL decay curves obtained for green light stimulation at temperatures from 50 to 225~ for a feldspar sample given 62 Gy and then pre-heated at 220~ for 10 s. Both sets of curves are normalised to the initial intensity (from McKeever et al., 1997a). Inset in (b) shows the integrated (0-20 s) OSL intensity as a function of temperature.
196
Optically Stimulated Luminescence Dosimetry
G6ksu, 1999). The LM-IRSL curves were obtained for grains with sensitivities that had been stabilised by repeated heating and irradiation, and given a laboratory irradiation and then heated to 200~ for 5 min to remove thermally unstable components. The LM-IRSL curves for sodium and potassium feldspars appear similar and both result in three firstorder peaks when standard curve fitting procedures were applied (Fig. 5.63). However, the peak maximum for the LM-IRSL from the sodium feldspar occurs at 76 s, compared with
=.,
6
_
..
(a)
K-Feldspar o Measurement Y
~
~4
-
S
-
S 1
......
S 2
. . . .
S 3
'
'
+S2+S
3
8 3 82 g. 1
,
s
J t// ~ "
9
\,
"'..._
0 I
0
....i -
,
'
' .... -I--
100
'
i-
,
,
I
200
'
'
'
I
300
~
'
'
400
Time (seconds) 12 (b)
.._.,10-
__
Na-Feldspar
r
o
"N
Measurement
--s,,s~+s~
5
. . . . .
S I
......
S 2
. . . .
S 3
==6
8, g2 0
I
0
'
'
'
'
I
1O0
'
'
~
'
I ' ""'
200
'" -'
'
I
300
'
'
'
v'
400
Time (seconds) Fig. 5.63. LM-IRSLcurves obtained with linearly increasing excitation energy over 400 s. (a) K-feldspar and (b) Na-feldspar. Also shown are the three components deduced by curve fitting (from Bulur and G6ksu, 1999).
OSL Properties of Natural Materials
197
91 s for the potassium feldspar. This implies that the IRSL of the sodium feldspar decays more rapidly, possibly due to a larger photoionisation cross-section at this wavelength. Bulur and G6ksu (1999) determined the dose response curves for the three components of each sample. The position of the peaks did not move as increasing doses up to 180 Gy were given, indicating that each component follows first-order kinetic behaviour. Over this dose range, each component of the sodium feldspar was more linear than that for the potassium feldspar. Bulur and G6ksu (1999) also measured the LM-IRSL at a variety of elevated temperatures, from 30 to 150~ and obtained thermal activation energies for each of the three peaks (see Section 5.2.9.2). 5.2.4. Pulsed OSL and IRSL
5.2.4.1. Pulsed OSL Sanderson and Clark (1994) obtained time-resolved spectra (with stimulation at 470 nm) for a potassium feldspar standard from the International Atomic Energy Agency and a feldspar from a sample of volcanic lava. They observed peaks in the spectrum, occurring at a few hundred nanoseconds, about a microsecond and several tens of microseconds. 5.2.4.2. Pulsed IRSL Clark et al. (1997) selected 850 nm stimulation light from a tuneable IR laser and observed the luminescence emitted after the end of the pulse. These time-resolved measurements were made at temperatures ranging from 25 to 100~ and the luminescence was observed at several different emission wavelengths selected using colour glass filters. No peaks were observed, but multi-exponential non-linear regression analysis suggested the presence of up to five exponentially decaying components with different lifetimes, ranging from 30 ns to more than 15 txs. This study was extended by Clark and Bailiff (1998) who used a set of bandpass interference filters to obtain greater resolution for the CW-IRSL emission spectrum, before measuring the time-resolved luminescence emitted at 550A40 nm. The 560 nm emission has previously been linked to electronic transitions in Mn 2+. However, intrinsic Mn 2+ transitions are known to have lifetimes --~ms these are long compared with the lifetimes of 22 ns up to 164 l~s found when a dwell time of 5 ns per channel was used in the time-resolved IRSL measurements (Fig. 5.64). 5.2.4.3. Optically stimulated afterglow OSA is the luminescence observed following a pulse of photons and is caused by retrapping at, and subsequent release from, shallow traps. Htitt et al. (1999, 2001) and Jaek et al. (1999) used a xenon lamp and monochromator to obtain selected wavelengths from 250 to 900 nm for stimulation. A shutter system was employed to obtain a pulse of 2 s. The OSA is observed at least 0.05 s after the stimulation pulse ends using a near-UV (--- 330 nm) filter. For measurements made on a Cu-doped single crystal of microcline, an emission peak was found in the near IR around 880 nm. An increasing OSA signal strength was observed as the wavelengths were varied from 750 to 360 nm (Fig. 5.65) and a stimulation peak was seen near 360 nm. The OSA measurements enable the stimulation response to be observed in the region of the spectrum where the luminescence is being detected. These data can be compared with the bleaching response spectrum for IRSL
Optically Stimulated Luminescence Dosimetry
198
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15
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nm
Fig. 5.65. OSL and OSA stimulation spectra obtained for a microcline crystal showing an IR stimulation peak at 880 nm and an OSA stimulation peak at 360 nm (redrawn from Hiitt et al., 2001).
OSL Properties of Natural Materials
199
(and OSL) (see Section 5.2.5.2) and the direct measurements of the IRSL excitation spectra (see Section 5.2.5.1). 5.2.5. Excitation spectra 5.2.5.1. Direct m e a s u r e m e n t s
In the visible part of the spectrum, the stimulation characteristics are similar to those of quartz, with the OSL signal intensity increasing rapidly as the energy of stimulation is increased. Htitt et al. (1988) and Godfrey-Smith et al. (1988) showed that luminescence could be stimulated from feldspars using wavelengths in the visible and in the near-infrared. Using a variety of laser lines, Ditlefsen and Huntley (1994) observed that the luminescence response of a K-feldspar extract from a sediment was not a linear function of stimulation energy, when the signal intensity was plotted on a log scale. They concluded that two traps contributed to the OSL signal. Htitt et al. (1999) obtained the OSL stimulation spectra for potassium-rich feldspars extracted from sediments and single crystals of microcline. In order to improve detection efficiency, the luminescence sensitivity was increased by doping with activators such as T1 (Jaek et al., 1997a) or Cu (Jaek et al., 1997b). Electrodiffusion, particularly of Cu, can be carried out at relatively low temperatures (--- 550~ and does not appear to alter the crystal structure. In Fig. 5.65, the OSL observed using a near-UV (---330 nm) filter is shown for stimulation from 1040 to 450 nm (Htitt et al., 2001). Bctter-Jensen et al. (1994b) obtained high-resolution stimulation spectra from museum specimens (Fig. 5.66a) and sedimentary feldspars (Fig. 5.66b) using a continuous scanning monochromator together with a tungsten halogen lamp (see Section 7.5.1). The luminescence was detected in the near UV (--~340 nm) as the stimulation wavelengths were varied from 400 to 1000 nm. In the visible region, the dominant feature is a steeply rising continuum, but between 450 and 650 nm there is evidence of stimulation peaks at 500 and 600 nm for the albite samples (Fig. 5.66a). Peaks at these wavelengths are also visible for the feldspar separates (Fig. 5.66b). Clark and Sanderson (1994) also used a scanning monochromator, together with a xenon lamp to study a selection of feldspars. They detected the luminescence in the wavelength region 300-400 nm. Excitation bands occurred at 500-540, 550-650 and 800-1000 nm; the relative strengths of these contributions varied amongst the 20 feldspars that were measured. Clark and Sanderson (1994) also measured the excitation spectra after the samples had been heated for 30 s at temperatures from 150 to 440~ in 50~ steps. The whole excitation spectrum was reduced progressively, but by 400~ only the excitation band at 500-540 nm remained. It is this component which was identified as having a measurable thermal activation energy (see Section 5.2.9.2). Both data sets show that besides the monotonic decrease in OSL with increasing wavelength, there is a peak for stimulation in the near infra-red (--~880 nm). Other studies, using a tuneable laser (Bailiff, 1993; Bailiff and Barnett, 1994; Barnett and Bailiff, 1997a), show a peak at about 855 nm (1.46 eV). Barnett and Bailiff (1997a) were able to show that this peak is well defined in a number of different types of feldspar, e.g., albite, oligoclase, orthoclase and sanidine. For some potassium-rich feldspars they also found a weaker
200
Optically Stimulated Luminescence Dosimetry
0.8 ~D 0.6 __J oo 0.4 O 0.2
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0 400
AMELIAALBITE ANORTHITE / / ~ ~ )
500
600 700 800 STIMULATION WAVELENGTH[nm}
900
(b)
/----PI.F
0,8
1000
d 0.6 -0.4 0 0.2 I..I..I oO
0 40O
500
600 700 800 STIMULATION WAVELENGTH(nm)
900
1000
Fig. 5.66. OSL stimulation spectra (corrected for power density at sample) observed with a Hoya U-340 filter; (a) museum specimens of the feldspars anorthite and albite, (iii) showing the difference between the measured spectrum for albite (i) and the continuum (ii) on an expanded scale; (b) sedimentary feldspar separates, with K.F* showing the difference between the measured spectrum for K.F and the continuum on an expanded scale (from BCtter-Jensen et al., 1994b).
stimulation peak at higher energies (---745-800 nm), whereas BCtter-Jensen et al. (1994) observed a stimulation peak in a sample of anorthite at 970 nm (Fig. 5.66a). These stimulation spectra have led to the use of IR LEDs emitting at 880A80 nm and a laser diode emitting at 830 nm for IRSL measurements in dating studies (see Sections 7.4.2 and 7.4.3). Stimulation in the near infra-red region enables detection of IRSL with a wide window in the visible region of the spectrum. However, using a filter with a broad transmission band, such as the Schott BG39 (see Fig. 7.2b), to observe the IRSL may lead to complicated responses since several emission bands will be observed (Rieser et al., 1997; see Section 5.2.6). A summary of published stimulation peaks and mineral types was authored by Krbetschek et al. (1997), together with the detection wavelength region used for the measurements. In a study of museum specimens of feldspars and feldspar-dominated sediment extracts, Godfrey-Smith and Cada (1996) found single stimulation peaks. The peaks occurred at 845 nm for all microclines, most plagioclases and all sedimentary feldspars and at slightly lower wavelengths (840 nm) for two nearly pure Na plagioclases. This led Godfrey-Smith and Cada (1996) to suggest the use of a semi-conductor diode laser tuned to 845 nm, instead of 880 nm LEDs for feldspar dating. This would provide increased photoeviction efficiency for microclines, feldspars that are less likely to be affected by anomalous fading (see Section 5.2.10).
OSL Properties of Natural Materials
201
Links between the IR stimulation peak and chemical composition were explored by Poolton et al. (1995a). They found that whilst sodium and potassium feldspars had similar stimulation spectra, calcium-containing feldspars exhibited a range of values for the stimulation peak wavelength. For low temperature, more ordered, plagioclase feldspars there was a direct correspondence between the OSL intensity and anorthite content.
5.2.5.2. Bleaching response spectrum Measurements of optical bleaching of the IRSL of four feldspar samples were carried out by Bailiff and Poolton (1991). They observed a resonance peak between 850 and 900 nm. Spooner (1994b) studied the effect of optical bleaching on the IRSL signal (stimulated at 880z~80 nm) using 28 interference filters to provide narrow wavelength bands ranging from 400.6A10.1 to 1065.3A15.0 nm. Fig. 5.67 shows, for a specimen of microcline, the energy required at each wavelength to reduce the IRSL signal by a given percentage. The IRSL was measured using a broad-band Schott BG39 filter. It can be seen that the amount of energy required to reduce the IRSL signal decreases rapidly with decreasing wavelength. In addition, a resonance can be seen at about 860 nm. 5.2.6. Emission spectra OSL emission spectra obtained from feldspars were published by Huntley et al. (1989) when stimulation was carried out using the 514 nm line from an argon laser and the 633 nm line from a H e - N e laser. Subsequently, IRSL spectra were obtained by Huntley et al. (1991) for IR diode stimulation (880A80 nm) and stimulation with the 647 nm line from a krypton laser. These authors reported that most feldspars showed emission bands at 330, 400 and 570 nm.
5.2.6.1. IRSL emission spectra BCtter-Jensen et al. (1994) used a small scanning monochromator to measure the IRSL emission spectrum from 370 to 640 nm for a museum specimen of albite and for feldspar extracts from sediments. They found two main emission bands centred at 400 and 550 nm, with the blue signal being dominant for the sedimentary feldspars. Krbetschek et al. (1996) reported emission bands at 280, 330, 410, 560 and 700 nm for feldspars from a number of museum specimens and sedimentary feldspars. Using IR as the optical stimulation source means that it is possible to observe the emission spectrum throughout the visible and near UV regions of the spectrum. Krbetschek et al. (1997) provided a comprehensive review of luminescence emission from feldspars, including TL and CL spectra as well as IRSL spectra. 5.2.6.1.1. 280-290 nm (near UV). The IRSL spectrum of alkali feldspars was studied by Clarke and Rendell (1997a), following a previous study of the TL and IRSL emission spectra for sedimentary feldspars (Rendell et al., 1995). Of particular interest are the results for the 280-290 nm emission, seen in laboratory-irradiated feldspars, but not in naturally irradiated feldspars, except for some from the low-temperature environment of Antarctica (Krause et al., 1997). The short thermal stability of this signal has been investigated by Clarke and Rendell (1997b). They found that the signal was removed by pre-heating at 220~ for 5 min after irradiation. Thus, it is removed by the typical pre-heat used in feldspar dating, and its effect can also be avoided by using a filter that does not
Optically Stimulated Luminescence Dosimetry
202
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800
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Wavelength (nm) Fig. 5.67. The bleaching energy required at various wavelengths to reduce the IRSL signal from the initial level to the percentages shown for a microcline sample (redrawn from Spooner, 1994b).
permit transmission of wavelengths from 280 to 290 nm. However, Clarke and Rendell (1997b) also showed evidence for changes in the magnitude of other luminescence signals as a result of application of this pre-heat.
5.2.6.1.2. 320-340nm (near UV). This emission is seen primarily in sodium-rich plagioclase and alkali feldspars, but not in calcium-rich feldspars (Krbetschek and Rieser, 1995; Krbetschek et al., 1997). However, using colour glass filters in dating studies, it is difficult to measure in this wavelength region, in isolation from the 280 and 390-440 nm emissions.
OSL Properties of Natural Materials
203
5.2.6.1.3. 3 9 0 - 4 4 0 nm (violet~blue). Emission in this range is found for all feldspars, with emission in the region 400-410 nm being the most common (Krbetschek and Rieser, 1995; Krbetschek et al., 1996). For potassium-rich feldspars, this spectral component is usually dominant for specimens from mineral collections (Huntley et al., 1991) and from sediments (Jungner and Huntley, 1991; Wiggenhom and Rieser, 1996). However, the latter was not found to be the case for the sediments examined by BCtter-Jensen et al. (1994). Krbetschek et al. (1997) suggested that additional peaks at 390 and 430 nm may be present, and that some combination of these three emission bands may relate to the signal observed when dating is carried out with violet/blue colour glass filters. Duller and BCtter-Jensen (1997) measured the emission spectrum as a function of stimulation temperature for IRSL stimulated at 880 nm. For the dominant emission peak at 400 nm (3.0 eV) they found a small, but consistent, shift of the peak emission energy to higher energies (from 2.992 to 3.015 eV) as the temperature was increased from 50 to 400~ They concluded that this small shift would have no effect on measurements of the thermal activation energy of the IRSL. 5.2.6.1.4. 5 5 0 - 5 7 0 nm (yellow/green). IRSL emission at 560 nm has been found in nearly all feldspars (Krbetschek et al., 1997). It will be observed, together with the violet/blue emission, when a filter such as the Schott BG-39 is used (Fig. 7.2b). IRSL emitted at 560 nm is bleached more quickly by daylight than the IRSL emitted at 410 nm (Krause et al., 1997). However, Krbetschek et al. (1996) provided evidence that the 560 nm IRSL signal was less thermally stable than that measured at 410 nm and may thus be unsuitable for dating samples over 10,000 years old. 5.2.6.1.5. 6 0 0 - 7 5 0 nm (red~far red). Krbetschek et al. (1996) reported IRSL in the red (690-750 nm), but could not obtain conclusive data because of interference from the IR diode stimulation source (880A80 nm). Poolton et al. (1995b) measured the OSL in the red (600-700nm) and found that it exhibited strong thermal quenching above room temperature (see Section 5.2.9.1). Studies by Fattahi (2001) suggest that it is possible to select detection filters and a photomultiplier tube to enhance detection of the far-red signal from natural feldspars. The advantage of this IRSL signal for dosimetry is that it appears not to exhibit anomalous fading, unlike the IRSL signals observed at other emission wavelengths. 5.2.6.2. TL emission spectra There have been many studies of TL spectra for feldspars from mineral collections, as well as for feldspars separated from sediments and volcanic rocks (e.g., Zink et al., 1995). A review is beyond the remit of this book and the reader is recommended to read the comprehensive review by Krbetschek et al. (1997). 5.2.6.3. RL emission spectra 5.2.6.3.1. Under X-ray stimulation at low temperature. As part of the study to understand IRSL production in feldspars at temperatures below room temperature (Bailiff and Barnett, 1994), Barnett and Bailiff (1997b) investigated the prompt luminescence during X-irradiation (RL) at temperatures from 80 to 300 K. Three detection windows were usedmblue/UV (350-500nm), green/orange (500-580nm) and red (580-610nm).
204
Optically Stimulated Luminescence Dosimetry
Of the six feldspars studied, all except one (a sanidine) showed a decrease in blue/UV RL as the temperature was raised from 90 to 200 K; thereafter the RL remained constant until 300 K. This can be explained in terms of thermal quenching (see Section 5.2.9.1). The quenching was observed to be higher for the potassium-rich feldspars (orthoclase and microcline microperthite), than for the plagioclases (albite, labradorite and oligoclase-the latter two having roughly equal Na and Ca contents). These data sets (Fig. 5.68) were used to obtain the thermal activation energy for quenching, W, by fitting to the equation r / - 1/(1 + C exp (-w/~)) given in Section 2.4.6, Eq. (2.64), where r/ is the luminescence efficiency, C a dimensionless constant, k the Boltzmann' s constant and T the absolute temperature. For the orthoclase and microcline microperthite, the values of W were 0.064 and 0.061 eV, respectively. For both the labradorite and the oligoclase, the value of W was 0.043 eV. Barnett and Bailiff (1997b) compared these values for the thermal quenching activation energy with the energies of the IR absorption bands for feldspars, which are in turn related to lattice vibrations related to the S i - O bonds. They concluded that in labradorite and
10
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60
80
100
Distance from surface of brick wall, mm Fig. 6.2. Dose-depth profiles measured with A1203:C chips across sections of bricks from (a) laboratory building (37 years old) and (b) village house (72 years old) (from BCtter-Jensen et al., 1999).
tell the difference between the gamma dose rates measured directly using A1203:C, and the quartz-derived gamma dose rates. 6.4.2. Estimation of the accident dose As discussed above, the total absorbed equivalent dose (De) is the dose absorbed by the mineral inclusions in the ceramics (e.g., quartz and feldspar) and is built up of two components: (1) the background dose accrued since the manufacture of the ceramics (brick) due to naturally occurring radioactive isotopes in the surrounding material and (2) the accident dose (also occasionally termed the fall out or transient dose in the literature) due to sources introduced into the local environment by the radiation accident. The accident dose is the difference between the total equivalent dose (De) delivered to the minerals (evaluated by luminescence measurements) and the accrued natural background dose. Thus, the total dose D e is expressed as:
De = Da + t(D~ + D~ + D.y + Dc)
(6.1)
where Da is the cumulative gamma dose observed by the ceramics due to the accident, t is the time since manufacture of the sample in years; D~, D~, D r and Dc are the effective annual alpha, beta, gamma and cosmic ray doses, respectively, due to natural sources of radioactivity. Evaluations of Da for quartz inclusions in ceramics can be related to dose in air at an external reference location by the use of conversion factors that are derived from computational modelling (Bailiff and Stepanenko, 1996). A large part of the OSL work performed to date has been concerned with dose evaluation in bricks. This is largely due to the predominance of that material at the sites studied so far (BCtter-Jensen, 1996; Bailiff, 1997; BCtter-Jensen and Jungner, 1999; Banerjee et al., 1999; BCtter-Jensen and Murray, 1999, 2001, 2002; BCtter-Jensen, 2000a,b).
250
Optically Stimulated Luminescence Dosimetry
6.5. Analytical protocols 6.5.1. Introduction All the measurement protocols used in accident dosimetry are based on those developed for dating archaeological materials (pottery, heated stones, etc.) and geological sediments. Until recently, evaluation of the equivalent dose (De) using OSL was undertaken using multiple-aliquot methods, either the additive-dose or the regenerative-dose procedures developed earlier for TL (Wintle, 1997). These methods require tens of sub-samples for a single estimation of D e. A single-aliquot additive-dose (SAAD) protocol was developed for feldspar (Duller, 1991, 1995), and more recently regenerative-dose single-aliquot methods have been developed for quartz (Mejdahl and BCtter-Jensen, 1994, 1997; Murray and Roberts, 1998; Murray and Mejdahl, 1999). In the latter, the OSL is first measured, and in the process, the light-sensitive traps are emptied. A regeneration or calibration dose is then given, approximately equal to the natural dose, and the OSL is measured again. Unfortunately, there is often a significant sensitivity change in such a cycle of measurements, especially if the sample is heated between irradiation and measurement, and this initially prevented the application of this very simple approach. 6.5.2. Multiple-aliquot protocols One of the earliest measurement protocols to be developed was that of multiple-aliquot additive-dose (MAA). The MAA protocol gets its name from the fact that many aliquots are needed, and also because laboratory doses are added on to the natural dose, to generate that portion of the dose-response curve, or growth curve, which lies above the natural dose. In its simplest form, this approach requires a minimum of two (in practice more, perhaps as many as 100) sub-samples (or aliquots) of identical characteristics. One sample is given a laboratory dose in addition to the natural dose, and the luminescence signal (TL or OSL) of both is measured. The two signals are plotted against laboratory dose, and the equivalent dose determined by extrapolation. Figure 6.3 illustrates this procedure with a practical example, in which were used 21 aliquots of quartz grains extracted from a Chernobyl brick, and six different laboratory doses. Because of the extrapolation, the actual value of De will clearly depend on the algebraic relationship used to fit the data. For the low dose levels encountered in accident dosimetry, the responses were usually found to be linear. More detailed descriptions of various multiple-aliquot techniques are given in Part II of this chapter (Section 6.11.1). 6.5.3. The single-aliquot regeneration and added dose protocol The Single-Aliquot Regeneration and Added Dose (SARA) method introduced by Mejdahl and BCtter-Jensen (1994, 1997) is based on repeated measurements on each of a small number of aliquots and has the advantages of: (1) being able to be applied to a small sample, (2) giving high precision, and (3) needing no normalisation. Duller (1991) applied single-aliquot measurements with both regeneration and added dose procedures, but abandoned regeneration because of the sensitivity changes found as a
Retrospective OSL Dosimetry
251
16
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200
,
|
,,
l
,
400
600 Dose ( mGy )
l
,
800
1000
1200
Fig. 6.3. Typical MAA growth curve obtained from extracted brick quartz. Aliquots were each about 4 mg and normalised using the OSL signal from a brief stimulation of all aliquots, prior to addition of any laboratory doses. The solid line is the best linear fit. The intercept gives De as 98 mGy (from BCtter-Jensen, 2000b).
result of the re-use of aliquots. Sensitivity changes associated with regeneration have been further studied by Jungner and BCtter-Jensen (1994), McKeever et al. (1996, 1997), Murray and Mejdahl (1999), who identified them to be due to the transfer of electronic charge (electrons and holes) between the various traps and recombination centres taking part in the TL and OSL processes. The SARA method requires a minimum of two aliquots and thus is not a true singlealiquot method. The procedure can be summarised as follows (Fig. 6.4): (1) add beta doses (0, B1, B2, B3) to aliquots containing their natural dose; (2) carry out single-aliquot regeneration measurements on these aliquots to obtain doses (Do, D~, D2, D3); (3) plot these doses as a function of the known added doses (0, B1, B2, B3); and (4) extrapolate the regression line through the points to intersect the added dose axis and Q~ r
.Q
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~
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252
Optically Stimulated Luminescence Dosimetry
obtain the intercept I. The intercept will then represent the true equivalent dose De (referred to as ED in Fig. 6.4), irrespective of any sensitivity change introduced during the regeneration procedure. There is one important restriction: any sensitivity change must be the same for the doses Do-D3 independent of the beta doses added initially. A simple procedure for testing this is as follows (see Fig. 6.4): From the two triangles O-Do-I and I-B3-D3 in Fig. 6.4, one can deduce that a condition for the sensitivity change being independent of the added dose is given by the following expression: Do~De = D 3 / ( D e -t- B3).
(6.2)
Three regeneration doses are usually used to determine each value of De; these are adjusted so that the "natural" signal falls within the signal interval determined by the regeneration doses. This is necessary because the regeneration growth curves are not always linear. By repeating the measurements, the dose interval can be narrowed so that the interpolation errors are negligible. The SARA method has been used with fired (archaeological) materials, ceramics, bricks and burnt stones, which are relevant to retrospective dosimetry. Mejdahl and BCtter-Jensen (1994, 1997) applied their OSL SARA protocol to both heated and non-heated quartz and feldspar samples and Murray (1996) used it on sedimentary quartz. Murray (1996) showed that the growth curve will be linear, even in the presence of some non-linearity in the OSL dose-response curve. Because SARA is a regeneration-based method, no inter-aliquot normalisation is necessary, and the precision of De estimates obtained using only a few aliquots (typically 9) was a significant improvement over the earlier multiple-aliquot methods. As described earlier, it is implicitly assumed that the operator has some prior notion of the equivalent dose (De) level of the material being studied. To overcome this problem, an automated version of the SARA procedure was developed by Duller et al. (1999) to automatically adjust the radiation dose levels that are administered to several aliquots in an automated reader. An initial value for the first radiation dose was entered, and then the algorithm adjusted this value for the subsequent aliquots until the induced OSL fell close (within + 2%) to the initial light level. Only one single regeneration measurement was used on each aliquot in order to eliminate the risk of non-uniform sensitivity changes during different regeneration cycles. The iterative procedure uses each measurement of the natural-to-regenerative luminescence signal ratio for each aliquot to make an improved estimate of De for the next aliquot. This improved estimate is used in tum to adjust the regeneration dose for the last aliquot to improve the matching of the natural and regenerative light levels. The iteration was completed in less than three measurements. 6.5.4. True single-aliquot protocols
6.5.4.1. Introduction The advantages of single-aliquot procedures over multiple-aliquot techniques are: (1) improved precision, (2) the ability to study the dose distribution within a sample, (3) rapid measurement, (4) no need for normalisation, (5) no correction for supra-linearity (in the case of regeneration protocols), and (6) smaller samples needed.
Retrospective OSL Dosimetry
253
Single-aliquot protocols allow all measurements required for the estimation of D e to be made on one sub-sample (or aliquot). There has been a rapid development in this area, with additive-dose protocols receiving attention first (Duller, 1991; Galloway, 1996; Murray et al., 1997) (see Sections 6.11.2.1.1 and 6.11.2.2.1). More recently, regenerativedose single-aliquot procedures have been developed for quartz (Murray and Roberts, 1998; Murray and Mejdahl, 1999; Murray and Wintle, 2000) (see Section 6.11.2.2.2). Regenerative methods are conceptually the simplest--the OSL is first measured, and in the process the light sensitive traps are emptied. A regeneration or calibration dose is then given, approximately equal to the natural dose, and the OSL is measured again. If there has been no change in sensitivity, then D e is given by the ratio of the two OSL signals, multiplied by the known laboratory dose. As was discussed above, in practice there is usually a significant sensitivity change, especially if the sample is heated between irradiation and measurement as may happen when pre-heating is used to empty thermally unstable traps. However, recent work has shown that a precise correction for sensitivity can be made, based on the OSL signal from a test dose given immediately after measurement of the natural or regenerated OSL signal. In this case, there is no heat treatment between irradiation and measurement (other than a fixed heat to 160~ to empty the 110~ TL trap). It has been shown (Murray and Wintle, 1998; Murray and Mejdahl, 1999) that the OSL from the test dose provides a signal that is proportional to the luminescence sensitivity relevant to the preceding natural or laboratory-induced OSL signal. Thus, dividing the latter by the test dose OSL removes the effects of any changes in sensitivity. This procedure has been extensively applied to estimate the equivalent dose of quartz extracts from brick in retrospective dosimetry (Banerjee et al., 1999; 2000; BCtter-Jensen et al., 1999).
6.5.4.2. Variation of OSL signal with pre-heat In a previous study aimed at determining the effect of pre-heating on the OSL signal of porcelain and brick quartz, Godfrey-Smith and Haskell (1993) observed very small sensitivity changes with pre-heats of 180 and 200~ Jungner and BCtter-Jensen (1994) found that in regeneration experiments using quartz, an increase in the sensitivity was observed when pre-heating for 10 s in the temperature interval of 200-250~ Above this temperature the sensitivity remained relatively constant. Keeping the quartz sample at an elevated temperature of 120~ during OSL measurements, to keep the trap associated with the 110~ TL peak empty, had no effect on the sensitivity change. However, Wintle and Murray (1999) found significant sensitivity changes in both the natural and regenerated OSL signals from a sedimentary quartz, after heating to various temperatures above 160~ for 10s.
6.5.4.3. Choice of OSL signal Murray and Wintle (1998) and Banerjee et al. (2000) have discussed which part of the OSL decay curve should be used for D e evaluation. The measurement procedure is based on the subtraction of an underlying background taken as the signal observed at the end of the stimulation period. Banerjee et al. (2000) found that for both dim and bright signals, the smallest statistical uncertainty in the net OSL signal is achieved using the first few seconds of the decay curve. Although the initial and the total OSL signal behave
254
Optically Stimulated Luminescence Dosimetry
similarly, it has been demonstrated that there may be a significant (10%) hard-to-bleach component in the total integrated signal. This contributes much less to the initial signal. It is important in a regenerative protocol using OSL that the signal used in calculations is dominated by the most rapidly decaying component of the quartz OSL signal (see Chapter 5), and thus the initial signal is used after subtracting the underlying slow component measured over an equivalent number of channels at the end of the signal ( Fig. 6.5a). Fig. 6.5b (after Banerjee et al., 2000) shows the random uncertainty arising from counting statistics as a function of total integration time, where the random uncertainty is
OSL Intensity (Counts/s) (a)
I~ I/
Channels (25) usedfor estimationof average backgroundper channel
Diode Exposure time (s)
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Stimulation time, s
(0.24 s channel width)
Fig. 6.5. (a) OSL decay curve indicating the initial signal and the underlying slow component (from Bctter-Jensen, 2000b). (b) Random uncertainty in the net OSL signal plotted as a function of integration time. See text for definition of the uncertainty (from Banerjee et al., 2000).
Retrospective OSL Dosimetry
255
estimated using the expression"
)1/2/Y.iS i -
or--
~'.iS i -Jr-2B n
(6.3)
Bn ,
where S i is the signal from the ith channel (i = 1,2, ...n) and Bn is the average background. 6.5.4.4. Sensitivity changes with regeneration cycles For any single-aliquot regenerative-dose method to be applicable, it must be demonstrated that luminescence sensitivity changes are negligible or that they can be corrected for by measuring the OSL sensitivity, or a proxy for it. Fig. 6.6 shows the dependence of both the 110~ TL peak area and the OSL signal (first few seconds) from the second test dose, after the measurement of the regeneration OSL for a heated brick quartz sample. The same regeneration dose of 2.5 Gy was given 10 times and a pre-heat of 160~ for 10 s employed (Banerjee et al., 1999). The OSL from the test dose correlates very well with the regenerated OSL, whereas an off-set is observed when using the 110~ TL peak. It has thus been shown that a single measurement of the OSL test dose signal can be used to correct for sensitivity changes and a sensitivity-corrected growth curve can be obtained by dividing natural and regenerated OSL signals by the subsequent test dose OSL signals (Banerjee et al., 1999; Murray and Mejdahl, 1999; Wintle and Murray, 1999). The use of either the OSL response or the 110~ TL response to a test dose, to account for sensitivity changes in sedimentary materials is discussed in detail in Section 6.11.2.3. 6.5.4.5. The SAR protocol The Single-Aliquot Regenerative-dose (SAR) protocol for bricks could employ as few as four OSL measurements. The sample extracted from the brick is first pre-heated to an arbitrary temperature between 160 and 300~ for 10 s. The material has already absorbed a dose before sampling, i.e., the sum of the accident and the natural background dose. The OSL signal (first few seconds) due to this dose is measured to give signal Ln. A test dose is then applied (10-20% of the natural dose) and the sample heated to 160~ to empty the 4x10 s
9
o'J
E 0
x_
110~
Test Dose.signal
v
8000 c
3xl 0 5
- 6000
2x105
4000
0
10 5
2OO0
oE
o o
r
o0 O.c-
-g
I-- o
0
-
,r- 0 ~'-'0
0
0
5000
10000
15000
0
20000
Regeneration dose OSL, counts Fig. 6.6. Dependence of test dose 110~ TL peak area and test dose OSL on the regenerated OSL for a brick quartz sample (from Banerjee et al., 1999).
256
Optically Stimulated Luminescence Dosimetry
charge from the 110~ TL trap; this thermal treatment is often referred to as the 'cut heat'. The OSL signal is measured again, to give Tn. A regeneration dose (Dr) is then applied, which is followed by pre-heating and measurement of the regenerated OSL (L0. The test dose is given again, heated to 160~ and the OSL signal measured to give Tr. Using the observation in Fig. 6.6 that the correlation between L and T is linear and passes through the origin, the natural dose D e is then given by: De -- ( t n / t r ) ( Z r / Z n ) O r .
(6.4)
This calculation assumes that the OSL d o s e - r e s p o n s e curve is linear, or that Dr "-" De. To avoid the need for this assumption, least three regeneration doses chosen to encompass De, are normally applied in sequence to the same disc and D e is then estimated by interpolation. To verify that the OSL has been adequately corrected for any sensitivity changes during measurement, a dose equal to the first regeneration dose is then given to the sample and its OSL measured. A ratio of the sensitivity-normalised signals of the first and the fourth regeneration measurements close to unity (within + 10%) confirms that sensitivity changes, if any, have been properly accounted for (i.e., within 10%) in the evaluation of the equivalent dose. Finally, the OSL signal is also measured without giving an additional regeneration dose before pre-heating and measurement (the "zero-dose" measurement). The sensitivity-normalised zero-signal gives an indication of the degree of thermal transfer from the hard-to-bleach traps to the OSL trap. The SAR protocol (Murray and Wintle, 2000) is outlined in Table 6.1. Banerjee et al. (2000) demonstrated the robustness of the SAR measurement protocol by giving several increasing regeneration doses in the range 0 . 5 - 5 6 Gy to a heated brick quartz sample. A pre-heat of 160~ for 10 s and a test dose of 24 mGy were applied. Fig. 6.7 presents the uncorrected and sensitivity corrected OSL growth curves. The main distinction between the uncorrected and the sensitivity-corrected growth curves is a clear removal of a supra-linear growth in OSL after sensitivity correction. Fig. 6.8a presents a routine application of the SAR protocol to the measurement of De for a heated quartz sample (Banerjee et al., 1999). Three sensitivity-corrected OSL signals
Table 6.1 Outline of a typical SAR measurementsequence Natural + accident dose (De) Regeneration dose 1 (< De) Regeneration dose 2 ( ~ De) Regeneration dose 3 (> De) 5. 6.
Regeneration dose 4 ( = Regenerationdose 1) Regeneration dose 5 ( = 0 Gy)
Pre-heat (180...280~ for 10 s), OSL at 125~ Test Dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test Dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~ Pre-heat (180...280~ for 10 s), OSL at 125~ Test dose, TL to 160~ OSL at 125~
Retrospective OSL Dosimetry 6x10 6
9 Uncorrected o Corrected
o
4xl 06
O0
1000
0000 ~
E O
o~176
0000000
O
-
257
o
O0 0 0 - 500
0
8 2x106 g
E
~ g~
0~ 0
0 20
40
m
60
Regeneration dose, Gy
Fig. 6.7. Sensitivity corrected and uncorrected OSL growth curves for a Chernobyl brick quartz sample (from Banerjee et al., 1999).
(S1, S2, S3) are plotted against their corresponding regeneration doses (Drl, Dr2, Dr3). The latter were chosen so that Drl < De, Dr2 "~ De and Dr3 > Dr2. The equivalent dose (De) is then interpolated from this limited section of the regenerated dose-response curve. A fourth regeneration dose (Dr4) equal to the first (Dr4 = Drl) is administered to the same aliquot. The corrected luminescence signal ($4) corresponding to dose Dr4 is shown as an open triangle in Fig. 6.8a. The ratio of the fourth corrected luminescence signal to the first ($4/S~) gives a measure of how well the sensitivity correction has performed over the first four regeneration cycles (in this case $4/S~ --0.998). A fifth regeneration dose (D r = 0) is then given. Ideally $5 should be zero, but some recuperation may be observed. The zero-dose-corrected regeneration signal is shown as a filled circle at the origin in Fig. 6.8a. After sensitivity correction, the fourth and first regeneration dose signals ($4 and S~) are indistinguishable, signifying that sensitivity changes have been satisfactorily corrected for. The sensitivity-normalised zero dose signal is negligibly small. The sensitivitycorrected growth curve is linear in this dose range (R2 = 0.998) and passes through the origin. Figure 6.8b presents a typical plot of the variation of De with pre-heat temperatures (each for 10 s) between 180 and 280~ (Banerjee et al., 1999). The equivalent dose is independent of pre-heat temperature. This result is consistent with earlier observations (Murray and Wintle, 1998; Murray and Mejdahl, 1999) that after sensitivity correction, there is no evidence of significant thermal transfer of charge from light-insensitive traps to lightsensitive traps when pre-heating before measurement of the OSL signal due to the natural plus accident dose. Had transfer occurred in nature, a systematic change would have been observed in De with increasing pre-heat temperatures until a constant value was reached. 6.6. Evaluation of dose-depth profiles in bricks In the retrospective assessment of accident doses using luminescence methods with bricks, measurements of the dose-depth profiles into the brick material give information about the energy of the incident photon radiation (BCtter-Jensen et al., 1995a). For this reason it is desirable to compare such dose-depth profiles with those obtained from bricks
258
Optically Stimulated Luminescence Dosimetry 4.0
i I i"
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~d r.,e 9 3.5
I--I
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. ,,.,~ .,,-~
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/
.,.,~ r.~
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T~.TJ De = 464 mGy I
0.0
//, 0
I 500
400
600
Regeneration Dose, mGy
500 -
B d 9
400300 -
,.o
200> .,..~
~r 100 Mean D e = 376 _ 4 mGy (n = 20) 0
160
I
I
I
200
240
280
Temperature, ~
Fig. 6.8. (a) Sensitivity-corrected OSL growth curve for a Chernobyl quartz sample following the sequence given in Table 6.1, ( 9 ) regenerated OSL, (A) 4th generated OSL, (T) natural OSL. R 2 = 0.998, D e = 464 mGy. (b) Variation of the equivalent dose with pre-heat temperature for another Chernobyl quartz sample (from Banerjee et al., 1999).
irradiated using known gamma sources in the laboratory. Laboratory-irradiated bricks also provide a basis for comparison with modelling using Monte Carlo simulations. Such Monte Carlo simulations have been performed for Chernobyl bricks (BCtter-Jensen et al., 1995a, 1999; Bailiff and Stepanenko, 1996); these simulations ultimately predict the absorbed dose in air at an external reference location for a given source energy and configuration. If assumptions on the source configuration can be made, the dose-depth profiles are expected to reflect the time-integrated energy spectrum of the external radiation field. Thus, measurements of dose-depth profiles are important because they provide support for the assumption used to convert from dose in brick to dose in air. 6.6.1. Continuous OSL scanning Although beta and alpha radiation are rapidly absorbed in the outer layers of brick, gamma radiation penetrates tens of centimetres. By monitoring the attenuation of the
Retrospective OSL Dosimetry
259
radiation-induced luminescence, information on both the dose and the energy spectrum of the gamma rays can be obtained. A method was developed for measurement of dosedepth profiles in brick, tile and porcelain cores, without the need for sample separation techniques (BCtter-Jensen et al., 1995a). Using brick cores, profiles were generated by laboratory radiation using different photon energies from 137Csand 6~ gamma sources; the measured depth dependency was then compared with theoretical calculations derived from Monte Carlo simulations, and with experimental measurements made using conventional optically stimulated luminescence methods of analysis. An automatic OSL scanner is described in Chapter 7 for these applications. Examples of dose-depth profiles obtained fromtwo brick cores that had received i37Cs and 6~ gamma laboratory doses, respectively, are shown in Fig. 6.9. 6.6.2. Determination of dose-depth profiles from Chernobyl bricks Optimum sensitivity is usually attained by using samples of pure minerals (quartz and feldspar) extracted from the bulk material. Such extraction techniques have been used extensively to measure the dose-depth profiles in a variety of brick samples, collected from inhabited sites in the Chernobyl accident area. Sub-samples for measuring the dosedepth profiles are prepared by slicing a cross-section of the brick into 10 mm thick subsections, coarse-grain (90-150 ~m) quartz samples are then extracted from each section. The thickness of each slice represents the limit on spatial resolution. Thinner slices can be used, but the cross-sectional area would have to be increased in proportion. Figure 6.10 shows three dose-depth profiles obtained from Chernobyl bricks measured using the OSL SAR protocol (BCtter-Jensen, 2000b). The two upper curves represent the results from bricks that have been exposed from one side to external accidental photon doses and the exponential decay rates compare well with that obtained from a brick irradiated with 660 keV 137Csphotons in the laboratory. The lower curve represents a brick that has not received any significant dose other than that from the internal radionuclides in the matrix and the ambient photon radiation including the cosmic radiation. 6.6.3. Absolute errors and estimated precision of the equivalent dose in bricks The absolute precision of the total dose estimates obtained using the SAR protocol has been demonstrated for known-age house bricks (of an age < 100 years) and a precision of < 3% is readily achievable. The absolute uncertainties in the accident dose (i.e., D e minus the natural background dose) are dominated by systematic uncertainties, such as those arising from the calibration using laboratory doses (typically --~ 3%), and the estimation of the background dose. The overall uncertainty associated with the latter component is around 4% for a known-age sample, depending on the analytical method used to determine the dose rate (Banerjee et al., 1999). The accident dose is given by Da - - D e - B , where B is the background dose. Thus, for a sample with D e - - 1 0 0 -+- 3 mGy, and background B = 50 +__2 mGy (equivalent to the natural dose in a 15-20 year old brick), the accident dose is 50 +__4 mGy. A typical minimum detection limit for a fallout dose in these circumstances would be about 12 mGy (three standard errors). This detection limit can be optimised most easily by selecting
Optically Stimulated Luminescence Dosimetry
260
(a) A
:i ei
O cI
0,75
0,5
_i
ILl IZ:
0,25
0
50
100
150
200
250
DISTANCE ALONG THE CORE (mm)
(b) A
~J
O E3
0,75
0,5
..i
LU n,"
0,25
0
50
100
150
200
250
DISTANCE ALONG THE CORE (mm) Fig. 6.9. Relative dose-depth profile into a brick from (a) 137Csand (b) 6~ gamma radiation from one side, as calculated by the Monte Carlo code MCNP (bold lines). For comparison, the relative dose-depth profiles measured with the automatic OSL core scanner system are also shown (from Bc~tter-Jensen et al., 1995a).
buildings that were built immediately before the accident. Obviously, measurement should be made as soon after the accident as possible. Close to detection limits, the largest single source of uncertainty will probably arise from the estimation of the background dose-rate, and it is in this area that effort should be concentrated to improve accuracy.
6.7. Retrospective OSL dosimetry using unheated quartz Most attempts to apply retrospective dosimetry to building materials have made use of heated (sensitised) items such as brick or tile ceramic. Unfired materials, such as concrete and mortar, are much more widespread in the office and industrial environments, but unfortunately these cannot be assumed to contain a negligible dose at the time of construction.
Retrospective OSL Dosimetry
261
500
400
E
300
0 200
r9
~"
-
--
100
I
0
~
'
I
30
'
'
'
'
I
60
~
~
'
'
I
'
90
'
'
'
120
Mean depth of section (mm) Fig. 6.10. Typical dose-depth profiles measured from Chernobyl bricks using the SAR protocol on extracted quartz. Note the error bars are within symbols. See text for further details (from BCtter-Jensen, 2000b).
Sand for building materials is quarried from geological deposits, which contains a natural dose, in some cases up to > 100 Gy depending on the age of the deposit. However, the sand is exposed to light during quarrying and use. As a result, grains of quartz extracted from a modern mortar or concrete will often show a wide distribution of doses, with only some completely bleached grains giving effectively zero dose. The challenge in using such materials as retrospective dosimeters is in identifying well-bleached grains at the time of the accident dose, which is then superimposed on the original dose distribution. 6.7.1. Dose distributions Analyses have been described of dose distributions derived from OSL measurements of a variety of unheated samples using techniques based either on small aliquots (i.e., < 100 grains per aliquot) or single sand-sized quartz grains (Olley et al., 1998; 1999; Duller and Murray, 2000). BCtter-Jensen et al. (2000) reported the use of small aliquots (---60 grains) to measure the dose distribution of quartz extracted from the bulk mortar in a wall of a low-level radioactive waste storage facility containing distributed sources of 6~ and 137Cs. The average value obtained compared very well with that derived from a dosedepth profile measured using OSL on extracted quartz from an adjacent brick, and from a separate TL dosimeter record. However, the availability of single-grain OSL apparatus (see Section 7.7.3) has made it possible to measure large numbers of quartz and feldspar grains extracted from building materials. Recently, Jain et al. (2002) and BCtter-Jensen
Optically Stimulated Luminescence Dosimetry
262
and Murray (2002) measured the dose distributions in quartz extracted from a crosssection of a mortar sample collected from an outer wall of a radioactive waste storage facility using both small aliquots (< 100 grains per aliquot) and individual grains. Examples of dose distributions obtained using the SAR method on small aliquots and single grains of quartz from a sample of mortar are shown in Fig. 6.11. Only those aliquots with uncertainties of < 15% on the dose estimate are included. Although, the expected dose is about 9.3 Gy (Jain et al., 2002) the doses measured using small aliquots form a broad, approximately Gaussian, distribution with an average value about 14 Gy and a standard deviation of---17%, considerably more than expected on the grounds of individual uncertainties. However, the doses derived from the single grain measurements of the same sample (also only using doses determined with uncertainties of < 15%) seem to show two distributions: one having approximately the fight average value of about 9 Gy and the other having an average value of about 14 Gy. Although, only 137 out of 11,000 25
9
(a)
>., 20" O C (D 15' 13" (1) Lt_ 10"
95 80 60
.... 5
10
~ , ~ .
15
20
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30
35
40
45
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~
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o r
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= O
Dose (Gy)
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ii
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0
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Dose (Gy) Fig. 6.11. Dose distributions from (a) small aliquots and (b) single grains of quartz extracted from a poorly bleached mortar sample. See text for explanation (from BCtter-Jensen and Murray, 2002).
Retrospective OSL Dosimetry
263
grains (---1.2%) provided results that meet the acceptance criteria, these results suggest that single grain analysis is capable of identifying two different dose populations that seem to be merged when using small aliquots. Jain et al. (2002) made a comprehensive comparison of small-aliquot and single-grain OSL measurements using quartz extracted from mortar and bricks taken from a cross-section of the same wall. The measured dosedepth profiles are shown in Fig. 6.12a. Quartz grains extracted from a commercial dry pre-mix concrete have also been studied using the SAR protocol on small aliquots (---60 grains) and single grains. Thomsen et al. (2002a) prepared a simulated concrete brick consisting of a number of 10-mm thick layers of pre-mixed concrete, inter-spaced with thermally annealed quartz to provide a dosedepth profile through the brick. The brick was irradiated in the laboratory with 662 keV 137Cs photons. In this experiment, the dose distribution in the concrete after the addition of an "accident" dose can be compared with that obtained before the accident dose. Olley et al. (1998) suggested the use of the lowest 5% of a dose distribution to identify the wellbleached grains. OSL data from both small aliquots and single grains can be plotted either as a histogram or as a radial plot (Galbraith, 1990). In a histogram, all data points are weighted equally, irrespective of the precision with which they are known. A radial plot, where each result is plotted together with its relative statistical error, may be more informative. Thomsen et al. (2002a) found that about 80% of the natural OSL comes from only 2% of the single grains of an aliquot and only 2.5% of the grains gave a statistical uncertainty on the natural test dose response of < 30%. A small aliquot normally contains about 65 grains, which means that on average each aliquot only contained 1 - 2 detectable grains. Fig. 6.12b shows a comparison of small-aliquot results derived from the first 5% of histograms, radial plots and single grain results and the Monte Carlo calculated dose-depth profile into the dry-concrete-mixture-simulated brick (Thomsen et al., 2002a). 6.7.2. Thermal transfer and sensitivity changes The SAR procedure for quartz (Murray and Wintle, 2000) has successfully demonstrated that the OSL signal can be corrected for sensitivity changes occurring during repeated measurement cycles by using the OSL response to a small test dose. In the case of poorly bleached materials, the pre-heat stages in these cycles can cause charge transfer from light insensitive but thermally stable traps to the main OSL trap (associated with the 325~ TL peak) (Spooner, 1994). This thermally induced charge transfer can, in some cases, create significant problems in dating young materials, i.e., aliquots with a small natural dose (Rhodes, 2000). In retrospective dosimetry, where it is desirable to measure doses as low as few tenths of a mGy with high precision, thermal transfer could give a significant dose offset (Jain et al., 2002). The optimal pre-heat temperature, constrained by minimum thermal transfer from stable traps, can be investigated by measuring the absorbed dose as a function of the pre-heat temperature. Jain et al. (2002) analysed the thermal transfer in unheated quartz taken from a mortar sample using two different grain size ranges. Also, the thermal transfer from the test dose itself was measured. The results are shown in Fig 6.13a. Thermal transfer is insignificant for temperatures up to 240~ subsequently, it increases to about 0.5 Gy at higher
264
Optically Stimulated Luminescence Dosimetry
Fig. 6.12. (a) Dose-depth profiles measured using small aliquots and single grains of quartz, and polyminerallic fine-grained aliquots extracted from cross-sections of mortar and bricks taken from a nuclear waste storage building. The obtained data are fitted with an exponential curve (bold line) and all results compare well with an independent environmental TLD record (from Jain et al., 2002). (b) Dose-depth profiles measured using extracted quartz from pre-mix concrete. Comparison of small aliquot (SA) results derived from the first 5% of the histograms and radial plots, and single grain (SG) results versus depth with the Monte Carlo calculated dosedepth curve (from Thomsen et al., 2002a).
t e m p e r a t u r e s . T h e dose contribution due to t h e r m a l transfer f r o m the test dose is seen to be insignificant at any t e m p e r a t u r e . T h e plot of p a l e o d o s e as a function of pre-heat for the s a m e s a m p l e is s h o w n in Fig. 6.13b and forms a plateau in the range 1 6 0 - 2 6 0 ~
As the
t h e r m a l transfer is insignificant at low t e m p e r a t u r e s and there exists a stable plateau up to 260~
a standard pre-heat at 200~
is a d e q u a t e for this sample.
Retrospective OSL Dosimetry
A 150-212 microns 0 250-300 microns A 150-212 microns (TD)
1.1 A
it_
I-
265
0.8
(a)
9 250-300 microns (TD)
0.5
"~
0.2
I--
-0.1
200
160
240
280
Preheat Temperature (*C) 12 A
10
>h
(,.5 q)
i
8
o "o o
6
m
4
Ix.
(b)
2 140
i
i
i
i
i
i
i
160
180
200
220
240
260
280
Preheat Temperature (*C) Fig. 6.13. (a) Thermal transfer for different quartz grain sizes. The standard error (SE) is calculated from five aliquots for each grain size. TD represents the thermal transfer signal from the test dose alone. (b) Pre-heat plateau for the 150-212 t~m grain size. The SE is calculated from at least 10 aliquots (from Jain et al., 2002).
6.8. Retrospective OSL dosimetry using household and workplace chemicals
There are other crystalline materials found in the domestic and industrial environment, which may also act as retrospective dosimeters. Bailey et al. (2000) and Bulur et al. (2001) investigated some OSL properties of common salt (NaC1) which seems to be the most obvious of these materials. The OSL characteristics of several other household and workplace chemicals have also been investigated, including washing powder, dishwashing powder, and water softener (Thomsen et al., 2002b). Such chemicals are often held in a light-tight packaging (important for stability of the OSL signal), and are likely to have been manufactured recently, which limits the size of the likely background dose. Figure 6.14 presents typical linearly modulated OSL (LM-OSL) data for common salt, Glauber salt, washing powder and water softener, all after a radiation dose of 500 mGy and a pre-heat of 150~ for 10 s. The CW-OSL curves obtained using constant stimulation power are also shown as insets in Fig. 6.14. All materials show a strong and easily
266
Optically Stimulated Luminescence Dosimetry
1200
Jxl0~4 ~-~ 30001
oo41 / I
"•
o/
"T
'oooik
1~~I
c
.d 100 CO O -o 0 . m
E t ~
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Time(s)
L
800-
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i
400
(b)
~ 4500il 0 3000
200
o
(a)
6000! 4000 2000 0 0
, m
6
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18
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6 12 18 Time(s)
2ooo|
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OI
200
L~
0
,
6.
,,
,
12
18
|
|
i
0
200
400
Time (s) Fig. 6.14. LM-OSLcurves for four commonhousehold chemicals(a) commonsalt; (b) Glauber salt; (c) washing powder; (d) water softener, with stimulationpower increasedfrom 0 to 100% in 500 s. The insets show CW-OSL decay curves, measured at a constant 100% stimulation power (from Thomsen et al., 2002b).
stimulated signal, which decayed to negligible proportions after < 4 s of continuous stimulation with blue light (30 mW/cm2). The LM-OSL curves demonstrate that a single trap dominates the decay curve, although a weak slow component is also visible. Thomsen et al. (2002b) investigated the stability of the signals from such materials over periods of 24 h and 2 weeks and they controlled the sensitivity changes before and after storage by monitoring the OSL response to a smaller test dose, to ensure that any signal loss during storage was not an artefact of sensitivity change. The results showed that for most of the materials tested, negligible fading over the two-week period was found.
Retrospective OSL Dosimetry
267
BCtter-Jensen and Murray (2002), and Thomsen et al. (2002b) also applied the SAR protocol to determine the growth curves and natural doses from a variety of chemicals. Common salt showed no significant sensitivity changes during the generation of the growth curve, whereas other samples showed overall sensitivity changes of about 20%. However, one can completely compensate for this effect by using the SAR method. As an example, a SAR growth curve from a common dish washing powder (Blue Care) is shown in Fig. 6.15. This material was given a dose of 500 mGy before measurement, and the evaluation of the dose using the lower region of the growth curve resulted in a value of 495 ___ 14 mGy, in good agreement with the given dose. It is clear that an accident dose of a few hundred mGy can be accurately measured using most of these materials. Furthermore, these measurements can be carried out several days after the accident, and in some cases much longer. A practical average lower detection limit found for several of the household and workplace chemicals was of the order of 10 mGy and the fading characteristics varied from 0 to 40% over two weeks (Thomsen et al., 2002b).
6.9. Retrospective OSL dosimetry using porcelain 6.9.1. Introduction Porcelain is potentially a very important material in retrospective dosimetry because it is widespread in the domestic and industrial environment (Bailiff, 1997). The potential of OSL for dose measurements on various porcelain ceramic materials has been investigated (Poolton et al., 1995; BCtter-Jensen et al., 1996), and Htibner and Grksu (1997) have reported their use of the OSL-pre-dose effect in porcelain from electric-power insulators to retrospectively assess accident doses. Although the principal raw materials used in the manufacture of porcelain are quartz, feldspar and china clay (kaolinite), A1203 is often added as a minor component. As described in section Chapter 3, A1203 can be a very sensitive OSL radiation dosimeter. However, the sensitivity of any of the potentially usable dosimeters contained within porcelain ceramic is likely to depend strongly on the production conditions (firing temperature, atmosphere, etc.), as well as the exact composition of the starting materials. 6.9.2. The origin of OSL in porcelain In general, optical stimulation of both the main porcelain matrix and the glaze gives rise to two types of luminescence signals. These are the time-decaying dose-dependent OSL signals, in which the stimulation energy is less than the emission energy, and the time-steady dose-independent photoluminescence (PL), in which the stimulation energy is greater than that of the emission.
6.9.2.1. Time-decaying dose-dependent OSL signals A link has been shown between the OSL signal and the TL peak at 110~ in quartz (Stoneham and Stokes, 1991; BCtter-Jensen and Duller, 1992; BCtter-Jensen et al., 1995b). In porcelain, an indication that at least part of the dosimetric information arises from the quartz phase of the material is obtained by monitoring the TL at 110~ both before and after OSL (Poolton et al., 1995). As shown in Fig. 6.16a, illumination of a porcelain
268
Optically Stimulated Luminescence Dosimetry
Fig. 6.15. SAR growth curves from a sample of dish-washing powder (Blue Care) that had been exposed to a gamma dose of 500 mGy. A pre-heat of 100~ was applied before each regenerated OSL measurement. (a) The sensitivity-corrected growth curve between 0 and 16 Gy. (b) The lower region of the same growth curve as in (a) from which a dose of 490 +__ 14 mGy was derived for the 'unknown' initial dose by interpolation. The error bars are hidden by the symbols (from BCtter-Jensen and Murray, 2002).
sample causes phototransfer to this low temperature TL trap (PTTL), a process that is typically associated with OSL in quartz (BCtter-Jensen et al., 1993, 1995b). However, the quartz component is certainly not the only OSL-active material present. Thermal annealing of fired porcelain samples to successively higher temperatures following irradiation indicates that the time decaying OSL signals are composed of at least three components. Fig. 6.16a also shows the result from an experiment where a porcelain sample (from a Chernobyl toilet tank) was given a 20 Gy dose using 6~ gamma radiation and OSL was measured at 20~ for 0.1 s, a period not long enough to significantly deplete
[/f~
Retrospective OSL Dosimetry
~, 125 f 1.oo_ t -
-
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0.75
269
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200 300 Temperature (~ I
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Ti me (seconds) Fig. 6.16. (a) The TL signals from a toilet porcelain sample irradiated to 20 Gy and thermally annealed at 120~ both before and after illumination with blue-green broad-band (420-550 nm) light: the photo-transferred TL signal (PTTL) at about 110~ is typical for that of quartz. Thermal annealing of the sample for 10 s at successively higher temperatures (dashed line) indicates that the OSL probably originates from several of the TL traps. (b) Time decay characteristics of OSL measured from a porcelain sample (toilet tank) following a 5 Gy radiation dose and thermal annealing for 10 s at the temperatures indicated. The OSL stimulation intensity was 16 mW/cm 2 in a broad-band (420-550 nm) (from Poolton et al., 1995).
the OSL signal. Subsequently, the sample was pulse-annealed in steps of 50~ in the range 50-350~ for 10 s, with the OSL monitored each time for 0.1 s at room temperature. For the thermal annealing between 50 and 150~ the OSL is greatly reduced and it can be deduced that a significant portion of the initial OSL probably arises from low temperature TL traps. However, the TL curve in Fig. 6.16a does not show these, since the sample here was heated to 120~ prior to measurement. For thermal annealing between 150 and 250~ no significant change in the OSL trap population is normally observed, but for heating beyond 250~ the OSL decreases rapidly. This indicates that an unstable OSL signal is present in a freshly irradiated porcelain sample and an appropriate pre-heat treatment is required for obtaining a stable OSL signal suitable for dosimetry (Poolton et al., 1995). Fig. 6.16b shows the time decay characteristics of OSL in porcelain after pre-heating at different temperatures.
Optically Stimulated Luminescence Dosimetry
270
6.9.2.2. Time-steady PL emission spectra from porcelain BCtter-Jensen et al. (1996) examined the emission characteristics of different porcelain samples by recording the time-steady PL emission spectra using a continuous scanning monochromator. UV stimulation was provided using a halogen lamp, filtered with a U-340 filter (peak transmission at 340 nm). Analyses of the spectral emission features of the crockery porcelain and glazes allow the possibility of identifying both the principal luminescent matrix, and luminescent defects contained within it. The PL emission spectra (excited by 340 nm light) from the bulk porcelain and the glaze are shown in Fig. 6.17a,b (BCtter-Jensen et al., 1996). The structures observed in the PL spectra of porcelain were identified by comparing these with TL emission spectra obtained from known artificial phosphors such as calcium sulphate doped with dysprosium (CaSO4:Dy) and aluminium oxide doped with carbon (A1203:C). Such TL spectra are shown in Fig. 6.17c. The bright emission peak near 700 nm from A1203:C is consistent with the observations by Akselrod and Kortov (1990) and Kortov et al. (1994) who identified this emission as an internal transition of Cr 3+, a very common impurity of this material (see also, Chapter 3, Section 1 0.8
1
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,
.
.
.
.
.
350 400 450 500 550 600 650 700 750 Wavelength (nm)
350 400 450 500 550 600 650 700 750 Wavelength (nm) 1-251~ I
[
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450
500
550
600
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Wavelength (nm) Fig. 6.17. (a) PL plotted against wavelength for two domestic bulk porcelain samples. The emission from A1203 is demonstrated by the typical peaks at 410 and 700 nm (from BCtter-Jensen et al., 1996). (b) PL spectra for two glaze samples. Sample 2G is a white glaze and shows emissions from A1203 and Dy 3+. Sample 6G is a clear glaze also showing emissions from A1203 (from BCtter-Jensen et al., 1996). (c) TL spectra from two artificial phosphors, namely (i) CaSOa:Dy and (ii) A1203:C. The results suggest a similarity of the porcelain with that of A1203:C, with some Dy 3+ impurities present (from Poolton et al., 1995).
Retrospective OSL Dosimetry
271
3.1). Typically, this comprises a main emission at 693 nm, with satellite lines at 670, 714 and 740 nm, at relative intensities depending on the doping concentrations (Lapraz et al., 1991). The broad emission band from A1203:C peaking at --~410 nm (also seen in Fig. 6.16c) corresponds directly with the well-known F-centre emission arising from the 3P ---, IS transition (Lee and Crawford, 1979; Akselrod and Kortov, 1990, and Chapter 3). The spectra obtained from CaSO4:Dy clearly show the sharp emissions at 490 and 580 nm, which represent the distinct blue-green and yellow emission signals caused by the Dy 3+ dopant (e.g., McKeever et al., 1995). The PL spectra from the bulk porcelain samples (see Fig. 6.17a) show identical emissions at 410 and 700 nm and thus indicate that the principal luminescent matrix of bulk porcelain is A1203. The PL spectra from the glaze (see Fig. 6.17b) show peaks at 410, 490 and 580 nm that identify emissions from both A1203 and Dy 3+. It is well known that A1203 is a frequently used component of bulk porcelain matrixes and both A1203 and Dy 3+ are components often included in glazes used as decorations on crockery porcelain.
6.9.2.3. OSL stimulation spectra The OSL stimulation spectra, i.e., OSL versus stimulation wavelength, for a Chernobyl toilet porcelain and the associated glaze have been obtained using a scanning monochromator in the visible range using a U-340 detection filter. These spectra are shown in Fig. 6.18a (Poolton et al., 1995). A prominent broad transition is observed peaking around 540 nm (particularly in the glaze), together with a rising continuum at lower wavelengths. It is noted that the occurrence of the 540 nm feature is unlikely to arise from quartz, where only structureless excitation characteristics have been reported previously (BCtter-Jensen et al., 1994). An OSL excitation spectrum from another typical porcelain sample is shown in Fig. 6.18b (BCtter-Jensen et al., 1996) and a similar smooth stimulation resonance is seen, but around 500 nm in this case, well matched to stimulation sources producing light around 470 nm. 6.9.3. OSL dose response of porcelain Typical OSL decay curves for a porcelain sample, given 6~ gamma doses from 30 mGy to 2 Gy, are shown in Fig. 6.19a (BCtter-Jensen et al., 1996). As quartz and A1203 are considered major OSL sensitive components in the porcelain, pre-heating at 150~ for 30 s is recommended to remove unstable components before any OSL readout in attempting to stabilize and reproduce the signal. The dose-response curves, i.e., OSL versus 6~ gamma dose, are shown for three porcelain samples in Fig. 6.19b. In general, the OSL sensitivity of porcelain glaze is more than one order of magnitude higher than that of bulk porcelain; this effect is ascribed to the high content of A1203 and Dy 3+. Unfortunately, glazes are not suitable for OSL dosimetry since the OSL signal from this surface material will, in most cases, be bleached by the ambient light. For most porcelain samples, the OSL signal increases linearly from 10 mGy up to about 20 Gy and shows a further sub-linear increase up to at least 200 Gy. Using blue-green light simulation with sensitive fired porcelain samples has allowed doses lower than 50 mGy to be measured with a statistical uncertainty of 10% and the lower detection level was determined to be
272
Optically Stimulated Luminescence Dosimetry 2.5 I~~
k l
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l
'
l
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I
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(a)
~. 2.0 r
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8
1.0 0.5 "p
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450
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Wavelength (nm)
5
(b)
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450
m
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9
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Fig. 6.18. (a) The dose-dependent OSL excitation characteristics (OSL versus wavelength) for a Chernobyl porcelain toilet tank sample (from Poolton et al., 1995). (b) The OSL stimulation spectrum obtained from a domestic porcelain sample (from BCtter-Jensen et al., 1996).
about 10 mGy (BCtter-Jensen et al., 1996). The fading of the OSL signal from a porcelain sample has been shown to be negligible over one month (Poolton et al., 1995). 6.9.4. Dose-depth profiles in porcelain and the effect of transparency BCtter-Jensen et al. (1997) measured the OSL dose-depth profile from a ceramic fuse collected at the nuclear accident site in Chernobyl. An 8 mm diameter (12 mm long) core was drilled across the fuse and sliced into lmm-thick discs. The normalised doses evaluated from each disc as a function of the depth into the material are shown in Fig. 6.20a. The dose-depth curve shows a bleaching effect on the OSL signal in the outer layers of the material. Thus, the transparency of porcelain and the consequent bleaching effect caused by ambient daylight has to be considered. BCtter-Jensen et al. (1997) consequently carried out an experiment using a 12 mm long porcelain core that was given a uniform 137Cs gamma dose of 2 Gy at fight angles to the long axis and subsequently placed in sunlight for 8 h so that only one end of the core was illuminated. Discs (1 mm thick) sliced from the core had their OSL signals measured. For comparison TL measurements were made on the same discs. The doses evaluated by OSL and TL are plotted against depth into the ceramic in Fig. 6.20b. It is clear that samples for both TL and OSL measurements must be taken at a depth of at least 2 mm in order to be unaffected by
Retrospective OSL Dosimetry
273
16 (a) o o o
o
12 ~
~
Gy 1.0 Gy 0.5 Gy 0.17 Gy
8
x _J
m 0
4 (
0
,
5
10
15 20 Time (s)
25
30
1 oo 0 0
"-0.8 xi. O
0.6 o -o 0.4 09
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t--
0.2
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Dose (Gy)
Fig. 6.19. (a) Typical OSL decay curves from domestic porcelain representing different doses from 30 mGy to 2 Gy 6~ gammaradiation. Stimulation: broad-band (420-550 nm), 16 mW/cm2. (b) OSL versus 6~ gamma dose for three different domestic porcelain samples (from BCtter-Jensen et al., 1996). the ambient daylight. This suggests that thin porcelain items (crockery) are unsuitable in dosimetry applications, because the entire body of the material will be significantly affected by daylight exposure. 6.9.5. OSL dosimetry using porcelain dental crowns Bailiff et al. (2002) investigated the OSL properties of porcelain dental crowns with the aim of using these as retrospective dosimeters after nuclear accidents. Dental ceramics, because of their intimate contact with the human body, are of interest as a luminescence dosimeter material because of their potential to provide a means of determining cumulative exposure to external gamma radiation arising from accidents or large-scale incidents involving population groups. The term dental ceramics is used to describe materials including porcelain and glass-ceramic materials that are employed in the construction of tooth crowns, restorative components of teeth and prosthetic teeth. Dental ceramics may have some luminescence characteristics in common with domestic porcelain, although the composition of the former generally differs from that of domestic porcelain, having a high proportion of feldspar (80% versus 15%) relative to kaolin (15% versus 70%) to achieve translucent quality (Bailiff et al., 2002). Previous work by Davies (1979) demonstrated the feasibility of the use of both thermally stimulated exo-electron
274
Optically Stimulated Luminescence Dosimetry
(a)
v ffl
a0
2
4
6
8
10
Depth (mm)
2,5
(.9
1,5
o
1
121
(b)
0,5 .
0
1
,
,
2
3
4
5
Depth (mm)
Fig. 6.20. (a) Relative accident OSL dose versus depth into a ceramic fuse. Procedure: slicing of cross-section core into discs and subsequent measurementof individual OSL signals. (b) Relative OSL and TL doses versus depth into a ceramicfuse after exposing a core to a uniform dose of 2 Gy 137Csgammaradiation and subsequently placing the core in daylight for 8 h such that only one end was illuminated. See text for details (from BCtter-Jensen et al., 1996). emission (TSEE) and TL techniques for determination of absorbed dose using dental porcelain. Later work by Mauricio et al. (1985) further underlined the potential of the material for accident dosimetry; a TL peak located at 270~ was found to be linear with the absorbed dose over a wide range (400 m G y - 4 0 0 Gy). Bailiff et al. (2002) measured OSL and IRSL decay curves from prosthetic tooth and crown enamel using different optical stimulation sources: (i) filtered spectrum ( 4 2 0 5 5 0 n m ) from a halogen lamp, (ii) IR LEDs (875 ___ 8 0 n m ) and (iii) blue LEDs (470 ___ 40 nm). The OSL intensity was generally found to be too weak for samples of crown ceramic using either halogen lamp or IR LED stimulation; however, significant improvements were obtained by using blue LED stimulation instead of the halogen lamp and by measuring the infra-red stimulated luminescence (IRSL) at an elevated temperature of 140~ (70% increase). The forms of the OSL and IRSL decay curves measured with either type of dental ceramic were not described by first-order kinetics, i.e., not of single exponentials. Examples of IRSL decay curves measured with prosthetic tooth are shown in Fig. 6.21a. However, the initial part of the decay approximates to an exponential form,
Retrospective OSL Dosimetry ,
.
,
9
|
.
,
.
275 |
.
|
3000 250O 2000
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500
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20
40
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80
100
Time of IRSL [sec]
,,___,
1o00oo
(b)
80000 60000 o
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40000 20000 0
,
0
,
1
,
2
3
Absorbed Dose [Gy] Fig. 6.21. (a) IRSL decay curves measured with aliquots of prosthetic tooth after administration of different beta doses. The aliquots were pre-heated at 160~ for 100 s before measurement and held at 140~ during IR stimulation. (b) IRSL growth characteristics obtained with aliquots of prosthetic tooth at room temperature (open squares) and at 140~ (filled squares) (from Bailiff et al., 2002).
with a halving of the initial intensity after 4 and 8 s for OSL and IRSL decay curves, respectively. The general form of the dose-response characteristics for the OSL and IRSL signals were found to be linear (experimental error of ___5%) within the dose range 100 m G y - 1 0 Gy for samples that had been [3-irradiated and then subjected to a pre-heat treatment. An example of an IRSL growth characteristic for prosthetic tooth ceramic is shown in Fig. 6.2 lb where the measurements were performed at both ambient and elevated (140~ sample temperatures.
6.10. Retrospective accident dosimetry--conclusions OSL techniques with ceramic materials, as discussed so far in this chapter, appear to have widespread suitability for retrospective accident dosimetry. The application of the SAR method for the estimation of equivalent dose in quartz extracts from modem house bricks exposed during a nuclear accident has been illustrated. High precision (1% S.E. for n = 15) in the measurement of equivalent dose is readily achievable, and with present methods a detection limit of about 10 mGy for the accident component on a natural
276
Optically Stimulated Luminescence Dosimetry
background of 50 mGy can be estimated. To improve this detection limit significantly, uncertainties in the estimates of the natural dose-rates must be reduced. The use of bricks from walls to obtain dose-depth profiles provides important information concerning the nature of the time-averaged incident external photon field. Because of their speed and high precision, the new OSL measurement techniques have shown major advantages in routine dose evaluations and in the evaluation of dose-depth profiles using cut brick sections. Porcelain provides a widely available dosimeter material for measurements in shielded locations and its glazed surface also provides the advantage of low fallout retention when used in exterior locations. In the case of populated areas that have received radioactive fallout, the combined use of luminescence and computational modelling provides a means of validating calculated values of absorbed dose in air for contaminated areas and to provide dose values for subsequent modelling of dose to population groups within the area studied. The validation and use of models supported by direct measurements is crucial to epidemiological investigations and subsequent arrival at a more accurate assessment of risk to members of the population exposed to ionising radiation. The selection of appropriate samples is one of the most important aspects of retrospective accident dosimetry since the interpretation and use of the results relies heavily on assumptions made concerning the relationship between the sample and the radiation sources contributing to the transient dose. So far a comprehensive sampling methodology is yet to emerge. Wider use of the method is likely to accelerate the demand for standard procedures to be established so that the selection of appropriate samples for both accrued dose and dose rate evaluation, according to the type of building and dosimetry problem, can be optimised.
P a r t II: G E O L O G I C A L
AND
ARCHAEOLOGICAL
DATING
Another major application of retrospective dosimetry is the dating of unfired sedimentary materials and heated archaeological ceramics. Luminescence dating offers the only direct method for dating geological and archaeological sedimentary events that have occurred in the last 250,000 years (250 ka) and it is becoming increasingly the method of choice (Murray and Olley, 2002). To calculate an age requires a knowledge of both the dose and the dose rate. The latter is derived from either direct measurement or radionuclide concentrations (Aitken, 1985). This section discusses methods of determining the dose received since the event of interest.
6.11. Measurement procedures Considerable detail regarding various OSL measurement procedures has already been given in Section 6.5 with reference to the measurement of nuclear accident doses, particularly those received by fired materials such as bricks. Here we consider both multiple-aliquot procedures, and various single-aliquot procedures that have been more fully developed for the dating of sediments.
Retrospective OSL Dosimetry
277
6.11.1. Multiple-aliquot methods Multiple-aliquot procedures for determining the radiation dose received by mineral grains involve the use of a number of nominally identical sample aliquots, some of which would have received only the dose that needs to be determined, whilst the others would have received a laboratory dose. This approach was adopted for OSL dating from preexisting methods used in TL dating. In TL dating, the luminescence signal is destroyed by the act of measurement, thus only one measurement can be made per aliquot, apart from the measurement of a signal derived from a subsequent test dose. This type of measurement was used to provide a method of normalisation based on the TL sensitivity of the grains making up each aliquot; it is termed "second glow normalisation". In OSL dating, the signal may also be totally removed by the measurement, with the decay curve being taken until the OSL is less than 1% of the initial value. These decay curves for the natural- or laboratory-irradiated aliquots can then be used to construct the growth curves. Either the integrated OSL signal can be used or the signal can be broken down into components that correspond to different parts of the stimulation curve. Two multiple-aliquot approaches have been derived, the so-called additive-dose and regenerative-dose methods. In regenerative-dose procedures the OSL signal is zeroed in a way that is analogous to the zeroing that took place in nature (i.e., exposure to light) and doses are subsequently given to construct an OSL growth curve up to and just above the natural OSL level. The equivalent dose is then obtained by projecting the natural OSL level onto the growth curve (Fig. 6.22a). In additive-dose procedures, the aliquots are given additional radiation doses that will increase the signal above the level due to the natural irradiation. In this method, the equivalent dose is obtained by extrapolation (Fig. 6.22b). Used in this simple way, both approaches have their limitations, which can be discussed in terms of the OSL properties of quartz that were discussed in Chapter 5. First, in order to be able to plot such curves, it is assumed that each aliquot is identical to every other, or that some method to normalise between the aliquots is available. Weighing every aliquot (--~5 mg) to the required level of precision (1%) to use as a correction factor is tedious. For many sandy samples, weighing would be inappropriate since only a small percentage of the grains give an OSL signal. Fig. 6.23 gives the total light sum for the natural OSL of three samples of dune sand. Only about 30% of the grains in each case had OSL signals that were distinguishable from the background signal of the photomultiplier tube. For all three samples, 10% of the grains would give rise to 9 0 - 9 5 % of the signal. These results imply that 9 0 - 9 5 % of the natural signal from a 5 mg multiple-grain aliquot would come from 50 grains out of the 500 being stimulated. This leads to the use of a normalisation procedure based on the initial natural OSL signal (e.g., resulting from a 0.1 s exposure to the light source) from each aliquot. This requires the grains to be firmly mounted on the discs, so that the grains do not move during subsequent irradiation and heating prior to each OSL measurement, or during light exposure for multiple-aliquot regenerative-dose measurements. This "natural normalisation" requires the grains making up each aliquot to have received the same dose during burial. Secondly, it is assumed that the response as measured for the laboratory-irradiated aliquot, is identical to that relating to a naturally irradiated aliquot. There are several reasons why this might not be the case. For example, the dose rates employed in the
Optically Stimulated Luminescence Dosimetry
278 coo 30 t F5-MAR 25
9 n~ 5 0 - F5-SAR uncorr.
i
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Fig. 6.22. Multiple and single-aliquot growth curves for quartz OSL from a Holocene dune sand from Germany. (a) Data set for MAR protocol, using 13 natural measurements (open circles) and 35 laboratory-irradiated aliquots (filled circles) giving a De of 9.7 +- 1.3 Gy, (b) data set for MAA protocol, giving a D e of 8.0 -- 0.7 Gy obtained using similar number of aliquots, (c) raw data for one aliquot obtained using SAR protocol, with the natural signal shown as a horizontal line, and (d) the data from (c) corrected using the OSL signal from the test dose given after each measurement. The D e obtained was 9.4 _ 0.3 Gy. The insets in (c) and (d) are the data for doses below 3 Gy, and demonstrate the minimal amount of recuperation (open squares) and good reproducibility after correction shown in (d) (open diamonds and triangles) (from Hilgers et al., 2001).
laboratory are m a n y orders of m a g n i t u d e higher than those experienced by grains in their natural environment. This m a y lead to thermally unstable traps being filled during a laboratory irradiation, and it has been suggested that laboratory irradiation should be carried out at elevated temperatures in order to maintain trapping conditions similar to those in nature (Bailey, 2001). Also, as concluded in Section 5.1.8.2, the efficiency of l u m i n e s c e n c e production for a naturally irradiated aliquot of quartz m a y be higher than that for an equivalent laboratory irradiated aliquot. M u r r a y and Wintle (2000) suggested that this is the result of a sensitisation process that occurs during storage at ambient temperature. It is of particular importance for samples that are more than a few thousand years old and are from hot climatic regions. The effect can be o v e r c o m e by using the O S L response to a test dose, as in the SAR protocol (see Section 6.5.4.5). Thirdly, any multiple-aliquot procedure requires a large n u m b e r of aliquots in order to obtain a single value for De. This m e a n s that it is only practicable to m a k e a single
Retrospective
OSL
100
Dosimetry
.~.. .~.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
//--
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Fig. 6.23. Distribution of the natural OSL intensity from over 1000 single grains from three samples. The percentage of the total light sum is plotted as a function of the specified percentage of all the grains measured (from Jacobs, pers. comm.).
determination of De and thus it is not possible to build in any checks that might permit assessment of whether an appropriate pre-heat has been applied. The data in Fig. 6.22 are for a sample of dune sand from north-eastern Germany (Hilgers et al., 2001). The laboratory-irradiated quartz grains used in the multiple-aliquot regenerative-dose (MAR) growth curve (Fig. 6.22a) were exposed to sunlight for several hours prior to gamma irradiation. Natural normalisation was used for the data shown in Fig. 6.22a,b, which is the equivalent data set for the MAA method. For this sample, the values of D e w e r e quoted as 9.7 __+ 1.3 Gy for MAR and 8.0 _ 0.7 Gy for MAA. It is clear from the figures that there is a large degree of scatter, despite the use of a normalisation procedure. For the 11 samples measured in the same way, similar scatter was found leading to uncertainty in D e o f "~ 10-50% (Hilgers et al., 2001). Given the degree of reproducibility found for SAR measurements on the same set of samples (see Sections 6.5.4.5 and 6.11.2.2.2), Hilgers et al. (2001) concluded that the scatter for the multiple-aliquot data was not caused by the presence of grains with a wide range of dose. This was to be expected intuitively for a clearly aeolian deposit. Another possibility for scatter could be related to the fact that the natural normalisation measurements were made on grains that had not been heated. Thus, grains with different degrees of natural sensitisation may have been present, and their properties were altered to different extents by thermal treatment applied prior to the main OSL measurement, namely 220~ for 300 s. This could have been taken into account if the OSL response to a test dose was obtained after each OSL measurement.
280
Optically Stimulated Luminescence Dosimetry
6.11.2. Single-aliquot methods 6.11.2.1. Feldspars 6.11.2.1.1. Additive dose. Single-aliquot procedures were first explored for the dating of sand-sized feldspar grains using the IRSL signal by Duller (1991; 1994; 1995) (see Section 5.2). Duller tried two approaches. One was based on an additive-dose method, which is feasible when only the initial part of the IRSL decay curve is used as the signal. The second procedure involved complete removal of the IRSL signal by the IR exposure used for measurement, followed by repeated cycles of irradiation and IRSL measurement in order to construct the regenerated OSL versus dose curve. In each case, only a single aliquot was needed to produce a growth curve. This possibility was first mentioned by Huntley et al. (1985). Both procedures require the application of a pre-heat to remove any thermally unstable signal. Studies by Li (1991) indicated that a pre-heat for 10 min at 220~ would be appropriate and this was widely adopted. The short stimulation required to make the measurement is typically an IR exposure of--~ 20 mW/cm 2 (e.g., 0.5 s with the power at the sample being --~40 mW/cm2). This stimulation will cause a 4% drop in the IRSL signal (Duller, 1994). In addition, this pre-heat causes the signal to undergo thermal decay. Repeated heating and IRSL measurement results in a decay curve similar to that shown in Fig. 6.24b, for which the initial drop is close to 20%. In the additive-dose method it is necessary to correct for the loss that occurs for every measurement used to construct the growth curve. The uncorrected OSL measurements are shown in Fig. 6.24c, which also shows the effect of the "luminescence correction" method described by Duller (1994). In this correction, the signal from each additional irradiation is treated separately. The correction factors applied to each component are derived from the decay curve shown in Fig. 6.24b, which is obtained using an additional aliquot of natural sample. This method is appropriate for those samples with a dose response that is close to linear. For non-linear growth of OSL with dose, another correction procedure needs to be appliedmnamely, the "dose correction" method (Duller, 1994), which is illustrated in Fig. 6.24d. The appropriateness of each of the correction procedures can be ascertained by continuing the cycles of pre-heat and IRSL measurement, but giving no additional irradiation. For the "luminescence correction" method, the values obtained after correction should be identical. For the dose correction method, the corrected data should fall on the growth curve, but not on top of each other (Duller, 1994). It should be noted that two sub-samples are actually needed for these additive-dose methods, one being required to quantify the decay, as in Fig. 6.24b. Galloway (1996) developed an empirical approach that used only one aliquot. It was based on there being a fixed signal loss as a function of each measurement cycle. This loss is determined using repeated cycles with no dose at the end of the additive-dose measurements. The loss per cycle is then applied to each data point used to construct the additive-dose growth curve. A similar approach was taken by Zhang et al. (2001). 6.11.2.1.2. Regenerative dose. The single-aliquot regenerative-dose procedure outlined by Duller (1994, 1995) is simpler to apply, since it does not require a correction for signal loss with repeated measurement. A growth curve is constructed as in the multiple-aliquot procedure; however, measurements are made consecutively. To determine
Retrospective OSL Dosimetry (a)
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Fig. 6.24. IRSL data set obtained for the SAAD protocol as applied to feldspars from a dune sand from New Zealand. (a) Raw data with no correction for the effect of pre-heating and the IRSL measurement, (b) the loss of IRSL due to repeated heating and IRSL measurement, with data obtained for seven other aliquots, (c) data from (a) corrected using the data in (b) with the luminescence correction method of Duller (1994), and (d) as for (c) but using the dose correction method of Duller (1994) (from Duller, 1995).
the correct value of De, no sensitivity changes must occur as a result of repeated cycling. This was not found to be the case, with the measured sensitivity increasing with cycle. There is also a dependence upon the spectrum of the light used to remove the IRSL signal (Richardson, 1994). This procedure was thus abandoned, until sensitivity monitoring within a SAR protocol was used (Wallinga et al., 2000a). With this procedure, reproducible values of De were given by potassium-rich feldspars that had been separated from sediments. However, ages derived using these values were consistently too young compared to both the quartz OSL ages and independent ages (Wallinga et al., 2000b). The age underestimation has been suggested to be related to changes in optical absorption as a result of pre-heating and light exposure (Wallinga and Duller, 2000), changes in electron trapping probability as a consequence of heating (Wallinga et al., 2000b) or anomalous fading (Huntley and Lamothe, 2001) or a combination of all three processes. 6.11.2.2. Quartz 6.11.2.2.1. Additive dose. The possibility of using single-aliquot procedures was put forward by Smith et al. (1986). They suggested that it would be possible to construct an additive-dose growth curve using very short stimulation times. Stokes (1994) presented
282
Optically Stimulated Luminescence Dosimetry
experimental results that showed an apparent lack of dose-dependent sensitivity changes. He thus deduced that it would be possible to derive a SAAD procedure. The experiments used a pre-heat of 16 h at 160~ which would not be practicable in routine dating. Galloway (1994) applied Duller's method to samples of heated quartz, but gave no initial dose. He used a short pre-heat (200~ for 1 min) and used 10 s exposure to the green LED stimulation source (approximate power at sample 0.2 mW/cm 2 and peak emission at 565 nm). The growth curves that he constructed for single aliquots were similar to those obtained using multiple aliquots. The approach developed by Galloway (1996) for feldspars (see Section 6.11.2.1.1) was adopted for quartz (Liritzis et al., 1997). Indeed the application to quartz was simpler than for feldspars. The decay of the OSL signal on repeated cycles of heating (220~ for 1 min) and OSL measurement was found to be exponential with cycle (Liritzis et al., 1997); thus, the percentage correction is the same for each component of signal resulting from each added dose. This exponential decay can be determined from additional measurements at the end of the additive-dose sequence, making it a true single-aliquot procedure. These data are used in an iterative least squares fitting correction (Galloway, 1996). This procedure has been applied to samples of sedimentary quartz and to quartz from small fragments of pottery taken from sub-surface drill cores. The OSL results for the ceramics fitted well with the radiocarbon chronology for the cores from which the fragments were taken (Liritzis et al., 1997, 2001). Other studies on pottery gave results in agreement with the archaeological evidence (Hong et al., 2001). However, for the sediments, no independent age control was available to confirm the results obtained for quartz (Hong and Galloway, 2000). However, Hong and Galloway (2000) demonstrated that values of De obtained using blue LEDs (420 nm and with 5 mW/cm 2) were in agreement with those obtained on the same sample using green LEDs (475 nm) giving the same power to the sample. Using the blue diodes gave better precision, however, owing to the higher OSL output achieved using this wavelength. Although these procedures gave acceptable results for the ceramic samples, it is necessary for the absolute luminescence sensitivities to be identical for both natural- and laboratory-irradiated aliquots. Wintle and Murray (1999) have shown the sensitivity for a sample of sedimentary quartz to be critically dependent upon time and temperature, whether in nature or relating to laboratory pre-heats. For the dating studies reported by Liritzis et al. (1997, 2001) and Hong et al. (2001), it may have been fortuitous that the 1min pre-heat at 220~ resulted in similar luminescence efficiencies. To support the method, Hong et al. (2000) carded out experiments to characterise the behaviour of the OSL signal when subjected to repeated cycles of pre-heat and OSL readout. Each measured decrease could be expressed as a function fin), where f(n) = exp[-c(n - 1)], where n is the number of measurements made on the aliquot and c is a parameter that depends upon pre-heat temperature and duration. This can be rewritten as f ( n ) = r (n-l~, where r = exp[-c] is the ratio of two successive measurements. Measurements made without the pre-heat showed the loss in OSL to be 5% from the optical stimulation alone. In 1997, Murray et al. also reported the exponential decay of OSL with stimulation cycle for several sedimentary quartz samples (Fig. 6.25). This led them to propose a single-aliquot additive dose (SAAD) method, again evaluating the exponential decay with
Retrospective OSL Dosimetry E
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Fig. 6.25. OSL signals obtained by 0.1 s stimulation after repeated 10 s pre-heats. (a) Data shown for two preheat temperatures, 200 and 280~ and (b) for another sample for 280~ only. The semi-log plot indicates the loss is exponential. Data are shown for the natural OSL and for aliquots given an additional dose ranging from 1.7 to 215 Gy. The data suggest only weak dependence upon the dose (from Murray et al., 1997).
measurements at the end of the sequence (Fig. 6.26). This approach is essentially identical to that of Liritzis et al. (1997). One of the assumptions in the SAAD method is that the depletion rate is dose independent. In fact, careful inspection of the data in Fig. 6.25 suggests that there is a weak dependence of the decay constant on dose, as was pointed out by Murray et al. (1997). For the temperatures shown in Fig. 6.25a, the direction of change is different for different temperatures. For the 200~ pre-heat (10 s) the slope decreases from 0.090 to 0.073 with increasing dose, whereas for 280~ the slope increases from 0.24 to 0.29 over the same dose range. For both the samples in Fig. 6.25, the laboratory dose of 215 Gy far exceeds the natural values of De, which are --- 3 Gy for DS2 (Figs. 6.25a and 6.27c) and --- 50 Gy for WIDG8 (Figs. 6.25b and 6.27e). If the effect were dose dependent at this level, then it would have a negligible effect on SAAD dating of DS2. Given the different response for the two pre-heats, the effect is most likely to be related to changes in the luminescence efficiency brought about by continuous application of the 10 s pre-heats. Additional studies by Murray et al. (1997) showed the value of De to be independent of the stimulation temperature employed (using 25, 110, 160 or 200~ However, different values of De were obtained when different pre-heat conditions were employed (Fig. 6.27).
284
Optically Stimulated Luminescence Dosimetry 2x104
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Fig. 6.26. Additive-dose growth curve obtained using a single aliquot of quartz. Squares show the data after correction using the data points from the decay cycle. De ("~ 3 Gy) is obtained by extrapolation (from Murray et al., 1997).
For young sediment samples, giving De values of 0.4-3 Gy (Figs. 6.27b,c) there is little dependence on the pre-heat temperature. For the very young sample (Fig. 6.27a) with De ~" 0.03 Gy, the De plateau is destroyed by thermal transfer effects (see Section 5.1.8.5). For the older samples, shown in Fig. 6.27d,e, the values of De for low temperature preheats (up to 280~ are severely overestimated. This overestimation was shown to be due to the effects of luminescence sensitivity change (Wintle and Murray, 1999), rather than thermal transfer from a peak at --~280~ as originally suggested by Murray et al. (1997). From such data, it can be seen that higher pre-heats are required for older samples in order for the laboratory sensitivity to be made equivalent to that pertaining to the naturally irradiated sample. Although the SAAD approach was reported to work on 13 (out of 15) Australian sedimentary quartzes (Murray et al., 1997), some problems have been reported. Stokes et al. (2000) report a 64% failure rate, particularly for fluvial quartz from Egypt and the River Loire in France. These samples could be identified by a dip in their uncorrected growth curve (Fig. 6.28). The problem could also be seen by analysis of the decay in OSL signal with cycle (Fig. 6.29). This graph shows the decay observed for 15 repeated cycles, using a pre-heat of 10 s at 250~ and blue LED stimulation with 24 mW/cm 2 at 125~ The exponent obtained for the initial five points and the last five points is different. After 6 cycles at the beginning of the decay curve (using exp- o.1122,,, where n is the cycle number) 51% of the signal would be left, whereas using the exp- 0.0839n, 60% of the signal would be left. Clearly this deviation from a single exponential function prevents the application of the SAAD protocol. A check for exponential decay over the 15 or so cycles used to construct the growth curve and the final decay curve should be carried out as a preliminary check. By measuring the 110~ TL peak generated by a small test dose, Stokes et al. (2000) demonstrated that the sensitivity did not remain constant for the duration of the measurement cycles. Having discovered the magnitude of the sensitivity changes caused by even a short preheat (10 s), and more importantly the different magnitudes of the sensitivity changes for
Retrospective OSL Dosimetry 0,2
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natural- and laboratory-irradiated aliquots of Australian quartz, Wintle and Murray (1999) attempted to correct for the sensitivity change. They used the 110~ TL response to a small test dose given at the end of each OSL measurement in the additive-dose measurement sequence. The OSL data were corrected by dividing by the subsequent TL response, and the additive-dose growth curve was then constructed as before. The result is shown in Fig. 6.30. It can be seen that D e is now independent of the pre-heat temperature and the mean value of De is close to that obtained using other methods, i.e., 52 Gy. 6.11.2.2.2. Regenerative dose. The SAR protocol for OSL dating of quartz has been described in Section 6.5.4.5, where it was applied to heated quartz. The main feature of the protocol is the use of an OSL response (Tx) to a test dose given immediately following the natural OSL measurement (Ln) and after each regeneration-dose measurement (Lx). It is implicitly assumed that both the main and test-dose OSL signals are derived from the same electron trap, i.e., that giving rise to the fast component of the OSL (Section 5.1.2.4).
286
Optically Stimulated Luminescence Dosimetry 50
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If other components are present, e.g., an ultra-fast component is present following laboratory irradiation or the fast component is not dominant, the SAR protocol may be inappropriate. The experimental procedure, outlined in Table 6.1 for heated quartz (Section 6.5.4.5), is that originally proposed for sediments. The pre-heat made prior to measurement of the main OSL signals is designed to detect, and allow the removal of, any thermally-unstable component of the OSL signal that may be induced by laboratory irradiation. The pre-heat, selected from within the range 160-300~ is applied for 10 s, a time convenient for automated OSL readers. Such thermal treatments have been shown to result in sensitivity change in some samples (e.g., Wintle and Murray, 1999; Section 5.1.8.2). Repeated application of the pre-heat during the course of the SAR cycle has been shown to result in
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Retrospective OSL Dosimetry
287
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50 mW/cm 2 (BCtter-Jensen et al., 2000). New features are (1) a dual green/IR (532/830 nm) solid-state laser based single grain OSL attachment that can accommodate 48 sample discs, each containing 100 grain hole positions in a 10 x 10 grid (Duller et al., 1999b; BCtter-Jensen et al., 2000) and (2) a RL attachment with an extra heater beneath the irradiator for providing temperature-dependent RL measurements (Poolton et al., 2001). A software-controlled beta irradiator attachment for in situ irradiation of samples is also provided. A recently developed sequence editor software has significantly extended the flexibility and measurement capabilities. The address is: Rise
OSL Measurement Technology
345
National Laboratory, Radiation Research Department, Building 201, DK-4000 Roskilde, Denmark. Landauer provides an InLight TM system that utilises OSL technology known as 'Sapphire Technology'. The system is based on a LED light source for stimulating the A1203:C detector material. The InLight TM system has a linear dose response up to 10 Gy, and uses two beams, one weak and one strong, for extending the dynamic range.
7.12. Future developments In this chapter, techniques and methods applied in luminescence dating and dosimetry at many laboratories around the world have been reviewed and an attempt has been made to describe the state-of-the-art in instrument and method development. One outstanding problem, which remains to be addressed in the development of combined TL/OSL instrumentation using different stimulation light spectra, is the design of a flexible optical detection filter changing system to allow for rapid (automatic) selection of the optimal detection window whether using IR or visible light stimulation. Changing of excitation or detection filters may, if not properly protected either by hardware or software, cause serious damage to the PM tube because of insufficient suppression of stray light from the stimulation light source. The growing industrial interest in ultra-bright LEDs as light indicators (e.g., from automobile manufacturers) may soon make visible LEDs commercially available with a greater variety of emission wavelengths and substantially higher emission power than is available today. These LEDs should provide sufficient power to be considered a real alternative to lasers and powerful incandescent lamps as light sources in OSL. Major effort will no doubt still be put into the development of sensitive systems capable of measuring luminescence from small aliquots, even down to single grains so that variations in dose from grain to grain can be studied in detail. The latter feature will be especially valuable in studies of incompletely bleached materials in dating and retrospective accident dosimetry. In particular, methods for single grain spectroscopy at doses found in field samples still have to be developed. Further developments in, and investigations of, luminescence imaging systems for obtaining spatially resolved TL and OSL signals from multi-mineral samples are also foreseen. These systems give rapid and valuable information about the mineralogy of the sample and enable individual analysis of luminescence signals from single grains of a sample. This has the potential to avoid the cumbersome mechanical and chemical separation processes presently required. Thus, it will be possible to map solid surfaces containing grains with different OSL sensitivities and different doses.
References Aitken, M.J., 1985. ThermoluminescenceDating. Academic Press, London. Aitken, M.J., 1990. Optical dating of sediments: Initial results from Oxford. Archaeometry32, 19-31. Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after heating. Quat. Sci Rev. 7, 378-393.
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Optically Stimulated Luminescence Dosimetry
Akselrod, M.S., McKeever, S.W.S., 1999. A radiation dosimetry method using pulsed optically stimulated luminescence. Radiat. Prot. Dosim. 81, 167-176. Akselrod, M.S., Agersnap Larsen, N., McKeever, S.W.S., 2000. A procedure for the distinction between static and dynamic radiation exposures of personal dosimetry badges using pulsed optically stimulated luminescence. Radiat. Meas. 32, 215-225. Alanso, P.J., Halliburton, L.E., Kohnke, E.E., Bossoli, R.B., 1983. X-ray induced luminescence in crystalline SiO2. J. Appl. Phys. 54, 5369-5375. Bailiff, I.K., 1993. Measurement of the stimulation spectrum (1.2-1.7 eV) for a specimen of potassium feldspar using a solid state laser. Radiat. Prot. Dosim. 47, 649-653. Bailiff, I.K., 2000. Characteristics of time-resolved luminescence in quartz. Radiat. Meas. 32, 401-405. Bailiff, I.K., Barnett, S.M., 1994. Characteristics of infrared-stimulated luminescence from a feldspar at low temperatures. Radiat. Meas. 23, 541-545. Bailiff, I.K., Morris, D.A., Aitken, M.J., 1977. A rapid interference spectrometer: application to low level thermoluminescence emission. J. Phys. E: Sci. Instr. 10, 1156-1160. Bakas, G.V., 1984. A new optical multichannel analyser using a charge coupled device for thermoluminescence emission measurements. Radiat. Prot. Dosim. 9, 301-305. Barnett, S.M., Bailiff, I.K., 1997. The temperature dependence of luminescence in some feldspars (80-300 K). J. Appl. Phys. 30, 683-689. Bernhardt, R., Herforth, L., 1974. Radiation dosimetry by optically stimulated phosphorescence of CaF2:Mn. In: Newiadomski, T. (Ed.), Proceedings of the Fourth International Conference on Luminescence Dosimetry, Krakow, Poland, pp. 1091-1104. Bortolot, V.J., 1997. Improved OSL excitation with fiberoptics and focused lamps. Radiat. Meas. 27, 101-106. Bortolot, V.J., 2000. A new modular high capacity OSL reader system. Radiat. Meas. 32, 751-759. Bos, A.J.J., Winkelmann, A.J.M., Le Masson, N.J.M., Sidorenko, A.V., van Eik, C.W.E., 2002. A TL/OSL emission spectrometer extension of the Ris~ reader. Radiat. Prot. Dosim. 101, 111-114. BCtter-Jensen, L., 1978. A simple, hot N2-gas TL reader incorporating a post-irradiation annealing facility. Nucl. Instr. Meth. 153, 413-418. BCtter-Jensen, L., 1997. Luminescence techniques: instrumentation and methods. Radiat. Meas. 17, 749-768. BCtter-Jensen, L., 2000. Development of optically stimulated luminescence techniques using natural materials and ceramics, and their application to retrospective dosimetry. Rise-R-1211 (EN), DSc. Thesis, p.185. BCtter-Jensen, L., Duller, G.A.T., 1992. A new system for measuring OSL from quartz samples. Nucl. Tracks Radiat. Meas. 20, 549-553. B Ctter-Jensen, L., Murray, A.S., 1999. Developments in optically stimulated luminescence techniques for dating and retrospective dosimetry. Radiat. Prot. Dosim. 84, 307-316. BCtter-Jensen, L., Ditlevsen, C., Mejdahl, V., 1991. Combined OSL (infrared) and TL studies of feldspars. Nucl. Tracks Radiat. Meas. 18, 257-263. BCtter-Jensen, L., Duller, G.A.T., Poolton, N.R.J., 1994a. Excitation and emission spectrometry of stimulated luminescence from quartz and feldspars. Radiat. Meas. 23, 613-616. BCtter-Jensen, L., Poolton, N.R.J., Willumsen, F., Christiansen, H., 1994b. A compact design for monochromatic OSL measurements in the wavelength range 380-1020 nm. Radiat. Meas. 23, 519-522. BCtter-Jensen, L., Jungner, H., Poolton, N.R.J., 1995. A continuous OSL scanning method for analysis of radiation depth-dose profiles in bricks. Radiat. Meas. 24, 525-529. B~tter-Jensen, L., Mejdahl, V., Murray, A.S., 1999a. New light on OSL. Quat. Sci. Rev. 18, 303-309. BCtter-Jensen, L., Duller, G.A.T., Murray, A.S., Banerjee, D., 1999b. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosim. 84, 335-340. BCtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523-528. BCtter-Jensen, L., Bulur, E., Murray, A.S., Poolton, N.R.J., 2002. Enhancements in luminescence measurement techniques. Radiat. Prot. Dosim. 101, 119-124. Braiinlich, P., Gasiot, J., Fillard, J.P., Castagn~, M., 1981. Laser heating of thermoluminescent dielectric layers. Appl. Phys. Phys. Lett. 39, 769-771. Bray, H.E., Bailey, R.M., Stokes, S., 2002. Quantification of cross-irradiation and cross-illumination using a Rise TL/OSL-DA-15 reader. Radiat. Meas. 35, 275-280.
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Bulur, E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiments. Radiat. Meas. 26, 701-709. Bulur, E., G6ksu, H.Y., 1999. IR stimulated luminescence from feldspars with linearly increasing excitation light intensity. Radiat. Meas. 30, 505-512. Bulur, E., BCtter-Jensen, L., Murray, A.S., 2000. Optically stimulated luminescence from quartz measured using the linear modulation technique. Radiat. Meas. 32, 407-411. Bulur, E., B0tter-Jensen, L., Murray, A.S., 2001. Frequency modulated pulsed stimulation in optically stimulated luminescence. Nucl. Instr. Meth. B 179, 151-159. Burggraaf, D., Haskell, E.H., 1994. A software package for TL/OSL spectrometry and extraction of glow curves from individual grains. Radiat. Meas. 23, 537 (abstract only). Chithambo, M.L., Galloway, R.B., 2000a. A pulsed light-emitting diode system for stimulation of luminescence. Meas. Sci. Technol. 11, 418-424. Chithambo, M.L., Galloway, R.B., 2000b. Temperature dependence of luminescence time-resolved spectra from quartz. Radiat. Meas. 32, 627-632. Clark, R.J., Bailiff, I.K., 1998. Fast time-resolved luminescence emission spectroscopy in some feldspars. Radiat. Meas. 29, 553- 560. Clark, R.J., Sanderson, D.C.W., 1994. Photostimulated luminescence excitation spectroscopy of feldspars and micas. Radiat. Meas. 23, 641-646. Clark, R.J., Bailiff, I.K., Tooley, M.J., 1997. A preliminary study of time-resolved luminescence in some feldspars. Radiat. Meas. 27, 211-220. Colyott, L.E., McKeever, S.W.S., Akselrod, M.S., 1999. An integrating UVB dosemeter system. Radiat. Prot. Dosim. 85, 309- 312. Cowens, M.W., Blouke, M.M., Fairchild, T., Westphal, J.A., 1980. Coronene and lumogen as VUV sensitive coatings for Si CCD imagers: a comparison. Appl. Optics 19, 3727-3728. Ditlefsen, C., Huntley, D.J., 1994. Optical excitation of trapped charges in quartz, potassium feldspars and mixed silicates: the dependence on photon energy. Radiat. Meas. 23, 675-682. Dravins, D., Fario, D., Nilsso, B., 2000. Avalanche diodes as photon-counting detectors in astronomical photometry. In: Iye, M., Moorwood, A.F. (Eds.), Optical and IR Telescope Instrumentation and Detectors, vol. 4008. SPIE, pp. 298-307. Duller, G.A.T., B0tter-Jensen, L., Markey, B.G., 1997. A luminescence imaging system based on a charge coupled device (CCD) camera. Radiat. Meas. 27, 91-99. Duller, G.A.T., BCtter-Jensen, L., Kohsiek, P., Murray, A.S., 1999a. A high-sensitivity optically stimulated luminescence scanning system for measurement of single sand-sized grains. Radiat. Prot. Dosim. 84, 325-330. Duller, G.A.T., BCtter-Jensen, L., Murray, A.S., Truscott, A.J., 1999b. Single grain laser luminescence (SGLL) measurements using a novel automated reader. Nucl. Instr. Meth. B 155, 506-510. Duller, G.A.T., BCtter-Jensen, L., Murray, A.S., 2000. Optical dating of sand-sized grains of quartz: sources of variability. Radiat. Meas. 32, 453-457. Erfurt, G., Krbetschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behaviour of A1203:C-potential for dosimetric applications. Radiat. Meas. 32, 735-739. Fattahi, M., Stokes, S., 2000. Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz. Radiat. Meas. 32, 479-485. Galloway, R.B., 1993. Stimulation of luminescence using green light emitting diodes. Radiat. Prot. Dosim. 47, 679-682. Galloway, R.B., 1994. On the stimulation of luminescence with green light emitting diodes. Radiat. Meas. 23, 547-550. Galloway, R.B., 2002. Luminescence lifetimes in quartz: dependence on annealing temperature prior to beta irradiation. Radiat. Meas. 35, 67-77. Galloway, R.B., Hong, D.G., Napier, H.J., 1997. A substantially improved green light emitting diode system for luminescence stimulation. Meas. Sci. Technol. 8, 267-271. Godfrey-Smith, D.I., 1991. Optical dating studies of sediment extracts, Ph.D. Thesis, Simon Fraser University, Canada. Godfrey-Smith, D.I., Cada, M., 1996. IR stimulation spectroscopy of plagioclase and potassium feldspars and quartz. Radiat. Prot. Dosim. 66, 379-385.
348
Optically Stimulated Luminescence Dosimetry
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SUBJECT
INDEX
Absorbed dose 1, 2, 4, 7, 8, 11, 27, 42, 37, 65, 71, 76, 80, 81, 84, 86, 90, 96, 104, 107, 109, 112, 117 Accident dose 249 Accident dosimetry 3, 7, 10, 11, 85, 245-249 Activation energy 25, 37, 39-42, 46, 47, 60, 88, 89, 95 Additive-dose 157, 250, 253, 277, 280, 281 Adularia 220 Afterglow 221,223 A1203:C 4-6, 19, 21, 24, 25, 31, 32, 37, 38, 47-49, 51, 54, 55, 58, 59-63, 71-81,101,102-105, 107-111,113, 114, 117, 248,270, 327, 340, 341 Albite 188, 199, 201,204, 206, 217-218, 220, 231 Alkali (feldspars) 188, 201,202, 205, 215-216 AIN 111 Alpha quartz 119, 150, 162 Annealing 162-169 Anomalous fading 218, 219-227, 303 Anorthite 188, 200 Anti-Stokes 1, 40 Argon-ion laser 123, 141, 316 Automated OSL readers 315- 316 BaFX (X= Br,C1,F) 87-89 BaS 90 BeO 51, 60, 92-95 Beta dosimetry 104 Bleaching of the OSL signal (quartz) 141-143 Bleaching at ambient temperature (feldspar) 201-202, 207-209 Bleaching by IR at elevated temperature (feldspar) 208-210 Blue LED stimulation 323-325 Brachytherapy 10, 112 Bremsstrahlung x-rays 89 Brick(s) 246-249, 253-260, 338-339, 340 Broad-band light stimulation 318-320 CaF2 60, 61, 86, 87 CaS 3, 90 CaSO4 60, 90, 270 Central age model 297, 299, 301 Ceramic(s) 246
Charge Coupled Device (CCD) 314, 334, 335 Chernobyl 250, 258, 259, 268, 272 Common age model 301 Concrete 260, 263, 297 Conductivity 120 Configurational coordinate 42-45 Continuous-Wave OSL (CW-OSL) 3, 4, 6, 19, 20, 26-29, 34-39, 44, 47-57, 65, 84-86, 92, 95, 96, 101, 102, 104, 112, 123-130,311 Cores 338-339 Cross-section 127, 184-186 Dating 3, 7, 10, 11, 21, 71, 84, 276-304 Decay curve(s) 123-130, 194-195, 271,273 274-275, 318 Deep dose (Hp(10)) 7, 102 Deep level transient spectroscopy (DLTS) 1, 2, 16 Delayed OSL (DOSL) 4, 59, 60, 140, 329 Delocalised bands 15, 16, 39, 45, 75 Dental crowns 273-275 Deterministic effects 8 Differential thermal analysis 1 Disordered structure 188 Donor-acceptor recombination 39-41, 75, 231 Donor traps 60-65 Dose correction method 280 Dose-depth profiles 248-249, 257-261, 263-264, 338-339 Dose dependence 157-161,234 Dose distributions 261,293 asymmetric (distributions) 293, 298 Dose equivalent 7, 8, 10 Dose recovery test 302 Dose response 75, 76, 94-96, 101, 109, 110, 113 E ~centre(s) 120, 164 Effective atomic number 3, 8, 93 Effective dose 7, 8 Electron spin resonance (ESR, EPR) 3, 162-165 Elevated temperature IRSL 191 Elevated temperature OSL 177-180 Elevated temperature infrared (ETIR) 209
352
Subject Index
Emission from quartz UVA-violet 121,150-151 blue 122, 153 orange 122, 153-155 Emission spectra quartz 149-157 feldspars 201-207 Emission spectrometry 332-334 Energy band diagram 16, 88, 93 model 31, 88, 93 Energy response 104 Environmental dose 3, 8, 10, 11, 71 dose rates 247-249, 250-257, 277-287, 293-302 dosimetry 5, 7, 8, 71, 75, 80, 107 radiation 2, 8, 11,109 Equivalent dose (De) 3, 7, 11, 157 Excitation spectra 141-148, 199-200 Eye dose (Hp(0.3)) 7, 102 Fadia 223-224, 299 Fading athermal 41,219-227 light-induced 71, 79 thermal 79, 83, 89, 90, 112 Fast component 127, 133-135, 151, 157-161 Fe 3+ 205-207, 221 Feldspar(s) 21, 38, 40-43, 50, 59, 71, 78, 188-235 Fermi level 16-18 Fermi-Dirac statistics 16, 17 Filling diagrams 16 Film badges 101, 102, 104 First-order kinetics 26, 29, 35, 49-51, 56, 214 Frank-Condon principle 42 Fredholm equation 31 Galactic cosmic rays 8 Gaussian 262, 293-294 General-order kinetics 50-54 Glass(es) 21, 95, 96, 101, 113 Glauber salt 265-266 Glaze 270-272 Gray-equivalent 8
Green LED stimulation 321-323 Growth curve 157-162, 166, 183-184, 250-252, 256-258, 267-268, 277-281,284-285 Half-life 35 Heavy charged particles (heavy ions) 109 Helmholtz free energy 18 High dose dosimetry 21, 65 Histograms 293-295, 299-300 Household chemicals 265-268 Huang-Rhys factor 23, 43 Hydrogenic trap model 22, 40, 218, 231 Imaging photon detectors (IPD) 313-314, 334 Infrared stimulated luminescence (IRSL) feldspars 3, 26, 189-203, 207-221, 223-227, 230-234 quartz 142, 148 Initial slope 127, 184 InLight TM102-- 104 International Space Station 8, 108 IR diodes 189, 316-318 Irradiated food 329 Irradiation, at elevated temperature(s) 173-174, 215 Irradiation, at low temperature(s) 120 Isothermal decay 169-170, 211,287 KBr 82, 83 KCI 81-83 Labradorite 204, 205-206, 219, 221 Lambert-Beer law 19 Laser 135, 178, 184, 211,316-330, 335-338 Laser diode (IR) 148, 200, 317- 318 Lattice vibrational modes 218 LiF 21, 61, 65, 71, 86 Lifetime excited state 44, 59, 56, 59, 77 free carrier 54, 56 luminescence 24, 46, 47, 57, 59, 60 radiative 45 recombination 28, 62, 169, 214 trapped charge 34 Linear energy transfer 109
Subject Index Linear modulation IRSL 194-197 Linear modulation OSL (LM-OSL) 4, 6, 19, 20, 26, 27, 48-56, 65, 84, 85, 92, 95, 130-136, 185, 265-266, 311,325-326 Linear modulation photoconductivity (LM-PC) 54, 56 Localised transition model 192, 219 Low-Earth orbit 8 Luminescence correction method 280 Luminescence efficiency 5, 109 Luminescence sensitivity change(s) 287-293 Luxel 72, 101,102-105, 109, 327 TM
Medical dosimetry 7, 9-12, 112-117 MgS 3, 90-93, 113, 117 MgSO4 90 Mn 2+ 205-207 Median value of the standard deviation 293 Medium component 127-128 Microcline(s) 191,192, 197-199,200, 201-202, 205-206, 217, 220 Microcline microperthite 204 Modelling 186-188, 230-234 Monte Carlo simulations 258-260 Mortar 260-264, 297 MOSFET detectors 112 Mott-Seitz 44, 47 Multiple-aliquot 157-159, 250, 277-279 NaC1 51, 52, 84-86, 265-266 Natural normalisation 277 Non-radiative 5, 28, 33, 34, 42, 44, 45, 62 Oligoclase 199, 204-206, 218, 219, 231 Optical bleaching 184, 201-202 Optical fibre dosimetry 112-117 Optically stimulated afterglow (OSA) 140, 146, 197-198 Optically stimulated exo-electron emission (OSEE) 2, 16 Optically stimulated phenomena (OSP) 1, 2 OSL emission spectra 149-150, 201-203 OSL scanner 338-339 Orthoclase 191,199, 205-206, 208-210, 215-218, 231
353
Personal dosimetry 3-5, 7, 9, 11, 80, 81, 101-107 Perthite(s) 188, 205 Phase transition 119, 150 Phosphorescence 48, 59, 60, 77, 86, 90, 92 Photochromic effect 81 Photoconductivity 1, 2, 54, 56, 73-75, 86 Photoelectron effect 15 Photoionisation cross-section 18, 19-26, 130-131,135, 184-186 Photoluminescence (PL) 1, 21, 86, 205-207, 215, 270-271 Photomultiplier tube 312-313 Phototransfer 21, 60-65, 71, 79, 85, 110, 124-125 Phototransferred thermoluminescence (PTTL) 124, 127, 268-269 Plagioclase(s) (feldspars) 188, 200, 202, 205, 215 Porcelain 246, 253, 267-275 Portable system 340 Pre-dose effect 121, 122 Probability density plots 295-296, 300 Pseudo-LM-OSL 131, 143, 185 Pulse annealing 170-173, 212-214, 229 Pulsed IRSL 197 Pulsed OSL (POSL) 4, 6, 19, 56-60, 75, 76, 79, 80, 101,102, 104-107, 136-140, 197, 311,326-330 Pulsed RL 140, 341- 344 Quality factor 7 Quartz 24, 25, 30, 32, 37, 39, 41, 42, 46, 47, 51, 59, 62, 64, 65, 71, 95, 96, 113, 119-188 Quasi-equilibrium 33, 62, 64 Quasi-Fermi level 16, 17 Radial plots 263-264, 296-298, 300-302 Radiochromic dye films 112 Radioluminescence (RL) 137, 155-157, 163-164, 174, 227-234, 340-343 Radioluminescence emission spectra 203-205 Radioluminescence dating 227-230 Radiophotoluminescence 1, 4, 65, 101 Radiotherapy 10, 112-117 RbI 85, 86
354
Subject Index
Red IRSL 220 Regenerative-dose 253, 277, 280-281, 285-287, 292 Resonance, in feldspar stimulation spectrum 201-202, 218, 316 Retrospective dosimetry 5, 7, 10, 11, 71, 85, 245-276 SAAD (single-aliquot additive-dose) 276, 282-285 SAR (single aliquot regeneration) 3, 159-160, 253, 255-257, 285-287, 291-293, 303-304 SARA (single aliquot regeneration and added dose) 250-252 Sanidine 199, 219, 220, 221,223 Scanning monochromator 199, 201,330-331 Sch6n-Klasens 44 Scintillators 50, 112 Self-trapped exciton 65, 88 Sensitivity change(s) with preheating 167-168, 255 monitoring of by OSL 291-293 ll0~ TL 167-168, 171,182 Sensitization in nature 167 Shallow dose (Hp(0.07)) 7, 102, 104 Shuttle 8 Single aliquots(s) 159, 252-257, 280-293 Single event upsets 9 Single grain(s) 160, 223-225, 229-230, 298-302, 334-338 Single grain laser OSL 160, 335-338 Single photon absorption 126, 143 SiO2 19, 119-120 Slow component 127, 130-135, 180-184 Solar particles 8 Space radiation 8, 107-110 SrS 3, 51, 90, 92 SrSe 3 Standard deviation 293 Standard error on the mean 293, 297 Stimulated relaxation phenomena (SRP) 15-17 Stimulation spectrum (spectra) 26, 73, 74, 81-87, 90-95, 141-148, 199-200, 330-331 Stimulation temperature 35- 39, 130, 189-194
Stochastic effects 8 Stokes' shift 1, 44 Stretched exponential 29, 30, 35 Superposition principle 18, 31, 49, 50 Supra-linear response 160-162 Teletherapy 10, 112 Thermal assistance general 37, 39, 41, 84-86 feldspars 216-217 quartz 130, 179-180, 184 Thermal quenching general 5, 44-48, 71-79, 84, 95 feldspars 204, 207, 215, 221 quartz 121,130, 137, 140, 151-153, 155-156, 177-179, 184, 204-205 Thermal stability 169-170, 181, 211,229 Thermal transfer 171, 174-177, 263-265 Thermally stimulated capacitance (TSCap) 16 Thermally stimulated conductivity (TSC) 1,2,15 Thermally stimulated exo-electron emission (TSEE) 1, 2, 16 Thermally stimulated phenomena (TSP) 1,2 Thermogravimetry (TG) 1 Thermoluminescence (TL) general 1, 2, 4-8, 71-81, 91-97, 102, 107, 109, 110-112, 189-193 emission spectra 150, 203 ll0~ 121-122, 123-126, 130, 267, 285, 287-291 325~ 143-144, 151-153, 177-178 375~ 153 Thermo-optical luminescence (TOL) 78, 189193 Time-resolved luminescence (TRL) 137-140 179, 181, 184, 220, 326 Tissue-equivalence 89 Trap depth 18, 23, 33, 39, 169-170, 172, 177-178 Traps deep 22, 31, 32, 34, 37, 38, 47, 59, 60-62, 65, 71, 74, 77, 79, 80 shallow 31, 35-39, 46, 47, 59-61, 71, 74, 77, 78, 80, 90
Subject Index Trapped radiation belts 8 Tuneable laser 199, 316, 331 Tunnelling 217, 218, 219, 220
Wavelength dependence 21-23, 24, 49, 52-54, 74 Weighted mean 293, 297
UV 15, 17, 61, 81-83, 87, 90-92, 110-112, 117, 340
X-ray storage phosphor 81
Washing powder 265-268 Water softener 265-266
ZnS 51 ZnSe 19
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