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
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TABLE OF C O N T E N T S
PREFACE
CHAPTER
.................................................................
1:
INTRODUCTION
..........................................
1.1 1.2
Optically s t i m u l a t e d l u m i n e s c e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical development of OSL dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3
OSL dosimetry ......................................................... 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . This b o o k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4
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 s c en c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T h e p h o t o i o n i s a t i o n cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O p t i c a l transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 W a v e l e n g t h dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2
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 .............................................................. M o d e l s a n d rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 The one-trap/one-centre m o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T h e r m a l quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 LM-OSL .............................................................. First- a n d general-order-kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Relationship between L M - O S L and C W - O S L .................... 2.5.2 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
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 60
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M a t h e m a t i c a l description and typical data . . . . . . . . . . . . . . . . . . . . . . . . Radiophotoluminescence ...............................................
60 61 65
59
2.7.1
2.7.2 2.8
2.8.1
Procedure
C H A P T E R 3: 3.1
3.2
......................................................
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 ..............................................................
84
3.2.3
3.2.4
3.3
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
...................................................
85 86 87 90 90 90 90 90 92 92 95
101 101
4.1.1 4.1.2 4.1.3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Landauer's L u x e U M personal dosimetry system . . . . . . . . . . . . . . . . . . Landauer's InLight T M personal dosimetry system . . . . . . . . . . . . . . . . .
101
4.1.4 4.1.5
Beta dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P O S L imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modelling processes giving rise to OSL in quartz . . . . . . . . . . . . . . . . 5.1.12 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 s t im a ti 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 c o 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous O S L scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 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 .................................................... CW-OSL ...................................................... 7.1.1 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P h o t o m u l t i p l i e r tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser stimulation .............................................. 7.4.1 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 .................................. Green L E D stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.6 7.4.7 Blue LED stimulation .......................................... Blue LED and cut-off filter characteristics ....................... 7.4.8 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
311 311 311 311 311 312 312
SUBJECT
351
CHAPTER
7.1
INDEX
........................................................
313 314
315 316 316 316 317 318
320 321 323 325 325 326 330 330 332 334 334 334 335 335 338 340 340 343 345
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.
Optically Stimulated Luminescence Dosimetry
I
-
,,..,
"1 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
0
9
0
710
0
O "(3
80
i-"
tO
<E
O
-o ._~
.Q E
-
690
I
I
I
I
I
I
I
I
I
I
I
I
I
Eo, with a maximum at hv = 1.4Eo. However, for such small photon energies in which the free electron is close to the ionised defect one should take into account the coulombic attraction between the freed electron and the ionised defect (Blakemore and Rahimi, 1984). For deep traps the hydrogenic model is inappropriate prompting Lucovsky (1964) to use a delta-function potential for the defect. Such a model leads to the following expression for the spectral dependence of the cross-section:
o. (hv, Eo) CC.[ 4(hv - E~176 ] 3/2 (hv) 2
(2.14)
Here, the cross-section reaches a maximum at h v = 2Eo, with o-increasing as ( h v - E o ) 3/2 for hv < Eo, and decreasing a s (hv) -3/2 for hv > Eo. It is to be noted also that the coulombic field should be accounted for when considering defect excited states. An explicit assumption in Lucovsky's analysis is that the effective mass me of the electron in the conduction band can be used also for the electron in the localised state. Grimmeis and Ledebo (1975a,b) preferred to use the electron rest mass mo for the localised electron, which, when used with a plane-wave final state and the assumption of parabolic bands, leads to: or (hv, Eo) co.
( h v - Eo)3/2 hv [ h v - Eo(1 - mo/me)] 2
(2.15)
Banks et al. (1980) discussed the photoionisation cross-section of deep traps and Lucovksy's assumption of a single band edge and derived the following expression for the cross-section by considering the possibility of nodal properties for the deep trap wavefunction: ( h v - Eo) 1/2 o'(hv, Eo) oc hu(hv +/3)2
(2.16)
Optically Stimulated Luminescence Theory
23
1.2
"O
O
o (1)
0
2
4 6 Photon Energy (eV)
8
10
Fig. 2.4. Comparisonof the shapes of various expressions for the photoionisation cross-section as a function of stimulation photon energy, obtained with a threshold energy Eo = 3.0 eV, and mo/m e 2. Each curve is normalised to a maximum of 1. =
with a range of possible values for/3 from/3 = 0 to/3 >> Eo. Other expressions for o- exist (Blakemore and Rahimi, 1984; Ridley, 1988; Landsberg, 1991) following different assumptions for the form of the potential and defect wavefunction. Dosimetric materials are usually wide-band-gap insulators, and stable OSL signals originate from the release of electrons from deep trapping states. Thus far, the expression most frequently used to represent the photoionisation cross-section of such centres has been either Eq. (2.15), as described by Grimmeis and Ledebo (1975a,b), or the Lucovsky (1964) expression (Eq. (2.14)) (e.g., Alexander and McKeever, 1998; Whitley and McKeever, 2000, 2001; Bailey, 2001). The shapes of some of the above expressions for the photoionisation cross-section are shown in Fig. 2.4 for a threshold energy of Eo = 3.0 eV, and mo/m e = 2. For each curve, o- -- 0 for hv --< Eo. The threshold optical ionisation energy Eo is larger than the thermal ionisation energy (thermal trap depth, Et) by an amount equal to the phonon energy, namely: Eph = Shoop
(2.17)
where S is the H u a n g - R h y s factor and Wp is the phonon vibration frequency. 2.3.3. Measurement of the photoionisation cross-section The photoionisation cross-section can be determined experimentally by a number of techniques. Consider a one-trap/one-recombination centre model for a luminescent material and illumination of an irradiated sample containing n trapped electrons at N traps, each with an optical ionisation threshold energy of Eo (see Section 2.3.2). Electrons are stimulated from the traps into the conduction band (viz. transition 4a in Fig. 2.3) from
Optically Stimulated Luminescence Dosimetry
24
where free electrons (concentration nc) may either be re-trapped or recombine with trapped holes to produce OSL. If the illumination flux at wavelength A is q~(A), then under steady-state conditions (i.e., dnc/dt = 0), we have qb (A)o" (A)n = A ( N - n)n c - Amncm
(2.18)
where A is the probability (in m 3 s - l) of capture of free electrons, and Am is the probability (also in m 3 s- l) of recombination of free electrons with trapped holes of concentration m. Under conditions of weak stimulation, n and m remain approximately constant during the stimulation period (i.e., the number of charge carriers removed is much less than the initial number, or An > n l, N2 >> n2 and m is approximated to a large constant. The latter assumption in effect means that n l 1) are populated. Since this is not the case with quartz, Spooner suggests a model based on an array of ground states energies, from where optical excitation to the conduction band can occur (i.e. Fig. 2.14e). Here AE is now identified with Eg. Earlier authors examined the necessary electron coupling to the phonon vibrational modes theoretically and its effect on the shape of the photoionisation crosssection has been discussed in detail, e.g., Jaros (1977) and Noras (1980). Such effects are important to consider when e l e c t r o n - p h o n o n coupling is strong since accurate interpretation of OSL and PC spectral dependencies depends upon the correct choice of the expression for the photoionisation cross-section. Electron-phonon coupling is usually described with the aid of a configurational coordinate diagram, such as that shown in Fig. 2.17 in which we see the defect energy state corresponding to a filled trap and an empty, ionised trap. The horizontal lines represent phonon vibrational levels. Electronic transitions are represented vertically due to the supposition that absorption of a photon occurs too quickly for simultaneous re-adjustment of the lattice coordinate, Q (the F r a n k - C o n d o n principle). Since ionisation of the trap will result in a lattice re-adjustment, the minima in the ground state and empty states are not coincident in Q. Thus, a vertical optical transition is followed by phonon emission and non-radiative relaxation. A f r e e - b o u n d transition occurs from the upper minimum
Optically Stimulated Luminescence Theory
43
t--
LIJ
Fig. 2.17. Configurational coordinate diagram showing the vibronic and potential energy curves for a filled trap, and an empty trap. The potential energy curves are plotted as function of the one-dimensional configurational coordinate, Q. The horizontal lines are vibrational levels with separation htop.
vertically to the lower filled state, with the further emission of phonons. The difference in energy between the upward vertical and the downward vertical transitions is given by the Huang-Rhys factor, S and is equal to 2Shwp (where the terms have been previously defined, in Section 2.3.2). Purely electronic transitions result in ionisation of the trapped electron into the conduction band when the optical excitation energy h v is greater than the optical threshold energy Eo. Expressions for the resulting photoionisation cross-sections were given in Section 2.3.2. For purely electronic transitions (vertical transitions on a configurational coordinate diagram) no change in the lattice configuration occurs. When phonon coupling occurs, transitions to the conduction band can occur at photon energies less than Eo as long as the lattice can supply (or absorb) the corresponding energy A (i.e., h v - - Eo + A). For strong phonon coupling the photoionisation cross-section can be expressed (B6er, 1990) as: o" ( h v) oc
~oo
( x/~ FA ) b e x p { - ( A - h v - Eo 0 hv(,4r2FA + Eo(m*/me)) a
(2.60)
with the phonon broadening factor given by
F ~--
(h
)2coth
2-~
(2.61)
Optically Stimulated Luminescence Dosimetry
44
from which the exponential temperature dependence is clear. The parameter a = 2 for a short-range &function potential, and a = 4 for a long-range Coulombic potential. Parameter b = 3/2 for forbidden transitions, while b -- 1/2 for allowed transitions (Brer, 1990). Similar expressions are derived by Noras (1980) and Jaros (1977). Note that for photon energies significantly above the threshold energy (hv >> 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)
Optically Stimulated Luminescence Theory
45
t~
r
iii
Ground state _ _ _
EQ~
%! i
I
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
Optically Stimulated Luminescence Theory
05
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
'=;
[
o
i
-.
.=_ E
E
"O Z
* vv 0.0
400x103
xi
~--
v
300x103 "g
~_/
200x103 .~ _J
I--.
oe--
0
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
.01
0
. ,
0,5
0 60
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; 160~ measurementtemperature.(b) A1203:C: 100 mGy; 180~ s pre-heat;75~ measurementtemperature.(c) 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
Optically Stimulated Luminescence Theory
~, b
,._: . J
0.4 0.2 I
(b)
~
I
I
I
I
I
15 10 5 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 =
0
2 1 0
~ 0.6
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 go
=
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
.
0
1000
-
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
OSL Properties of Natural Materials
171
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).
,e--
~
do ..-, o
5x10 3
if) L..
LI..
~
L..
0
o
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_..!
0.8
O N
E
0.4
k-\\
o Z
"
0.0
I
I
V.
-"
~,,
_.**L,~Lo~ ~ b :
i
o.i
- I i'll'
\
''It
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
OSL Properties of Natural Materials
173
0.3 L
~
0.0
0
oo
-0.1
200
300
400
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
175
4O 3O
g~
3
~
2
(.9 4 v
o
.
.
.
.
a
-
p.
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
"~ 0.08 ~
o ~
220~
o.o6t
J 200 ~
260~
|
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).
0
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 O
s
xlI
t" I1) t~
,
m
O" UJ !
!
--
!
t
w
!
w
i
Pre-heat Temperature, ~ >,
60 -
@ if) 0
121
40 -
1-i1) >
.m
ID"
20 -
LLI
|
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.
~r.- 10-13-
6
5'0
16o
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).
OSL Properties of Natural Materials
179
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
8
o.6
d
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
I
>"
0.30
0.20 "N
9
~"~
0.15
"~ 0.10
0.05 1~ i--
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()
............................
10
100
1000
1 hr
10hr
1 ........
,1 ......
10000
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
300
400
O.. ~
500
600
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
= 278_+37Gy De = 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
N
10 -12
r"
E 0
0
0 tO
10-19 % ~0
0
x= 10-la __o
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
O
400
0 0
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
OSL Properties of Natural Materials
187
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
E
-~g500t o~ 4001 ,--=~
100 i
|
|
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
~
"
v
a0 fID >
--
9
-
LU
..1
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
25000-
~ D"
/
\~
O.
E -'s ...1
t].
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
120-
60ffl
.~_
E
150
250
350
450
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
20 10
|
|
,
|
|
|
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
" ~ , ~ /
....
I
7s~
i
I
I
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
~4
8 3 82 g. 1 0 I
....i -
,
'
' .... -I--
'
i-
,
,
I
'
'
'
'
I
'
~
'
'
(seconds) 12
.._.,10-
5
r
"N
--s,,s~+s~ . . . . .
S I
......
S 2
. . . .
S 3
==6
8, g2 0
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
l0 s ra~
10 2
9 .
.: ..-.-
:..........
l01
10 ~
.
0
10
20
.
.... . :-. : : .-: . . . . . . . . . .
30
._-,.., . . .
._.,
40
50
Time / Fig. 5.64. Time-resolved luminescence from a sample of Amelia albite observed at 550A40 nm and obtained for pulsed stimulation at 850 nm and using a 5 ns dwell time (from Clark and Bailiff, 1998).
:
:
~
" 0
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
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
t
9
~r
!
,'\
I
9
,
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
~
~JL,,,.v q J-
|
....
i
!
|
i
u
i
Fig. 5.68. Temperaturedependence of blue/UV RL resulting from X-irradiation for two samples of orthoclase (ff], II), oligoclase (~) and labradorite (~). Data fitted with equations for thermal quenching (from Barnett and Bailiff, 1997b).
OSL Properties of Natural Materials
205
oligoclase, thermal quenching is correlated with deformation of S i - O - S i or S i - O - A 1 bonds, while, in albite, it is correlated with stretching of the N a - O bonds. As in the case with quartz (see Section 5.1.9.1), thermal quenching will affect measurements of thermal activation energies (see Section 5.2.9.2) in the temperature region that is affected by thermal quenching. 5.2.6.3.2. Under X-ray stimulation above room temperature. Rendell and Clarke (1997) obtained RL spectra for a suite of well-characterised alkali feldspars. They reported that the RL, TL and CL, emission spectra were similar for each of the samples. The relative strengths of the emission bands at 290, 340, 380-450, 550 and 700 nm varied. In a related study, Clarke et al. (1997) showed the RL spectra for perthitic feldspars (potassium feldspar with separated phases of sodium feldspars) to be dominated by the emission from 380 to 450 nm, although there was also a contribution at 290 nm. 5.2.6.3.3. Under beta stimulation from a t~7Cs source. Spectra have also been obtained for the prompt luminescence observed when feldspars are stimulated using a 137Cs internal conversion (beta) source, which forms part of the RL dating instrument (see Section 7.10 and Fig. 7.16). Spectra were obtained using a spectrometer (200-800 nm) linked to a liquid nitrogen cooled CCD detector. Trautmann et al. (1998) investigated about 15 feldspar samples from a museum collection and a sedimentary sample of labradorite. The RL spectra were collected over 600 s for irradiated and non-irradiated (thermally emptied) samples, shown in Fig. 5.69 as light and dark lines, respectively. The potassium feldspar, microcline and orthoclase, show very strong IR emissions. Other emission bands can be seen at about 3.0 eV (410 nm) for all samples and at 2.2 eV (560 nm) and 2.7 eV (330 nm) for the albite and plagioclase feldspars (Fig. 5.69). A red emission at 1.7 eV (730 nm) can also be seen, though this is unstable at room temperature, as demonstrated when an external beta source was used to give a laboratory dose. More detailed spectral measurements from 600 to 1000 nm have also shown that this emission peak at 700 nm increases its relative intensity and interferes with the 865 nm emission when a 500 Gy dose is added (Krbetschek et al., 2000). This effect is particularly pronounced since the IR peak intensity decreases when the dose is added and the red peak increases. Although, this peak has been shown not to be stable at room temperature (Trautmann et al., 1998), decaying in a matter of minutes after RL stimulation is stopped, it needs to be avoided in routine RL dating. This is particularly true if using an IR-sensitive photomultiplier or avalanche photodiode detector as proposed by Krbetschek et al. (2000). In this case, a narrow band interference filter would be needed in front of the detector to reduce the effect of red emission. 5.2.6.4. Photoluminescence emission spectra Photoluminescence (PL) is also observed for feldspars. These PL signals do not require exposure to ionising radiation and do not decay with time during optical stimulation (see Section 1.1). Under UV (340 nm) stimulation, two main emission bands have been observed, corresponding to internal transitions in the transition metal ions Mn 2+ and Fe 3+. The Mn 2+ emission usually occurs in the yellow/green (--- 560 nm) with the exact energy having little dependence upon mineralogy. Mn 2+ is often found to be the dominant luminescence defect relating to OSL. This information is derived from cathodoluminescence (CL) (G6tze et al., 1999) and optical absorption measurements (White et al., 1986).
Optically Stimulated Luminescence Dosimetry
8 m o
8 P
8 8
8 X
8 Z
sgun .qae I ma
0 0
0 0
0 0
0 0
0 0
0 0
g a g w a s sgun eq.181
OSL Properties of Natural Materials
207
The Fe 3+ emission occurs in the far-red (around 750 nm), with the exact wavelength depending upon the chemical composition of the feldspar (White et al., 1986; Brooks et al., 2002). The PL emission peaked at 745 nm for the alkali feldspars investigated by Poolton et al. (1996). This Fe 3+ emission was found to have two stimulation resonances, at 2.95 (420 nm) and 2.75 eV (450 nm). These resonances can be related to two transitions from ground to excited states in Fe 3+. An additional resonance at 1.9 eV (650 nm)is observed for alkali feldspars, but was absent from the calcium-rich plagioclase feldspars, thus providing information on mineral identification. Based on a comparison of OSL data and PL data (energy separations), Poolton et al. (1996) concluded that the Fe 3+ and OSL defects were to be found in the same part of the crystal. Under visible light stimulation, Poolton et al. (1995b) showed that most feldspars showed strong luminescence in the far-red. Under UV stimulation, this luminescence was obscured by broad luminescence emission resulting from other transitions, thought to arise from direct recombination between the conduction band and the ground state of the defect. For one sample of alkali feldspar this was not the case, and thermal quenching of the 780 nm PL was observed when stimulation at 340 nm was carried out at temperatures from room temperature to 300~ (Fig. 5.70a). The activation energy was determined to be W -- 0.34 eV. Poolton et al. (1995b) considered that this energy was too large for lattice vibrations. In a plagioclase sample, PL emission from both Fe 3+ and Mn 2+ was observed. The Fe 3+ emission at 780 nm showed thermal quenching, but the Mn 2+ emission at 560 nm was relatively unaffected by raising the temperature from 17 to 227~ (Fig. 5.70a). As previously mentioned, UV (340 nm) stimulation tends to produce broad PL spectra that span the visible region (Fig. 5.70b). These broad-band emissions are also thermally quenched in the temperature region from room temperature to 400~ When observed at 500 nm, the two samples shown in Fig. 5.70b have energies for thermal quenching of W = 0.14 and 0.05 eV. These are close to the energies of the lattice vibration modes. 5.2.7. Effects of previous optical treatment 5.2.7.1. Bleaching at ambient temperature The effects of IR (875A80 nm) and green light (515-560 nm) on both the IRSL and OSL (stimulated at 515-560 nm) signals are shown in Fig. 5.71a,b, respectively (Duller and BCtter-Jensen, 1993). Also shown are the effects of the same exposures on the two main TL peaks at 270 and 330~ Under IR exposure, the OSL signal falls less rapidly than the IRSL and at the end of the experiment (after 1000 s IR exposure), a larger fraction of the initial OSL signal remains, 13% compared with 0.3% of the IRSL. Both TL peaks are reduced, but by no more than 10% which occurs in the first 10 s of IR exposure. Under green light illumination, a greater reduction in the TL signals occurred and both IRSL and OSL decayed at more similar rates than under IR illumination. Duller and BCtter-Jensen (1993) concluded that green light was able to evict charge from traps that remained un-emptied by IR. Galloway (1994) also examined the effect of IR stimulation at room temperature on the OSL and found a similar level of depletion (--~10%) for extended periods of IR exposure. The same room temperature exposures did not affect OSL from
Optically Stimulated Luminescence Dosimetry
208
)
80
i 0 i 4OO
i
i
Fig. 5.70. (a) PL emission spectra for a plagioclase feldspar stimulated at 340 nm and recorded at the temperatures stated. The Mn2+ and Fe3+ emission bands are identified (from Poolton et al., 1995b). (b) Room temperature PL emission spectra stimulated at 340 nm for samples of albite (R34) and adularia (R28) (from Poolton et al., 1995b). quartz and this feature was used by Liritzis et al. (1997) to reduce the effect of any feldspar contamination in quartz OSL dating. 5.2.7.2. IR bleaching at elevated temperature Jain and Singhvi (2001) investigated the relationship between OSL (stimulated using blue/green 4 2 0 - 5 6 0 nm light) and IRSL (880A80 nm). When IR is applied during a preheat, the OSL and IRSL signals are depleted at different rates, depending upon the temperature of the pre-heat, leading Jain and Singhvi (2001) to conclude that there were two distinct trap populations. Fig. 5.72a shows the percentage OSL remaining as a function of the percentage IRSL remaining following IR stimulation carried out at 100, 150 and 220~ for a laboratory-irradiated sample of orthoclase. Fig. 5.72b shows the reduction in OSL as a function of IR stimulation time at the temperature indicated. Rapid depletion occurs in the first 100 s, but continued loss is seen for longer exposure times, particularly at higher temperatures (e.g., 220~ Jain and Singhvi (2001) concluded that
209
OSL Properties of Natural Materials
rL
E
E ..I
Fig. 5.71. Reductionin luminescence signals, TL (at 270~ /~ and 330~ x ), OSL (4') and IRSL (D), as a function of exposure time to (a) IR (875Anm, 42 mW/cm2) and (b) green light (515-560 nm, 6.5 mW/cm2) (from Duller and B~tter-Jensen, 1993).
elevated temperature infra-red (ETIR) exposure at 220~ could reduce the OSL signal in feldspars, thus leading to improved selectivity in measuring the OSL from a sample of quartz contaminated with feldspar. However, Bailey (1998) reported a 12% depletion in quartz OSL following IR stimulation for 300 s at 200~ Hence, a lower temperature ETIR may be more appropriate.
210
Optically Stimulated Luminescence Dosimetry
Fig. 5.72. (a) Reduction in OSL (stimulated at 420-560 nm) as a function of the reduction in IRSL (stimulated at 880A80 nm) observed after exposing to IR at the three temperatures shown for 100, 300, 500 and 700 s. (b) Reduction in OSL as a function of IR stimulation time at the temperatures given. The orthoclase sample was given a 30 Gy dose and then pre-heated before IR exposure (from Jain and Singhvi, 2001).
Similar measurements of OSL (470 nm stimulation) as a function of IR stimulation time at 50, 125 and 225~ were made by Poolton et al. (2002b). They compared their experimental data with the effects of such bleaching predicted by their model of luminescence production in feldspar. They predicted that the rate of bleaching with IR is increased at higher stimulation temperatures because more recombination sites are accessed at elevated temperature. This has implications for the choice of IRSL measurement temperature for dating (see Section 5.2.2.2.1).
OSL Properties of NaturalMaterials
211
5.2.8. Effects of previous thermal treatment The OSL and IRSL signals of feldspars are not derived from the same set of traps, as shown by their different responses to IR exposure (see Section 5.2.7). In addition, the optical decay curves for each are not exponential, suggesting that there is more than one component in each signal. For these reasons, isothermal decay curves have not been successful in the investigation of thermal stability (Trautmann et al., 1997). There is also evidence from the TOL curves that shallow traps contribute to the OSL signal (see Section 5.2.2.2), but not to the IRSL. When using feldspars for dating, it is important that a thermally stable signal can be isolated.
5.2.8.1. Pre-heating of laboratory and naturally irradiated samples One way of investigating the thermal stability is to compare the effect of progressively higher pre-heats on laboratory and naturally irradiated samples. Li (1991) measured the responses for laser (514 nm) stimulated luminescence from feldspars, and suggested that once the ratio between the two samples was constant, a thermally stable component had been isolated. Duller and BCtter-Jensen (1993) carried out similar experiments for TL (using the signal integrated from 0 to 450~ OSL (stimulated at 515-560 nm) and IRSL (stimulated at 875A80 nm) with 10 min pre-heats at a range of temperatures up to 250~ Fig. 5.73 shows that the depletion is greatest for the TL signal for which there was a pronounced peak at 150~ The OSL signal reaches a constant value after pre-heats of 200~ or higher, thus confirming the contribution to the OSL signal from shallow traps. No such effect is seen for IRSL, suggesting that it is derived from a trap that is thermally stable on the timescale of a few tens of thousands of years, the depositional age of the sediment
Fig. 5.73. The ratio of (N + 36 Gy)/N signals (A, TL integrated from 0 to 450~ El, IRSL and , , OSL) as a function of pre-heat temperature (from Duller and BCtter-Jensen, 1993).
212
Optically Stimulated Luminescence Dosimetry
from which the feldspars were separated. This result suggests that pre-heating after laboratory irradiation should not be necessary in order to isolate a thermally stable signal.
5.2.8.2. Pulse annealing Instead of isothermal decay measurements, the effect of heating samples of feldspar to progressively higher temperatures has been employed. In this procedure, known as pulse annealing, the OSL or IRSL signal is measured at a fixed temperature, e.g., 50~ BCtter-Jensen et al. (1993) showed such curves for the OSL and IRSL of the same sedimentary feldspar that had been given a laboratory dose to induce a TL peak at 150~ (Fig. 5.74). Both signals appeared to relate to the emptying of a TL peak that was found at 270~ when the grains were heated at 8~ in contrast to the data shown in Fig. 5.73. However, the interpretation is complicated by having normalised the data to the initial measurement, for which the OSL will have an additional contribution relating to the 150~ TL trap. Similar plots of percentage signal remaining can be used to compare natural and laboratory-induced signals and the effect of pre-heats on these signals. Duller (1994) obtained data for the IRSL of two aliquots of sedimentary feldspar (D e --- 40 Gy), one of which had been given an additional dose of 45 Gy (Fig. 5.75a). Both IRSL signals remained constant up to 250~ indicating no contribution from low-temperature traps. However, there is an offset between the two data sets above this temperature, with the natural IRSL signal staying higher, suggesting that the laboratory-irradiated sample has a small contribution from a thermally unstable trap. The effect of this trap can be removed
[] +
9
500
Fig. 5.74. Percentage of the OSL and IRSL remaining as a function of pre-heat temperature for a natural geological sample given 10 Gy (from BCtter-Jensen et al., 1993).
OSL Properties of Natural Materials
213
.~ 80t--
" 0
400
12
ci ~ ~ i..- 8"~arr-~ 9 c 4c~
2 _
~-
o-
s
0
100
200 300 Temperature (~
400
500
Fig. 5.75. (a) Pulse annealing curves for IRSL from sedimentary K-feldspars with pre-treatment as stated; preheat was 220~ for 10 min. (b) Data from (a) plotted as the percentage of the IRSL signal that is lost for each annealing run (from Duller, 1994).
by pre-heating aliquots at 220~ for 10 min. Following this treatment, both signals behave identically. This suggests that this pre-heat should be used when dating this sample. Additional information can be obtained by plotting the percentage of IRSL lost for consecutive measurements during the construction of each pulse anneal curve (Fig. 5.75b). Two components can be seen in the natural signal, with an additional component in the laboratory-irradiated sample. Li et al. (1997) obtained similar pulse anneal curves for the natural IRSL of a sedimentary K-feldspar, but using different heating rates (Fig. 5.76a). This results in the maximum loss in signal (expressed as percentage IRSL lost per ~ moving to higher temperatures with increasing heating rate (Fig. 5.76b). The relationship
Optically Stimulated Luminescence Dosimetry
214
12
] (a)
'~'~
1
~
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--
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100
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200
300
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i
~
400
-
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500
Temperature (~ 1.2
(b)
--o-
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200
300
400
500
Temperature (~ Fig. 5.76. (a) Pulse annealing curves, obtained using different heating rates, for natural IRSL from sedimentary K-feldspars; pre-heat was 220~ for 10 min. (b) Data from (a) plotted as the percentage of the IRSL signal lost per ~ (from Li et al., 1997).
between this peak temperature and the heating rate can be used to determine the activation energy and frequency factor of the signal. Tso et al. (1996) applied this approach to the natural IRSL from K- and Na-rich feldspars extracted from the same sediment. Much lower trap depths were obtained for the Na-rich feldspars than for the K-rich samples, with predicted lifetimes at 10~ of 0.9 x 104 year for the Na-rich feldspar and 2.4 x 109 year for the K-rich feldspar. Li and Tso (1997) used a similar approach on IRSL and OSL signals from the same K-feldspars extracted from a sediment. They found the OSL signal to be two orders of magnitude less stable than the IRSL signal, with the predicted lifetimes at 10~ of 1.3 x 106 year and 6.6 x 108 year, respectively. It should be pointed out that this method of analysis is based on the signal behaving as a single trap with first-order kinetics.
OSL Properties of Natural Materials
215
5.2.8.3. Irradiation at elevated temperature In their study of the TL of feldspars, Mejdahl et al. (1992) suggested that the trap corresponding to the TL peak at 130~ competed for charge in the natural environment, but not during laboratory irradiation once it had been filled. They predicted an underresponse of the growth of the high temperature TL. Wallinga et al. (2002) investigated whether this TL trap competed with those traps giving the IRSL. For samples irradiated at a range of temperatures from 35 to 260~ no step was seen in the IRSL signal and the small steady change observed was not dose-dependent. The former piece of evidence suggests that the shallow trap does not compete and the steady change is probably due to small temperature-dependent changes in the trapping cross-section, as suggested for quartz (Wallinga et al., 2002).
5.2.9. Raised temperature IRSL and OSL 5.2.9.1. Thermal quenching Thermal quenching refers to the changing efficiency of luminescence production as a sample is heated (see Section 2.4.6). For measurements made at temperatures between 90 and 200 K, Barnett and Bailiff (1997b) observed thermal quenching of the blue/UV signal when they observed the prompt luminescence during X-irradiation (see Section 5.2.6.3). No change in luminescence efficiency was found from 200 to 300 K (room temperature). Above room temperature, effects due to thermal quenching are swamped by the effects of thermal assistance (see Section 5.2.9.2.1). However, Poolton et al. (1995b) used UV and visible light to stimulate Stokes-shifted PL from a range of well-characterised alkali and plagioclase feldspars from a museum collection. These experiments showed the thermal quenching behaviour of feldspars to be extremely complex in the temperature region from 20 to 400~ As discussed in Section 5.2.6.4, the activation energy for thermal quenching of the Fe 3+ emission is W = 0.34 eV. For the broad emission band observed under UV (340 nm) stimulation of most other feldspars, thermal quenching was observed, but the energies were smaller, ---0.1 eV. More complex behaviour was found for two samples characterised by single PL emission bands at 610 (red) and 435 nm (blue), emission bands commonly observed in the IRSL spectra. Both these showed thermal quenching to occur in two steps, with the higher temperature quenching processes being seen to be irreversible when the measurement cycle was repeated. The irreversible change is suggested by Poolton et al. (1995b) to be caused by the thermal destruction of the defect itself. Poolton et al. (1995b) demonstrated that the PL (340 nm stimulated) and IRSL (850 nm stimulated) spectra were similar, irrespective of whether narrow- or broad-band emission occurs. The similarity of the PL and IRSL behaviour is shown by the thermal quenching measurements made for both types of signal (Fig. 5.77). The PL data in Fig. 5.77 can be linked to PL data on orthoclase feldspar obtained by White et al. (1986). They observed a large ( x 80) increase in red luminescence efficiency as feldspars were cooled from room temperature to 78 K. The implication is that IRSL sensitivity will be increased if measurements are made below room temperature. Visocekas et al. (1994) observed a reduction factor of 10 in red-IR phosphorescence as samples previously irradiated at room temperature were cooled to 80 K (see Section 5.2.10.5).
216
Optically Stimulated Luminescence Dosimetry
90 A
m
PL
F 30
Fig. 5.77. IRSLobservedat wavelengthsof 440 nm (O) and 600-700 nm (m) from an alkali feldspar measured as a function of temperature and PL (340 nm stimulation) observedunderthe same conditions (fromPoolton et al., 1995b).
5.2.9.2.
Thermal assistance
5.2.9.2.1. Above room temperature. As shown in Fig. 5.57, thermal activation is the
dominant process, when IRSL is observed, on raising the temperature from room temperature to about 200~ Indeed, Poolton et al. (1995b) concluded that the thermal activation energies obtained were hardly affected by the weak thermal quenching found in this temperature region. Bailiff and Poolton (1991) determined a thermal activation energy of 0.10 + 0.02 eV when stimulating using monochromatic IR at 930 nm. Duller and Wintle (1991) calculated a value of 0.15 _+ 0.02 eV for IRSL measured above room temperature for their particular feldspar samples. Htitt and Jaek (1993) measured the thermal activation energy as 0.2 +_ 0.1 eV in the temperature range from room temperature up to 200~ Clark and Sanderson (1994) measured the thermal activation energies for the three excitation bands that they obtained with their scanning monochromator (see Section 5.2.5.1). They obtained 0.2, 0.17 and 0.15 eV, respectively, for stimulation at 910, 624 and one component at 500 nm. 5.2.9.2.2. Below room temperature. As part of their study of IRSL stimulated from
orthoclase at temperatures between 80 and 300 K, Bailiff and Barnett (1994) found a component that had a thermal activation energy of 0.10 eV for temperatures above 220 K. In the temperature region from 80 to 140 K, Bailiff and Barnett (1994) could not calculate a thermal assistance energy because of the effects of thermal quenching. Subsequently, Barnett and Bailiff (1997b) obtained an activation energy for thermal quenching (W) of 0.064 _+ 0.003 eV for the IRSL for this feldspar in this temperature region. This allowed them to calculate an activation energy for thermal assistance of 0.061 _+ 0.004 eV (Barnett and Bailiff, 1997b).
OSL Properties of Natural Materials
217
Poolton et al. (2002a) re-plotted the orthoclase data of Bailiff and Barnett (1994) (Fig. 5.78a). They found that above 220 K they were consistent with a thermal activation energy of 0.1 eV, as found by Bailiff and Barnett (1994). Below this temperature Poolton et al. (2002a) calculated a thermal activation energy of 0.02 eV. Rieser et al. (1997) also made measurements below room temperature and obtained two activation energies for microcline, 0.15 and 0.05 eV. Poolton et al. (2002a) believe that these two components (--- 0.14 and --- 0.01 eV) indicate that non-tunnelling and tunnelling processes co-exist and can be identified (see Sections 5.2.9.2.4 and 2.4.5).
5.2.9.2.3. Wavelength dependence. BCtter-Jensen et al. (1994) used a scanning monochromator to measure thermal activation characteristics over the entire stimulation range of 400-1000 nm (3.0-1.25 eV). They made TOL measurements at 25 nm intervals on a sample of Amelia albite over the temperature range 50-180~ As previously shown in Fig. 2.7 (Chapter 2), at each energy where there is a peak in the excitation spectrum, there is a dip in the thermal activation energy curve (e.g., at 1.5, 2.1 and 2.5 eV). Poolton et al. (1994) suggest that these features reflect the complex nature of the conduction bands in feldspars. Poolton et al. (1995c) made further measurements on this and other feldspar
t,-
~
_
-4.5
0.11113
(K -1)
Fig. 5.78. Arrhenius plots of the thermal dependence of IRSL. (a) Data taken by Bailiff and Barnett (1994) from 80 to 300 K for orthoclase. (b) Data taken from 300 to 480 K for a range of single-crystal mineral (M) and sedimentary (S) feldspar samples (from Poolton et al., 2002a).
218
Optically Stimulated Luminescence Dosimetry
samples (orthoclase and oligoclase) in the temperature region from 20 to 200~ Fig. 5.79 shows the thermal activation energies obtained for the three samples as a function of the optical stimulation energy. These thermal activation energies can be compared with the energies at which the main lattice vibrational modes occur in feldspars (Salje, 1993); these phonon modes are shown in Fig. 5.79 and are based on data from Raman and infra-red absorption spectroscopy experiments. Also shown are the optical transition energies for albite, calculated using the Bohr hydrogen model (see Sections 2.4.5 and 5.2.12.1). 5.2.9.2.4. Link to anomalous fading. It thus seems that the value of thermal activation energy obtained varies rapidly near the peak of the IR resonance. This situation can be used to advantage. Since tunnelling is most likely to occur at the resonance, the determination of low thermal activation energies for IRSL measurements (preferably made below room temperature) could be linked to anomalous fading (see Section 5.2.10). Values of thermal activation energies have been suggested by Poolton et al. (2002a) as a means of predicting whether anomalous fading is likely to occur. If the thermal activation energy is low (e.g., ---0.02 eV), then this is assumed to be related to tunnelling between excited states. Conversely, if the thermal activation energy is around 0.12 eV, then this is related to a non-tunnelling process. Poolton et al. (2002a) observed the IRSL (stimulated at 830 nm) as a function of temperature from room temperature to 200~ for several sets of samples. The Arrhenius plots are given in Fig. 5.78b for single-crystal minerals (identified as M) and sediment extracts (identified as S). The samples with the lowest values of activation energy (---0.02 eV), exemplified by Ca26NaT1K (M), faded by ---65% on storage for 7 days at 100~ following a 100 Gy laboratory irradiation. The group with the highest values of thermal activation energy ( > 0 . 1 2 e V ) , exemplified by K(M) and K(S) showed no
Fig. 5.79. Thermal activation energy as a function of optical stimulation energy for three different types of feldspar, namely: Ab---albite, Or---orthoclase and An26Ab74--oligoclase. Horizontal lines represent calculated energies of different phonon modes. Vertical lines on the optical energy axis represent the allowed optical transitions of a donor defect calculated for albite. All energies abovethe optical ionisation energy of 1.896 eV are allowable (from Poolton et al., 1995c).
OSL Properties of Natural Materials
219
observable fading under the same conditions. The intermediate group with energies of 0.04-0.08 eV, exemplified by Na(M) and Na(S), showed some fading (around 10-15%). 5.2.10. Anomalous fading 5.2.10.1. TL, OSL and IRSL In some dating studies using either TL or IRSL signals from feldspars, ages are underestimated compared with the ages given by radiocarbon dating, or with ages based on OSL of quartz (Wallinga et al., 2001). The age underestimation is normally attributed to anomalous fading, a phenomenon that was first reported for TL of feldspars from volcanic rocks (Wintle, 1973). The term anomalous fading was used to indicate behaviour at odds with the predicted thermal stability deduced from laboratory-based kinetic studies. Feldspars of volcanic origin (e.g., sanidine) were observed to have a reduced TL signal at all glow curve temperatures, with losses as great as 50% occurring in a few days storage at room temperature (Wintle, 1973). However, measured losses are frequently much less, particularly in the case of K-feldspars extracted from sediments, although precise measurements of TL signal loss were hampered by the limited reproducibility that could be achieved using multiple aliquots. Templer (1986) suggested that the thermal dependence of the fading rate for zircon was consistent with localised transitions. However, quantum mechanical tunnelling has been proposed as the more likely mechanism for anomalous fading in feldspar (Wintle, 1977; Visocekas, 1985; Aitken, 1985 Appendix F), since it would result in the type of athermal behaviour that is reported. 5.2.10.2. Attempts to remove anomalous fading 5.2.10.2.1. Using a pre-heat. The application of elevated temperature storage as a means of removing anomalous fading in zircon TL signals was proposed by Templer (1985) by assuming that the anomalous fading observed could be explained in terms of a localised transition model (Templer, 1986). In this model, spatially localised electron traps and recombination centres share energy levels, allowing electrons to move from the trap to the recombination centre without going into the conduction band. This model would predict that a thermal treatment that would cause these electrons to recombine could be found, leaving stable electrons at other traps. Tyler and McKeever (1988) considered this mechanism to apply to the TL of an oligoclase. However, Spooner (1992) reported that the loss of signal for a labradorite for the OSL and IRSL was similar, irrespective of whether the sample was stored at 10 or 100~ behaviour that is consistent with the mechanism being quantum mechanical tunnelling. For one sanidine and an oligoclase, slightly faster loss of IRSL with storage time was found for 100~ however, no stable level was reached within 2 months, and only 50% of the initial signal remained. In a further study, Spooner (1994b) systematically examined OSL signals from a set of 24 well-characterised feldspars. Twenty showed similar fading rates for OSL (stimulated at 514 nm) and IRSL (stimulated at 880A80 nm) for storage at 10~ with signal loss at 100~ being slightly faster than at 10~ over the storage period of 15 months. This slight thermal dependence can be explained within the terms of a tunnelling model (Huntley, 1985).
220
Optically Stimulated Luminescence Dosimetry
Spooner (1994b) concluded that the anomalous fading of IRSL and OSL that he observed was consistent with quantum mechanical tunnelling, not a localised transition model. According to Visocekas (1985) quantum mechanical tunnelling is likely to occur on a wide range of time scales, since the range of trap lifetimes is likely to be infinite, with tunnelling distances ranging from 5 to 15 nm (Visocekas, 2002). Thus, the use of a preheat will not remove anomalous fading; it will lessen its effect in a dating study, but will also reduce one's ability to observe fading in laboratory experiments.
5.2.10.2.2. Using an optical treatment. In the study of zircon, Templer (1985) observed that the unstable TL signal could be removed by bleaching with red light (--- 600-660 nm). With a view to developing a similar approach, Spooner (1994b) measured the reduction of IRSL as a function of wavelength of the bleaching light. He found that the bleaching response was similar for those feldspars that faded and those that did not. In both cases, there was no resonance in the visible region (up to 750 nm) that could be linked to the removal of a specific component of the IRSL signal. Besides not providing a way to remove an unstable signal, the lack of resonance was taken as further evidence for there not being a localised transition. 5.2.10.3. Attempts to avoid anomalous fading 5.2.10.3.1. Using time-resolved measurements. Sanderson and Clark (1994) investigated the pulsed OSL signals (stimulated at 470 nm) from feldspars with the aim of identifying a luminescence component that was associated with long-range charge transport; such a component would not be affected by anomalous fading, according to the localised transition model. They carried out their experiments on a sample of feldspar from volcanic lava that was shown to lose more than 50% of its TL signal in 4 days storage following laboratory irradiation. They concluded that signals associated with components on the 40 n s - 8 ~s time scale showed the greatest fading. Both faster and slower components did not appear to show fading, at least not over the 4 day storage period. However, further studies using IR (850 nm) stimulation did not support the claim that it would be possible to select a signal component that did not fade (Clark et al., 1997; Clark and Bailiff, 1998). 5.2.10.3.2. Using different detection wavelengths. Spooner (1992, 1994b) used a detection filter (Schott BG39) that passed wavelengths from 340 to 610 nm for his IRSL studies that showed anomalous fading. However, there have been claims that better agreement of IRSL ages with independent ages can be obtained with particular detection filters (Aitken, 1998; Appendix D). More recently, it has been suggested that a non-fading IRSL signal can be obtained by observing red IRSL from 665 to 700 nm (Fattahi, 2001). This may provide a way to avoid anomalous fading in dating studies, whilst making use of the simple IR stimulation system and single aliquot measurement procedures. 5.2.10.4. CL and TL spectra of fading feldspars Visocekas et al. (1994) compared the room temperature CL spectra of six samples of sanidine that showed fading, three museum specimens (microcline, albite and adularia) that did not fade and three sedimentary K-feldspars that had been dated by IRSL and TL to over 100,000 years (and thus presumably did not fade). All samples showed a range of spectral peaks in the visible region, with emission at about 420 nm being dominant.
OSL Properties of Natural Materials
221
However, a major difference was seen in the infra-red, with all the sanidine samples having an emission peak at 720 nm (Fig. 5.80). It thus appears that there is a connection between the presence of infra-red CL and anomalous fading. Zink et al. (1995) measured the TL spectra of several volcanic sanidines that showed strong anomalous fading. For TL glow curves above RT, they observed well-separated peak emissions at 710 and at about 450 nm. They attributed the IR emission to Fe 3+ substituted for A13+ in the feldspar lattice.
5.2.10.5. Low-temperature phosphorescence Strong support for quantum mechanical tunnelling as an explanation for anomalous fading was provided in measurements of phosphorescence when recently irradiated samples were taken down to LNT. This phosphorescence was termed "tunnelling afterglow" by Visocekas (1985) when he cooled a labradorite from room temperature down to 80 K and similar behaviour was later reported for sanidine (Visocekas, 1993). This luminescence is observed at wavelengths from 590 to 890 nm, but is not seen when optical filters are used to restrict the observation to wavelengths from 305 to 590 nm. When the "red-IR" is obtained as a function of time during a typical experiment, the behaviour is typical of that shown in Fig. 5.81. The sample was irradiated at RT, following which a decaying signal is seen as traps just above room temperature empty. As the sample is cooled, the signal at first decreases, as these traps can no longer be thermally emptied. However, the signal does not reach zero, indicating the presence of an additional source of luminescence, the tunnelling afterglow. A minimum level is reached at about 250 K and on further cooling the signal increases. This increase is due to reduced thermal quenching of the red luminescence centres; as the sample is cooled, more luminescence centres produce photons when electrons recombine at those sites. On storage for 2 h at LNT, the tunnelling afterglow decays with time. At the end of the experiment, the sample is heated up to 400 K; from LNT to 250 K the luminescence decreases, due to thermal quenching, and then the TL glow curve is obtained. During the experiment "blue" luminescence is also observed, but is only significant when the final TL glow curve is measured. The glow curves for the two emissions peak at different temperatures, a fact previously noted by Visocekas (1985) for labradorite. Visocekas et al. (1994) carried out these experiments on 23 additional feldspars and compared the presence, or absence, of afterglow with the measured fading behaviour. Tunnelling rates, in terms of the percentage loss per decade of time, were calculated and a correction for thermal quenching was employed in order to permit comparison with the IRSL fading rate observed for RT storage (Spooner, 1994b). Visocekas et al. (1994) found a correlation between those samples that showed tunnelling afterglow and those for which fading had been detected. They also found that samples that did not fade did not give significant afterglow (corrected rates of < 2%). It thus appears possible to detect the likely presence of anomalous fading by monitoring the luminescence at LNT. This process is not only quicker than carrying out storage experiments, but is also more sensitive since one is not looking at small decreases in large absolute values and since the luminescence efficiency at LNT is enhanced by the lack of thermal quenching of the red-IR emission.
222
Optically S t i m u l a t e d L u m i n e s c e n c e D o s i m e t r y .................
-
_..t
:
.~
.
9
-
~:"
"~
..... ""
i .....~.
"
.." t ~ . .
. . . . . . . . .
"
-'/
~
'.x \ ""
9 \
"%
\
I
\
......
II"
.~. ...........
"\
360
460
560
660
760
860
9 . ..."
9
'" \
~X~-~
'-. " \
..~
..................
~5.[
',
. . . . . . . . . . . . . . . . .-... . . . . . .. .~. . . . . . . . .
. , ~ ..... 31 J2" "1
.................
%\
I
\
~:.I'~. .... " , II \-.. \\ ..... .
t~ t 9 / ....
1.0
/
"'k.
\
I ...
OSL Properties of Natural Materials
223
Fig. 5.80. Cathodoluminescencespectraof some feldspars: (a) Sanidines known to display tunnel afterglowand fading; (b) feldspars known to display neither fading nor tunnel afterglow; and (c) sedimentaryK-feldspars with low tunnel afterglow, dated to about 100 ka (redrawn from Visocekas et al., 1994).
5.2.10.6. Single grain IRSL fading and fadia plots Lamothe et al. (1994) found two populations of equivalent dose for individual Kfeldspar grains from a marine sand using the single-aliquot additive-dose method. Most of the grains gave values of De that were in excess of the values expected on the basis of the age and radioactivity of the sample, and Lamothe et al. (1994) concluded that these were not fully bleached at deposition. The remaining grains gave De values that were about half the expected value and it is suggested that the cause of this latter phenomenon was anomalous fading. Lamothe and Auclair (1997) measured the initial IRSL signal (LN) and the IRSL signal measured immediately after an additional g a m m a irradiation (LN+~), using a pre-heat of 250~ for 1 min before each measurement. They determined the ratio of these values as RI and found that it varied from grain to grain, suggesting differing amounts of fading for each grain. Lamothe and Auclair (1999) applied a gamma dose of 1.25 kGy to a Pliocene sample for which the IRSL would be expected to be in saturation, thus giving R] -- 1. However, three groups of grains that had values of RI < 2, 2 < RI < 4
-50
E ...1
5-
oO
i
--I .....
-I . . . . .
( ....
-i . . . . .
i .....
s ....
-I . . . . .
i .....
i. . . . .
-I . . . . .
[ .....
I. . . . .
"l . . . . .
,
,
i
i
\
0 200
Fig. 5.81. Typicalafterglow for a sanidine sample. The lower scale shows the time since the end of a 30-min beta irradiation. The upper scale is temperature (redrawn from Visocekas et al., 1994).
224
Optically Stimulated Luminescence Dosimetry
were observed, and RI > 4, thus indicating fading (Fig. 5.82a). Measurements over storage times of up to 50 days showed the IRSL signal loss to be characterised by a power law. The ratios measured at 4 h, RI (tl), and 10 days, RI (t2), for 48 individual feldspar grains from a much younger sediment were plotted against each other (Fig. 5.82b). Lamothe and Auclair (1999) called this a "fadia plot". All except three data points lie on a line that falls below the 1:1 line, but which intersects it for Ri(tl) - Ri(t2) = 5.18 --- 0.16, representing a point where no fading occurs and which represents the ratio of the IRSL signal (LN+~) divided by (LN) for the unfaded sample. The three open circle data points are from grains that were not fully bleached at deposition as shown by the ratio of their natural and regenerated IRSL signals (shown by arrows in the inset). The value at which R~(tl) = R~(t2) can be used to correct growth curves obtained for multiple-grain aliquots, and for this sample yielded an age of 68 + 7 ka, compatible with the U - T h date on a fossil coral from the same unit. This is twice the IRSL age estimate of 35 ka obtained using routine multiple-aliquot additive-dose procedure.
5.2.10. 7. Logarithmic signal decay Luminescence data for feldspars stored at room temperature are best plotted using semi-logarithmic axes, such that the percentage signal remaining is plotted as a function of the log of the storage time (Spooner, 1994b). TL data for two feldspars are shown in Fig. 5.83 (Wintle, 1973; Visocekas, 2000), together with lines for the theoretical fading rate, expressed in terms of percentage per decade of time. This approach was proposed by Aitken (1985), who defined these rates as g. From Fig. 5.83 it can be seen that age underestimation of--~ 20% would be expected for a sample of 1 million years in age if g = 1.5% per decade. If samples had g = 5% per decade, then age underestimation between 40 and 60% would be expected for samples in the range from 1 ka to 1 Ma. For the feldspar TL signals that have g--~ 20%, dating would be impossible. Thus, it is important to monitor the fading rate of any luminescence signal to be used for dating and determine whether the loss is greater than 1.5% per decade.
5.2.10.8. Correcting for anomalous fading Huntley and Lamothe (2001) developed further the model proposed by Aitken (1985) in order to predict the extent of fading of the natural signal compared with the signals produced by laboratory irradiation and used to obtain D e . They derived an equation
Tf/T = 1 - K[ln(T/tc)- 1],
(5.1)
where T is the true age and can be obtained by iteration. Tf is the age obtained when the laboratory-irradiated aliquots are measured at a time tc after the laboratory irradiation. K is related to g as K - g/100 In(10). K is obtained in a separate experiment to measure fading on the appropriate laboratory time scale and is given by the equation I = Ic[1 - K ln(t/tc)],
(5.2)
where I is the luminescence intensity at a time t after a short irradiation, lc is the intensity when t = to. It should be noted that the value of K depends upon the choice of to. For their study, Huntley and Lamothe (2001) chose tc to be 2 days, and Fig. 5.84 shows the
223
OSL Properties o f Natural Materials
RI
RI< 2 (n=28)
0"-"
(a)
---0"---
2 4(n=10) R I multiple grains
%
% % %
%
%
% %
~"~176 ,, - . . . . 9 . . . 0 1
.....
%
%
%
%
~_
103
%
10 6
10 9
Storage time (hours)
RI (t2) 60 I
50
20 (b)
15
.X
~=10 5 0 -2
40 -
/
0 2 4 6 8 Natural/Regenerated IRSL /
/
/
0
I 10
f
f
/
lO[/f 0
/
0
R I = 5.18 + o.16
1 20
I 30
.1. 40
I 50
I
60 Ri(tl)
Fig. 5.82. (a) RI (ratio of delayed to immediate IRSL) as a function of room temperature storage time for groups of typical single grains and for aliquots made up of multiple grains. (b) RI for a delay of 10 days plotted versus RI for a delay of 4 h for individual feldspar grains following a gamma dose of 1005 Gy. The inset is a histogram for the same grains showing the ratio of natural IRSL to IRSL regenerated by a beta dose of 100 Gy; grains indicated by arrows were not fully bleached (from Lamothe and Auclair, 1999).
226
Optically Stimulated Luminescence Dosimetry
_
44,9
'-. \ \
'-.
"'~'~....~.
\\
"~'-.~
",,
-...... .~.
\\
4~4 -
4, 4 "444
\ -
\
"444 4.
444,9 4~4~, 4 t~ UJ r .*..,
.c_ E
0
~
~
'".... A :
O
\ \
t(s) Fig. 5.83. Percentage TL left after anomalous fading as a function of storage time in semi-logarithmic coordinates, showing experimental data for labradorite (V from Wintle, 1977) and sanidine (A from Visocekas, 2000) samples and theoretical fading rates (redrawn from Visocekas, 2002).
measured loss in IRSL (corrected for optical depletion) for samples given a dose, preheated for 16 h at 120~ and then measured at different times. The samples shown cover the range of behaviour exhibited by K-feldspar separates from a number of sites in North America. Values of g ranged from 2 to 10% per decade. Huntley and Lamothe (2001) used Eq. (5.1) to correct for fading of a number of samples they had attempted to date. A test study was carried out on four samples with independent age control and for which the natural IRSL was in the linear part of the dose response curve. Corrected ages were also calculated for a further 24 samples from one site with four radiocarbon ages for peat marker horizons. The upper sample (SN4s) in Fig. 5.83 represents their behaviour, with g = 3.4 + 0.4% per decade. The uncorrected ages were 10-20% lower than expected; on application of the correction, the radiocarbon ages were found to lie very close to the trend line for the IRSL ages. For the sample SW6-01, with
OSL Properties of Natural Materials
1.0
t
i
i | i iiii1
i
227
i i iiiii I
I
Fig. 5.84. Fractional IRSL left after anomalous fading as a function of laboratory storage time, showing data for five samples of K-feldspar separated from sediments collected in North America (from Huntley and Lamothe, 2001).
g = 4.7 ___0.3% per decade, the correction increased the age from 4.19 +__0.20 to 5.3 +__0.3 ka, in far better agreement with the quartz OSL age of 5.4 +__0.6 ka.
5.2.11. Radioluminescence 5.2.11.1. A new dating method A strongly increasing RL signal was observed as the limit of the spectrometer (800 nm) was reached in a study of museum specimens of potassium-rich feldspars (Trautmann et al., 1998) when stimulated using a 137Cs source (see Section 5.2.6.3.3). Using a spectrometer with an extended spectral range (from 300 to 1000 nm), Trautmann et al. (1999) were able to observe the peak of the RL emission in the near infra-red at 865 nm (1.43 eV) for K-rich feldspars extracted from sediments. Fig. 5.85a shows RL spectra obtained for one such sample. The most important feature is the decrease in the IR-RL signal when it is exposed to laboratory irradiation and the increase in the IR-RL when it is exposed to sunlight. This is completely opposite to the behaviour of the RL emitted at other wavelengths, as also seen in Fig. 5.85a. Any sediment dating method (relying on exposure to sunlight and environmental radiation) based on the IR-RL signal would thus work in the completely opposite sense to those based on TL (Aitken, 1985) or OSL and IRSL (Aitken, 1998).
Optically Stimulated Luminescence Dosimetry
228
wavelength / nm
1000800
600
5~I .........
"~ 40 ~ :1~" [ i/l:" L :1 l"
1.5
500
400
300
/ Ca) 1
9. . . . sun-bleached (18 hours) natural - - - irradiated (50 hours)
2.0
2.5
3.0
| I 1
3.5
4.0
Ephoton] e V
Ookl (> 1 Ma) 9 H61 (> 200 ka) ---------- Gr68 (- 100 ka) . . . . . . Bur4 (~ 40 ka) Co3 (~ 35 ka) 9 Esl (~ 30 ka)
1.0
0.6
:t 0
200
400
600
800
1000
Dp/Gy
Fig. 5.85. (a) RL emission spectra for a sand sample (--~35 ka) treated as indicated in legend, and (b) normalised dose response curves for IR-RL from six samples (ages given in legend) for K-rich feldspars (apart from Gr68) (from Trautmann et al., 1999).
This is demonstrated in Fig. 5.85b, where laboratory irradiation is given to several naturally irradiated feldspar samples. Using the 137Cs source, not only to produce the RL but also as the laboratory-irradiation source, results in continuous plots of IR-RL as a function of dose. For the younger samples, the reduction in IR-RL is the greatest. Conversely, exposure to 2 - 3 h of sunlight results in an increase in the RL to a level that cannot be increased by further sunlight exposure; in this case, the greatest increase is for the oldest sample. Trautmann et al. (1999) used these concepts to obtain values of De for
OSL Properties of Natural Materials
229
these samples and the values were in agreement with those obtained using standard IRSL (observed at 410 nm and using a pre-heat) methods.
5.2.11.2. Practical considerations Krbetschek et al. (2000) constructed dose response curves for 30 samples using the 137Cs source (0.05 Gy/min) and found them to be characterised by a single exponential function that allowed precise De determinations to be made up to about 500 Gy. More recent studies have indicated that a stretched exponential function gives an even better fit (Krbetschek, pers. comm.). It should be noted that to reach 500 Gy using the 3.7 MBq 137Cs source would take 7 days. The maximum IR-RL signal level, attained by light exposure, could be obtained either by placing the grains in sunlight for 5 h or by using a 300 W sunlamp. Movement of the 4 mg aliquots out of the equipment for light exposure led to scatter in the maximum value of measured IR-RL. This was a major source of error in D e determination, and appears to be induced by changes in grain position. This scattering was particularly important when samples from recently deposited sedimentary environments were investigated (Krbetschek et al., 2000). For routine dating an instrument that would not require moving the sample aliquot for each exposure to light is needed. An advantage of the relatively high sensitivity for the IR-RL signal at low doses, provided that the bleached level can be obtained with precision, is that it should be easier to obtain depositional ages for young samples. The rate at which the IR-RL signal is altered by light exposure depends upon the power and wavelengths present. Krbetschek et al. (2000) found that a 200 W mercury vapour lamp (combined with a heat absorbing filter) at 20 cm from the sample would cause the IRRL to reach its maximum value within 10 min. Trautmann et al. (2000a) found that wavelengths below 500 nm were necessary to "zero" the IR-RL, with the procedure increasing in efficiency the higher the UV content. 5.2.11.3. Methods of De determination In addition to the additive-dose/total-bleach procedure for determining D e described above and exemplified in Fig. 5.86a, Krbetschek et al. (2000) also used the total-bleachregeneration procedure (Fig. 5.86b). Similar values of De w e r e obtained by the two approaches for two sedimentary samples. Because of the large radiation times involved (about 5 days for each dose response curve), IR-RL measurements in these examples were not made continuously, thus freeing the spectrometer to be used for other measurements. 5.2.11.4. Thermal stability Trautmann et al. (2000a) investigated the thermal stability of the IR-RL signal (and other RL emissions) by pulse annealing. They found that between 100 and 450~ there was no change in the IR-RL, thus indicating high-thermal stability. In addition, no fading of the IR-RL was observed when irradiated samples were stored at room temperature for several months. 5.2.11.5. Single grain measurements By increasing the spectral accumulation time from 300 to 900 s, Trautmann et al. (1999) were able to obtain RL spectra for naturally irradiated single grains (200-300 txm
230
Optically Stimulated Luminescence Dosimetry
(a)
1600;:
.~ 1400r
,ooo 1i
I~ 11oo
e
I
9ool 8001
De
-I
13-dose in Gy
1
(b)
De '
1()0
200
'
3()0
400
500
'
6b0
Fig. 5.86. (a) Additive-dose and (b) regeneration-dose IR-RL dose response curves for K-feldspars from a fluvial sediment, giving values of De of 120 and 100 Gy, respectively (from Krbetschek et al., 2000).
in diameter). All the K-feldspar grains showed an emission peak at 865 nm, though there was a factor of two variation in intensity and the peak structure for visible wavelengths showed even more variation. In a later study, Trautmann et al. (2000b) used an accumulation time of 300 s to measure single grain spectra after exposing the grains to sunlight for 10 h. They also constructed dose response curves for the different RL signals. 5.2.12. Models for IRSL, OSL, IR-RL in feldspars
5.2.12.1. IRSL Having observed the IRSL as a function of temperature from room temperature up to 200~ Htitt et al. (1988) proposed a model in which electrons were optically
OSL Properties of Natural Materials
231
stimulated by IR photons to an excited state of the electron trap, and from where they were elevated to the conduction band by thermal energy. Electrons from the same trap could also be excited directly into the conduction band. However, energies for thermal activation for IRSL of around 0.1-0.2 eV (see Section 5.2.9.2) seem far too small, when compared with the energy gap of about 0.8 eV suggested by Htitt et al. (1988). Clark and Sanderson (1994) found at least two stimulation peaks for the feldspars examined by them, and proposed a model consisting of at least two traps. The traps have one or more excited states from which electrons can be ejected into the conduction band by the lower energy photons. In their more detailed study of thermal activation energies for three feldspars, Poolton et al. (1995c) show that for the peaks of infra-red transitions, around 1.44 eV for albite and oligoclase and at 1.23 eV for orthoclase, thermal activation occurs (Fig. 5.79). The thermal activation energies are 0.105 eV for orthoclase, 0.065 eV for albite and 0.03 eV for oligoclase. For optical transitions at higher energies, above the optical ionisation energies of 1.896, 1.921 and 1.633 eV for albite, orthoclase and anorthite, respectively, a correspondence of the experimentally determined thermal activation energies and the lattice vibration frequencies can be seen (Fig. 5.79). As already discussed in Chapter 2, Poolton et al. (1995c) used the Bohr hydrogen model, and estimated that for albite the minimum thermal activation energy for the infra-red transition at 1.422 eV (the lowest excited state shown in Fig. 5.79) is 0.175 eV. This is three times larger than the measured value of 0.065 eV. From this Poolton et al. (2002a,b) concluded that the simple model put forward by Htitt et al. (1988) needed modification and instead suggested a donor-acceptor hopping model for the electron transport. See Section 2.4.5 for further discussion. 5.2.12.2. OSL As discussed in Section 5.2.2, the OSL decay curves do not alter their shape with increasing stimulation temperature. This behaviour led McKeever et al. (1997a) to propose that visible light is able to stimulate electrons directly into the conduction band. 5.2.12.3. IR-RL IR-RL is the infra-red RL emitted when potassium feldspars are exposed to ionising radiation, e.g., beta radiation from a 137Cs source (Trautmann et al., 1999). The IR-RL is observed when an electron in the conduction band is trapped at an electron trap. These trapped electrons can then be excited with IR to produce IRSL. A major difference exists between these two signals as they are used in the dating of sediments. IRSL is assumed to result from electrons being released from traps that are thermally stable over the period of interest and then recombining at recombination centres that are similarly thermally stable. IR-RL is observed as photons that are released when electrons from the conduction band are trapped at a thermally stable trap and provides a direct measure of the number of electrons in that trap. This fundamental difference results in IRSL dose response curves that grow with dose and are reduced to a near-zero level by exposure to light, whereas IRRL dose response curves decrease to a fixed level as the dose increases and increase to a fixed level as the sample is exposed to light (see Section 5.2.11). Trautmann et al. (1999) explained the production of IR-RL and IRSL in terms of a band model, with one trap and two recombination centres (Fig. 5.87). In Fig. 5.87a, ionising
232
Optically Stimulated Luminescence Dosimetry
Fig. 5.87. Band model for (a) RL production under beta stimulation and (b) IRSL production under IR stimulation. The model includes one trap (N2) which has an excited state (N2*) and two recombination centres (M4 radiative and M5 non-radiative) between the valence band (VB) and the conduction band (CB). Transition pathways are described in the text (from Trautmann et al., 1999).
radiation produces electrons and holes. The electrons in the conduction band may recombine at a non-radiative centre (Ms), at a radiative centre (M4) with the resulting luminescence being in the UV/visible part of the spectrum, or may be trapped at an electron trap (N2). When the latter occurs, an infra-red photon is emitted and IR-RL is observed. For IRSL to be observed, electrons in the trap (N2) need to be stimulated by IR photons (---1.45 eV, 855 nm) (Fig. 5.87b). The measurement of a thermal activation energy for stimulation in the infra-red (see Section 5.2.9.2) suggests that not all electrons are excited directly into the conduction band, but some recombine with a neighbouring recombination centre via an excited state (N2*), i.e., via donor-acceptor-pair recombination (viz., Poolton et al., 2002a,b). Electrons that reach the conduction band are able to recombine either non-radiatively at M5 centres or radiatively at M4 centres. Both these type of centres are made active by the trapping of holes formed in the valence band during irradiation (Fig. 5.87a). The prompt recombination (path [b] in Fig. 5.87a) and trap filling (path [c]) are competing processes. During irradiation (either in the natural environment or in the laboratory) the number of empty traps (N2) will decrease and the number of transitions via path [c] will also decrease. This explains the decrease in the dose response curves of Figs. 5.85b and 5.86. Since fewer traps become available during irradiation, more conduction
OSL Properties of Natural Materials
233
1.6
1.0
N
E o
r"
1.0
_.1
n,|
--,--
365nm
--o--
m
@ 0.8
575 nm
, \ \0
Fig. 5.88. RL signals, (a) IR (855 nm) RL and (b) blue (410 nm) RL, as a function of bleaching time when samples are exposed to light at the wavelengths given in the legend (from Trautmann, 2000).
band electrons are free to recombine at the recombination centres (Ms or M4) and thus the RL in the UV/visible emission bands (from type M4 recombination centres) increases with added dose (Fig. 5.85a). When exposed to light, electrons are removed from the trap (N2) and thus more traps are available. This is the cause of the high IR-RL levels following exposure to sunlight. Path [f] (Fig. 5.87a) is required to permit the observation of a high, but finite, level of IR-RL following optical stimulation.
5.2.12.4. Comparison of lR-RL and IRSL (or OSL) The ability to observe the RL spectrum, not just for IR but also for visible wavelengths, has provided much information about the recombination processes. This can be compared with the IRSL measured at different emission wavelengths. Trautmann et al. (1999) made
234
Optically Stimulated Luminescence Dosimetry
comparisons of thermal stability. For one sample they demonstrated that whilst the IR-RL was independent of temperature up to 450~ both the yellow and blue IRSL signals decreased rapidly between 260 and 360~ Trautmann (2000) investigated the effects of previous light exposure on IR-RL and blue-RL for times of 10-1000 s (Fig. 5.88). IR exposures had no effect on the IR-RL (Fig. 5.88a), in spite of the fact that this exposure should have caused a large reduction in the IRSL. Only wavelengths of 435 nm, or less, caused the IR-RL to increase. In contrast, the blue (410 nm)-RL decreased for all visible wavelengths (Fig. 5.88b). Trautmann (2000) further developed the approach of Trautmann et al. (1999) by introducing a localised transition into the model, consistent with the concepts proposed by Poolton et al. (1995a, 2000a,b). Based on measurements of a very old (> 1 Ma) sample, Trautmann (2000) concluded that only a very small number (< 1.5% for this sample) of electron traps were involved in the production of IRSL. In the localised transition model, IRSL can only be produced when there is a neighbouring recombination centre. Electrons that are in traps with no neighbouring recombination centre will still be excited by IR into the excited state of the trap, but cannot escape from it. However, they can be ejected into the conduction band if higher energy photons are used. Measurements of the dose dependence of the blue (410 nm)-RL and the IR-RL for samples of different age suggest that the recombination centre (M4) responsible for the blueRL is thermally unstable. As this is the recombination centre used in measurements of IRSL, this is a cause of concern for IRSL dating of K-feldspars. Trautmann (2000) suggested incorporating the thermal instability of the blue (M4) centres into a modified model. Trautmann (2000) stimulated the IR-RL and IRSL using the modified model and two ionisation rates, one related to the low-dose rate in the environment and the other to the higher dose rate used in laboratory irradiations. Trautmann used IR-RL to provide information on the trapped electron population and the blue (420 nm)-RL to provide information on the recombination centres. The IRSL signal was predicted for such conditions as environmental irradiation over different time periods (25-600 ka) and for pulse annealing experiments. The thermally unstable M4 radiative recombination centres provided the dominant control on the resulting IRSL signal. The trapped electron population (monitored by the IR-RL) continued to increase during environmental irradiation, long after the number of M4 centres reached an equilibrium level. Thus, Trautmann (2000) concluded that only IR-RL will be a suitable method for dating samples over 20 ka.
5.3. Conclusions
In the period since the publication of Aitken's book (1998) on optical dating, much has been learnt about the fundamental OSL properties of both quartz and feldspars. Almost all natural quartz samples have been shown to have a strong fast component in the OSL signal, best seen in LM-OSL measurements (Section 5.1.3). Provided this signal is dominant, sedimentary or heated quartz can be dated back to about 100 ka, with the limit related to the saturation of the electron trap (Section 5.1.7). The luminescence sensitivity can be altered by thermal treatment (Section 5.1.8), and any sensitivity changes that occur during a dating procedure can be monitored and corrections made. In particular, reliable
OSL Properties of Natural Materials
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ages are obtained using the SAR protocol (see Sections 6.5.4.5 and 6.11.2.2.2). To go back further in time, one of the slow components (Section 5.1.10) may be useful as it shows greater range in OSL response to dose (Section 5.1.10.2). However, being a slow component it is less easily optically bleached in nature (Section 5.1.10.3) and requires longer laboratory measurement sequences. Many advances have also been made in the understanding of feldspar IRSL behaviour (Section 5.2.12). The likely presence of anomalous fading can be detected using the red-IR phosphorescence (tunnelling afterglow) measured at LNTs following room temperature irradiation (Section 5.2.10.5). It has also been suggested that anomalous fading will be observed for samples that have IRSL thermal activation energies of around 0.02 eV, rather than 0.12 eV, for IRSL stimulated at the peak of the IR resonance in the stimulation spectrum (Section 5.2.9.2.1). Quantification of IRSL fading rates over laboratory storage times has led to two new ways of dealing with anomalous fading, namely a graphical approach (fadia) for single grains with different fading rates (Section 5.2.10.6) and a quantitative approach based on a tunnelling model (Section 5.2.10.8). The latter is limited to regions of linear growth with dose, with extrapolation of anomalous fading curves being restricted to four decades of time beyond the laboratory fading measurements used to obtain the fading factor. A more fundamental limitation of IRSL or OSL dating of feldspars is strongly suggested by the RL emission spectra (Section 5.2.12.4). The radiative recombination centres used in IRSL dating emit in the visible, and the RL measurements made following pre-heats suggest that they are only stable enough to date samples back to 20 ka. This would imply that apart from samples for which anomalous fading is a problem, ages back to 20 ka should be accurate, but older samples would give age underestimation. Thus, the development of correction factors for anomalous fading in samples over 20 ka would still not yield correct ages. For these samples, dating using the IR-RL emission appears to be more appropriate and more reliable (Section 5.2.11).
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Li, S.H., Tso, M.Y.W., Wong, N.W.L., 1997. Parameters of OSL traps determined with various linear heating rates. Radiat. Meas. 27, 43-47. Li, S.H., Tso, M.Y., Westaway, K.E., Chen, G., 1999. Choice of the most appropriate thermal treatment in optical dating of quartz. Radiat. Prot. Dosim. 84, 495-498. Liritzis, I., Galloway, R.B., Hong, D.G., 1997. Single aliquot dating of ceramics by green light stimulation of luminescence from quartz. Nucl. Instr. Meth. B 132, 457-467. Luff, B.J., Townsend, P.D., 1990. Cathodoluminescence of synthetic quartz. J. Phys. D: Condens. Matter 2, 8089-8097. Markey, B.G., BCtter-Jensen, L., Duller, G.A.T., 1997. A new flexible system for measuring thermally and optically stimulated luminescence. Radiat. Meas. 27, 83-89. Martini, M., Spinolo, G., Vedda, A., 1986. Radiation induced conductivity of as-grown and electrodiffused quartz. J. Appl. Phys. 60, 1705-1708. Martini, M., Paleari, A., Spinolo, G., Vedda, A., 1995. Role of [A104]~ centers in the 380-nm thermoluminescence of quartz. Phys. Rev., B 52, 138-142. McKeever, S.W.S., 1984. Thermoluminescence in quartz and silica. Radiat. Prot. Dosim. 8, 81-98. McKeever, S.W.S., 2001. Optically stimulated dosimetry. Nucl. Instr. Meth. B 184, 29-54. McKeever, S.W.S., Chen, R., Halliburton, L.E., 1985. Point defects and the pre-dose effect in natural quartz. Nucl. Tracks Radiat. Meas. 10, 489-495. McKeever, S.W.S., BCtter-Jensen, L., Agersnap Larsen, N., Duller, G.A.T., 1997a. Temperature dependence of OSL decay curves: experimental and theoretical aspects. Radiat. Meas. 27, 161-170. McKeever, S.W.S., BCtter-Jensen, L., Agersnap Larsen, N., Mejdahl, V., 1997b. OSL sensitivity changes during single aliquot procedures: computer simulations. Radiat. Meas. 27, 75-82. Mejdahl, V., Shlukov, A.I., Shakhovets, S.A., Voskovskaya, L.T., Lyashenko, H.G., 1992. The effect of shallow traps: a possible source of error in TL dating of sediments. Ancient TL 10, 22-25. Murray, A.S., Clemmensen, L.B., 2001. Luminescence dating of Holocene aeolian sand movement, Thy, Denmark. Quat. Sci. Rev. 20, 751-754. Murray, A.S., Olley, J.M., 1999. Determining sedimentation rates using luminescence dating. GeoResearch Forum 5, 121-144. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single-aliquot protocol. Radiat. Meas. 29, 503-515. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiat. Meas. 29, 65-79. Murray, A.S., Wintle, A.G., 1999a. Isothermal decay of optically stimulated luminescence in quartz. Radiat. Meas. 30, 119-125. Murray, A.S., Wintle, A.G., 1999b. Sensitisation and stability of quartz OSL: implications for interpretation of dose-response curves. Radiat. Prot. Dosim. 84, 427-432. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerativedose protocol. Radiat. Meas. 32, 57-73. Murray, A.S., Roberts, R.G., Wintle, A.G., 1997. Equivalent dose measurement using a single aliquot of quartz. Radiat. Meas. 27, 171-184. Nanjundaswamy, R., Lepper, K., McKeever, S.W.S., 2002. Thermal quenching of thermoluminescence in natural quartz. Radiat. Prot. Dosim. 100, 305-308. Nassau, K., Prescott, B.E., 1975. A reinterpretation of smoky quartz. Phys. Stat. Sol. (a) 29, 659-663. Nuttall, R.H.D., Weill, J.A., 1980. Two hydrogenic trapped-hole species in alpha-quartz. Sol. State Commun. 33, 99-102. Poolton, N.R.J., BCtter-Jensen, L., Ypma, P.J.M., Johnsen, O., 1994. Influence of crystal structure on the optically stimulated luminescence properties of feldspars. Radiat. Meas. 23, 551-554. Poolton, N.R.J., BCtter-Jensen, L., Johnsen, O., 1995a. Influence on donor electron energies of the chemical composition of K, Na and Ca aluminosilicates. J. Phys. Condens. Matter 7, 4751-4762. Poolton, N.R.J., BCtter-Jensen, L., Duller, G.A.T., 1995b. Thermal quenching of luminescence processes in feldspars. Radiat. Meas. 24, 57-66. Poolton, N.R.J., BCtter-Jensen, L., Johnsen, O., 1995c. Thermo-optical properties of optically stimulated luminescence in feldspars. Radiat. Meas. 24, 531-534.
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Wiggenhorn, H., Rieser, U., 1996. Analysis of natural IRSL and TL emission spectra of potassium-rich feldspars with regard to dating applications. Radiat. Prot. Dosim. 66, 403-406. Wintle, A.G., 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143-144. Wintle, A.G., 1975. Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astr. Soc. 41, 107-113. Wintle, A.G., 1977. Detailed study of a thermoluminescent mineral exhibiting anomalous fading. J. Lumin. 15, 385-393. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769-817. Wintle, A.G., Murray, A.S., 1997. The relationship between quartz thermoluminescence, photo-transferred thermoluminescence and optically stimulated luminescence. Radiat. Meas. 27, 611-624. Wintle, A.G., Murray, A.S., 1998. Towards the development of a preheat procedure for OSL dating of quartz. Radiat. Meas. 29, 81-94. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiat. Meas. 30, 107-118. Wintle, A.G., Murray, A.S., 2000. Quartz OSL: Effects of thermal treatment and their relevance to laboratory dating procedures. Radiat. Meas. 32, 387-400. Woda, C., Schilles, T., Rieser, U., Mangini, A., Wagner, G.A., 2002. Point defects and the blue emission in fired quartz at high doses: a comparative luminescence and EPR study. Radiat. Prot. Dosim. 100, 261-264. Yang, X.H., McKeever, S.W.S., 1990. The predose effect in crystalline quartz. J. Phys. D: Appl. Phys. 23, 237-244. Yoshida, H., Roberts, R.G., Olley, J.M., Laslett, G.M., Galbraith, R.F., 2000. Extending the age range of optical dating using single 'supergrains' of quartz. Radiat. Meas. 32, 439-446. Zimmerman, J., 1971. Radiation-induced increase of the 110~ thermoluminescence sensitivity of fired quartz. J. Phys. C: Sol. State Phys. 4, 3265-3276. Zink, A., Visocekas, R., Bos, A.J.J., 1995. Comparison of 'Blue' and 'Infrared' emission bands in thermoluminescence of alkali feldspars. Radiat. Meas. 24, 513-518.
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Chapter 6
Retrospective OSL dosimetry Part 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 6.1. Introduction
In the event of a large-scale nuclear radiation accident, a quantitative assessment of the radiation dose to the general population requires the availability of suitable techniques and procedures for reconstruction of doses. The main purposes of dose reconstruction, or retrospective dosimetry, in relation to the local population after a nuclear accident, can be summarised as follows: 9 to guide the provision of proper medical treatment and protection for people exposed to radiation, 9 to provide input data for epidemiological studies, 9 to provide information to the population, and 9 to help carry out research to improve dosimetry and preparedness. The methods used for dose reconstruction have been based on: (1) Dose modelling, e.g., Monte Carlo simulations based on direct measurement results obtained from local active dose rate meters (GM counters etc.) (e.g., Meckbach and Chumak, 1996). (2) Application of luminescence methods with ceramics (thermoluminescence (TL) and optically stimulated luminescence (OSL)) including modelling and photon transport calculations (Bailiff and Stepanenko, 1996; BCtter-Jensen, 2000a). (3) Direct measurement of accumulated doses in human tissues using: (a) Electron paramagnetic resonance (EPR) on tooth enamel (Wieser et al., 2000). (b) Chromosome analysis of lymphocytes in blood; Fluorescence In Situ Hybridisation (FISH) painting methods (Lloyd et al., 1996). Solid-state dosimetry methods based on luminescence, including TL and OSL, are particularly useful because they enable the integrated absorbed dose to be measured. In the case of external sources of radiation, materials found within the accident area can be used, e.g., bricks, tiles and pottery collected from local buildings. The absorbed dose may be evaluated many years after the accident. Consequently, luminescence methods have the potential to provide data essential for dose reconstruction in areas and locations where radiation-monitoring measurements were not carried out. Haskell
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(1993a,b) and Bailiff (1995, 1997) have recently reported the basis and operation of TL techniques and critical factors concerning their use. In the mid 1990s the application of OSL techniques to ceramics for the retrospective assessment of accident radiation doses was suggested (BCtter-Jensen, 1996, 2000b). The following sections describe OSL techniques and analytical procedures, particularly for the measurement of doses from materials collected in a nuclear accident area.
6.2. Materials and sampling The types of ceramic generally found to be suitable for retrospective dosimetry include fired materials such as bricks, glazed and unglazed tiles, roof tiles, interior floor tiles, porcelain fittings (e.g., sanitary ware) and exterior fittings such as lamp holders and electrical power line insulators (see Fig. 6.1). These materials can usually be found in various locations, allowing an investigation of both the nature of the external field and the degree of shielding within the interiors of buildings. Brick buildings offer the highest degree of flexibility in choice of samples because ceramic material is available at a range of depths within the wall. The composition and form of suitable material varies according to the geographical location and the nature of the building environment. Ceramic materials have so far proved to be the only candidates for measurement of doses at the 10 mGy level. In most cases, bricks contain a high proportion of quartz and variable amounts of feldspar of 9 0 - 1 5 0 Ixm grain diameter, the size range most suited to the use of coarse-
Fig. 6.1. Schematic of a house showing ceramic materials potentially usable as dosimeters for retrospective dose evaluations.
Retrospective OSL Dosimetry
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grain techniques (see Sections 6.5 and 6.11). However, the quality of the bricks may vary significantly depending upon the manufacturing procedures used. Some manufacturers add roughly crushed quartzite to the clay and this can result in poor characteristics from the point of view of dosimetry. A homogeneous distribution of quartz grains is essential when, for example, a dose-depth profile is to be measured.
6.3. Sample preparation and experimental details To estimate the dose in mineral grains, the sample must be extracted without exposure to light to avoid any bleaching of the luminescence signal. In the laboratory, the samples are handled in dim red or orange light (similar to that in a photographic darkroom). Any outer bulk material that may have been exposed to daylight must be removed; this may be kept in reserve for dose rate determinations and particle size analysis. Bricks can be sliced into 10 mm sections that are crushed, and the sand-sized grains (90-180 Ixm) extracted by sieving and treated with HC1. Quartz grains are then concentrated by heavy-liquid separation using sodium polytungstate (2.62-2.65 g/cm3). These are then etched in 40% HF to remove any residual feldspars. Acid-soluble fluorides are subsequently removed in 15% HC1. If only quartz is to be measured, satisfactory results can be obtained without heavy-liquid separation; the selected particle size range is treated with HC1 and H202, and then placed directly in concentrated HF.
6.4. Determination of the accident dose All bricks contain radionuclides that contribute to the dose of all mineral grains that make up the brick. This dose component will increase with time. Following a nuclear accident in the vicinity of the bricks, an additional dose will be recorded, namely, the accident dose. It is the aim of retrospective dosimetry studies to obtain the accident dose as accurately as possible. This requires measurements to determine the natural dose rate and the age of the building, either from documentary information or other measurements. 6.4.1. Retrospective assessment of environmental dose rates The dose absorbed by a mineral grain is a function of the radioactivity both in the grain and in the surrounding material. For quartz the internal dose rate, derived from within the grain, is usually very small (Aitken, 1985). The external dose rate depends mainly on the concentrations of 4~ 238U and 232Th series radionuclides in the surrounding material, plus a component due to cosmic rays. Consequently, to determine the dose rate, it is necessary to measure the radionuclide concentrations in the surrounding material. These calculations are summarised by Aitken (1985, 1998). The water content of the material may affect the dose derived from the natural radioactivity surrounding the grain, by absorbing some energy, and thereby decreasing the dose rate to the mineral grain. Thus, the water content must be estimated and a modified dose rate calculated. The dose received by quartz grains from natural radionuclides within the brick material can be as much as 5 mGy/year. The background dose can thus make up a considerable part
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Optically Stimulated Luminescence Dosimetry
of the total dose, particularly if bricks are collected from older buildings. As the background component is a major source of error in estimating the accrued accident dose, it is important that this background component be minimised and accurately determined. The annual dose rate resulting from internal emitters can be determined in different ways, e.g., by gamma spectrometry (Murray et al., 1987) and beta counting (BCtter-Jensen and Mejdahl, 1985, 1988). The cosmic ray contribution is calculated from relationships given by Prescott and Hutton (1988). The thickness of the overlying material, and the latitude and longitude of the location are required for these calculations. Having measured the radionuclide concentrations in the surrounding material and calculated the cosmic ray contribution, the total dose rate is calculated using the conversion factors of Olley et al. (1996). Sensitive artificial TL/OSL phosphors such as A1203:C (Akselrod et al., 1990) have also been used to perform in situ measurements of the integrated gamma doses over short periods without the need to correct for differences in mass absorption characteristics at low photon energies (BCtter-Jensen and McKeever, 1996; BCtter-Jensen et al., 1997, 1999). Such measurements help to confirm the modelled natural dose rates, and thus support the calculation of the natural background contribution to the total dose recorded after an accident (except in the case of very short lived contamination, where this background cannot be measured directly after the accident). Experiments have been designed where A1203:C dosimeters were placed in house walls. The observed doses were compared with: (1) doses derived from OSL measurements of quartz samples extracted from the same bricks, and (2) dose rates determined from laboratory measurements of the natural radionuclide concentrations. BCtter-Jensen et al. (1999) placed A1203:C dosimeters in bricks in two different house walls with known ages of 37 and 72 years, respectively, for a period of time to integrate the environmental dose rates. Aluminium tubes, 12 cm long, with an inner diameter of 6 mm and a wall thickness of 1 mm (to absorb beta particles) were packed with 30 freshly annealed A1203:C pellets (arranged in groups of three) distributed over the length of the aluminium tube using 10 mm plastic spacers; this provided 10 measurement points over the entire cross-section of the brick. The aluminium tubes were then placed in holes drilled in the middle of bricks in the two walls and left there for 18 days. At the end of the exposure period, the dosimeters were immediately removed from the aluminium tubes and their OSL signals measured. The bricks that held the aluminium tubes were then removed from the walls and quartz grains extracted for OSL measurements. Remaining bulk material was crushed and the natural radionuclide concentrations were determined using a high-resolution gamma spectrometer (Murray et al., 1987). The two dose-depth profiles measured with A1203:C are shown in Fig. 6.2. A1203:C measurements include a cosmic ray component (Prescott and Hutton, 1988), which in this case was estimated as 0.27 mGy/year, and which has to be subtracted to give the observed gamma dose rate. The quartz-derived dose rate includes the same cosmic component, an infinite matrix beta dose rate (derived from the measured concentrations) and an internal alpha component, estimated using 10% of the bulk concentrations, and an 'a' value of 0.15 (this alpha contribution amounts to approximately 5% of the total). When these contributions are subtracted from the total quartz dose rate, the so-called quartzderived gamma dose rates are obtained. Experiments have shown that it is not possible to
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Retrospective OSL Dosimetry
>'
4,5
_
j
.TL
=
i
~ 0 0
+ E
2,52,0; 0
i
i
i
i
20
4O
60
80
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).
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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
c-
8
(_1
m O
4
,
i
,
|
,,
l
,
l
,
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
L.
E O
J
r--
~
J
D2
~
Added beta dose Fig. 6.4. Principle of the SARA method, schematically. The figure shows the doses determined by regeneration versus the added doses. See text for details (from Mejdahl and BCtter-Jensen, 1997).
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
1
E 0.1 c"
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" or--
)1/2/Y.iS i -
~'.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
05
0
10 5
I-- o
2OO0
oE
,r- 0 ~'-'0
0
0
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
257
0000000
o
O0 0 0
0
8 2x106 g
E
~ g~
0~ 0
0
m
60
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 "2".
~d 3.5
I--I
I 3.0
r
/
/, /
T~.TJ De = 464 mGy I
0.0
//,
I 500
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
260
Optically Stimulated Luminescence Dosimetry (a)
(b)
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
100
0
30
60
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
(a)
95 80 60
40
o r o-
~
20
U..
5
="
]
~
E
(b) >~
6
t
5
,1
E 0
50
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
265
A
12 A
10
>h
(,.5 q)
8
o "o o
6
m
4
Ix.
2
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
"T
c
(c) E
2ooo| OI
L~
,
,,
,
|
|
i
0
200
400
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
Retrospective OSL Dosimetry
_ t
269
-
\\
" I
0.50I .c_ E o.25 _J
0" 0
20
40
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
1
(a) 5
~9 0.4 .d 13.
0.2 ,
,
,
,
.
.
.
.
.
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
8
0.5 I
75O
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
8 x _J
m 0
4 (
,
1 oo 0 0
"-0.8 xi. O
0.6 o -o 0.4 09
.4--, t~ X--
0
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
v ffl
a0
2,5
121
1 0,5
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
275
Time of IRSL [sec]
d~
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
~ 0 ,
,
,
,
,
,
,
,
~.0 -
I
~ ,
,
1 ,
~. " ~." - .
.
7
t'O
0
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
279
Dosimetry
.~.. .~.. . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
~
40
i
20
,
.
/
/ // _ _ .
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
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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
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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
285
Fig. 6.27. De obtained using SAAD as a function of pre-heat temperature for five samples of Australian quartz (from Murray et al., 1997).
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
Fig. 6.29. Depletion of OSL signal caused by repeated pre-heating and optical stimulation (but no added dose) showing that the data points are not fitted by a single exponential (from Stokes et al., 2000).
Retrospective OSL Dosimetry
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317
OSL Measurement Technology
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