Prompt Gamma Neutron Activation Analysis

Library of Congress Cataloging-in-Publication Data Prompt gamma neutron activation analysis / edited by Zeev £3. A l f w i and Chien Chung. p. cm. Includes bibliogrilphical references m d index. ISBN 0-8493-5 149-9 1. Nuclear activation analysis. I. Alfassi, Zeev B. 11. Chung. Chien, 1950- .

QDGOG.P76 1995 543'.08824c20

94-24929

CIP

This book contains information obtaincd from aulhcntic imd highly regardcd sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their usc. Neither this book nor any part may be reproduced or transmitted in any Corn1 or by any means, electronic or mechanical, including photocopying, microfilming, and rccording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use. or the personal or internal use of specific clients, may be granted by CRC Press, Inc., provided that $.SO per page photocopied is paid directly to Copyright Clcnrilnce Ccntcr, 27 Congress Strecl, Salc~n.MA 01970 USA. Thc fce code for uscrs of the Transactional Reporting Scrvicc is ISBN 0-8493-5 149-9/95/$0.00+$.50. The fee is subject to change without notice. For organizationsthat have been granted a photocopy license by the CCC, a separate systenl of payment has been arranged. CRC Press, Inc.'s consent does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press for such copying. Direct all inquiries to CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, Florida 33431. O 1995 by CRC Press, Inc.

No claim to original U.S.Government works International Standard Book Number 0-8493-5149-9 Library of Congress Card Number 94-24929 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

CONTENTS

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Chapter 3 ~ e & hDamage n and Induced Effects on Nuclear Instruments Used for PGAA chi% Chung

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pr&ipt Gamma Neutmn Activation Analysis with Reactor Neutrons Zeev B. Aljiassi

.................

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Chapter 5 PGNAA with Radioisotopic Sources, Neutron Generators, and Charged Particle . Acceler{ators ................................................................................................................................. Zeev B. Avassi Chapter 6 Prompt Gamma Actitration Analysis with Guided Neutron Beams Riclrfird M. Lirrdstron? arid Chus.hirn Yoneznrvn CZmptcr 9 . . Prompt G,zmma Activation Analysis

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I n Sitrt 'rrk~qrlicationls......................................................................................................................... Jiunn-h'sihR Chao Chapter 9 On-Line Applications Jiunn-Hsing Chao

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Appendix P - Thermal Neutron Capture Gamma-Rays J. K. TLili

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Appendix 31 Otlaer Rndiation-ReZated Properties of Prompt Gamma Activation Facility Chien Chrng

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THE EDITORS Zeev B. Alfassi, Ph.D., is professor of Radiochemistry and the Chairman of the Nuclear Engineering Department in the Ben Gurion University, Beer Sheva, Israel. Professor Alfassi received his B.Sc. and M.Sc. degrees from the Hebrew University in Jerusalem in 1964 and 1965, respectively, in the fields of chemistry and biochemistry. He received his Ph.D. from the Weizmann Institute of Science and the Soreq Nuclear Research Center in 1970. Professor Alfassi is a member of the council of the Israel Nuclear Society. He has published more than 150 scientific papers and edited the CRC books Chemical Kinetics of Small Organic Radicals, Activation Analysis, and Elemental Analysis by Naclear Methods and Prcconcentration Techniques of Trace Elements. Last year he edited Chetnical Analysis by Nuclear Methods (John Wiley & Sons) and Determination of Trace Elements (VCH). His current research interests include chemical analysis by nuclear methods, radioisotope production and uses, radiation chemistry and.chemica1kinetics of radicals . in solution, and solubility of electrolytes in water-miscible organic-solvents mixture.

Chien Chung, Ph.D., is chairman of the ~ e ~ h e ofn Nuclear t Science, National Tsing Hua University, Taiwan, ROC. He is also the adjunct professor of the National Defense Medical College in Taipei. Dr. Chung obtained his B.Sc. degree in 1972 from the Department of Nuclear Engineering, NTHU and his Ph.D. degree in 1980 from the Department of Chemistry, McGilI University, Montreal, Quebec, Canada. Dr. Chung is also a permanent member of the American Nuclear Society, divisional member of the American Chemical Society, and life-time member of the Chinese Nuclear Society. Dr. Chung served the NTHU as Institutional Director, Research Center Director, and the Dean of the Student Affairs. Before he came to NTHU as Associate Professor in 1983, he served both at the-Brookhaven National Lab as visiting scientist and the University of Maryland as research associate. Dr. Chung was promoted to full professor in 1986 at the age of 36. Dr. Chung has published more than 100 original papers in scientific journals recognized by the Citation Index; he has also presented over 50 invited papers at international conferences to whkh he served as chairman in numerous cases. His current major research interests include nuclear instrumentation, nuclear medical science, health physics, nuclear and radiochemistry, and environmental monitoring -of radiation.

Richard M. Lindstrom Inorganic Analytical Research Division Nationa! Institute of Science and Technology Gaithersburg, Maryland

J. K. Tuli Development Center ational Tsing Hua University Hsinchu, Taiwan

National Nuclear Data Center Brookhaven National Laboratory Upton, New York

Chien Churng

Chushiro Yonezawa

Department of Nuclear Science Natiorlal Tsing Hua University Hsinchu, 'Ijiwan

Eepartrnent of Chemistry Japan Atomic Energy Research Institute 'lbkai-mum, Ibaraki-ken, Japan

Chapter 1

Introduction Zeev B. Alfassi CONTENTS 1. Introduction ................................................................................................................................. 1 11. Neutron Sources .......................................................................................................................... 3 A. Research Nuclear Reactor ................................................................................................. 3 1. Neutrons from Accelerators and Neutron Generators .................................................... 4 2. Radioactive Neutron Sources ..........................................................................................4 111. Gamma-Detectors ....................................................................................................................5 A. Scintillation Detectors NaI(T1) and BGO ............................................................................ 5 B. Solid-state Ionization Detector ............................................................................................ 6 IV. The Shape of the y-Spectrum ................................................................................................. 7 V. Detection Systems .......................................................................................................................10 A. Anticoincidence Compton Suppression ................................................................................ 10 B. Coincidence Double-Escape Counting ................................................................................. 11 References ........................................................................................................................................ 12

f. INTRODUCTION All nuclear techniques of chemical alialysis involve the interaction between an incoming projectile (neutron, charged particle, or y-photon) and a target nucleus. As a consequence of this interaction, there is usually formation of two products-the light one and a heavier product. The light one is mainly yphoton, neutron, or small charged particle. We can write this interaction in the chemical form of reactions: projectile C target nucleus

+ light product + heavy product

or, it can be written in the shorter formalism of nuclear physicists:

target (projectile, light product) heavy product This formal writing does not exclude the possibility that :he reacting target projectile is forming initially a compound nucleus which later disintegrates into the two products. However, it limits the length of time between the interaction and the formation of products. For activation analysis, as long as this time is shorter than 1 p s (in most cases, it can even be 1 s or 1 min), we can treat the final products as our products, neglecting the intermediate steps. The rate of the nuclear interaction, R (number of interactions per unit time), is given by an equation similar to that of a bimolecalar chemical reaction:

where N = the density of the target nuclei (target nuclei per unit volume) I = the intensity of the incident projectiles (number per unit time)

X

-

=

a =

thickness of the target the reaction cross-section (a measure of the reaction probability)

0-84911-5149-9/951$0.00+$.50 0 1995 by CKC Press. Inc.

.

Cross-section has the dimensions of area and, since most cross-sections arc of the order of cm2, the cross-sections are reported in units of barns, defined as: ,"2 .

1 barn = lo-" cm2

Equation 1 shows that R is proportional to N, and hence by measuring R, we can calculate N-the number of target nuclei. This is the basis for all methods of activation analysis. R is measured by measuring the number of newly formed products species, either of the heavy product or that of the light one; and from it, we deduce the number of nuclei of that type present in the sample. Since the reaction is with the nucleus, and the surrounding electrons have no effect, the analysis tells only about the existing atoms, but not their chenlical speciation. It is therefore an elemental analysis. The heavy product can be a stable nuclide or a radioactive one. In cases where the heavy product is unstable, the number of nuclei of the heavy products formed can be measured by following the decay of the heavy product. This measurement is done after the end of the nuclear activation of the target. Consequently, this type of analysis is called delayed analysis. This method of analysis is limited to cases of unstable (either excited or radioactive nuclide) heavy product, with appropriate half-life. If the half-life is too short, either its saturation activity can be too low or it is technically too short for measurement. If the half-life is too long, activation during a reasonable length of time does not produce sufficient activity. Even in cases where the formed heavy product has a half-life of reasonable length, it cannot be used in delayed analysis if it is only a P- emitter due to the difficulties in distinguishing between p- particles emitted from different radionuclides. These problems do not exist in the measurement of the light particles. Another phase of measurement of the number of formed particles is by measurement, during the interaction, of the emitted light product. These light products have to be measured during the experiment and cannot be delayed, since they either disappeared in a very short time (as is the case of y-photons or neutrons) or can be measured only due to their high kinetic energy. If we wait, they will lose their high kinetic energy and can no longer be measured. Their amounts cannot be measured chemically due to the small amount formed (in the case of light product which is a small nucleus as e.g., protons or a-particles). Chemical measurement is not as sensitive as measurement of decaying nuclei or high kinetic energy charged particles. The measurement of a-particles by counting is several orders of magnitude more sensitive than chemical measurement of He gas. The same is true for accelerated protons vs. hydrogen molecules. Since in this case, the measurement of radiation (yphotons or small particles) occur during the nuclear interaction, this form of analysis is referred to as prompt analysis. In the case of prompt y-neutron activation analysis (PGNAA), the projectile is a neutron and the light product ia a y-photon. The reaction is written T(n,y)P, where the T is the target nucleus and P stands for the product. This reaction is called radiative capture of neutrons. There is one exception to the usual prompt y-measurement, in which the y measured is not due to the (n,y) reaction, and this is the case for boron. Boron (like lithium-6) does not react with neutrons by emission of photons, but rather by emission of a-particles, i-e., via the reaction 'OB (n,a) 7Li. However, the lLi formed is only partially (6.5%) in the ground state. Most of the 7Li nuclei (93.5%) are formed in thc excited state, which is decxcilcd with a half-life of 7.3 X 10'" s by emission of 477-keV yrays. Considering our agreement-that everything that occurs within less than 1 p,s is included in the interaction process-we can write this process as 'OB (n,cry) 'ki. The cross-section for this reaction is 0.935 of the conventional 'OB (n,a) 7Li cross-section. In summary, PGNAA offers a nondestructive, relatively rapid method for determination of trace and major elements that cannot be determined by conventional activation methods due to either only Pemission or inconvenient half-life (either too short or too long, including stable nuclides). The energy of neutron capture y-radiation ranges from about 50 keV to about 10 MeV and the spectra of most nuclides are fairly complex. PGNAA has found many uses in research, medicine, and industry. It was reported that in the coal industry in Canada and the U.S., there are 26 commercial PGNAA gauges.' However, this is not limited only to the coal industries but PGNAA gauges are found also in cement and other mineral industries. The main technical factors leading to the success of these gauges in recent years are:2

1. The development of a very stable high count rate y-spectroscopy 2. The availability of large radiation-hardened G e detectors (although most gauges use NaI(T1) detectors)

3, The availability of inexpensive, small but reliable and powerful multichannel analyzers (PC-based) 4. careful study and understanding of the transport processes of neutrons and the associated 7-rays in various bulk media

11. NEUTRON SOURCES ain sources of neutrons are available: (I) research nuclear reactor (nuclear reactors used as n sources); (2) ion and electron accelerators including neutron generators; and (3) radioactive es. Research nuclear reactors have the highest neutron fluxes, but are limited concerning on-site emination, price, and availability. Consequently, nuclear reactors are used predominantly for delayed neutron activation analysis of very minute amounts or for sensitive neutron radiography. However, it is used also for PGNAA. When site irradiation is important, neutron generators or radioactive sources are used.

A. RESEARCH NUCLEAR REACTOR Research nuclear reactors are usually Iarge devices in which fissionable material, almost exclusively is fissioned into two nuclides with simultaneous emission of neutrons that induce further fissions jn a chain reaction. The fission-produced neutrons are very energetic. The cross-section for neutroninduced fission of fissionable nticlides increases with decreasing energy of the neutrons; in order to increase the neutron activity, moderators that slowed down the neutrons are added to the reactor. To reflect back some of the neutrons that leaked from the reactor core, reflectors are used. The fission process releases large amounts of energy, mainly due to the stopping of the recoiling two fissioned and the system is cooled by a coolant (either liquid or gas). The nuclear reactors are categorized according to their fuel, moderator, coolant, reflector, and configuration. Almost all research nuclear reactors (neutron sources) are heterogeneous reactors in which the fuel is in the form of rods. The fuel (natural uranium has or!ly 0.7% *'TJ-the fissile material). Most research reactors have is enriched 235U 93 to 99% W. Many of the reactors have rods that are U-A1 alloys; however, some of the newer designs (mainly those converted to 20% 2'5U) are of the uranium-silicide type. Triga reactors operate with uranium-zirconium hydride fuel that, due to its large negative temperature coefficient of reactivity, allow the operation of the reactor in pulses. In the Light Water Reactor (LWR), ordinary water (H20) is used both as a moderator and as a coolant. The reflector is mainly graphite, but there are also Be or H20 reflected reactors. The construction is either pool type or tank-in-pool type. Due to the relatively high cross-section for capturing thermal neutrons by H atoms, the flux of neutrons in LWRs always contains large fractions of fast and epithermal neutrons. f i e available power is in the range of 10 to 5000 kW,with neutron fluxes of 5 X lOI4 to 1.5 X lot8d m 2 . s. The neutrons are usually divided into three groups according to their energy: (1) thermal energy with most expected energy of kT = 0.025 eV at room temperature; this group is usually extended up to the Cd (thermal neutrons absorber) cutoff of 0.5 eV (2) neutrons with energy between 0.5 eV and 1 MeV are called epithemal neutrons; and (3) neutrons with energies above 1 MeV are called fast neutrons. Many reactors are unique in their design; however, there are some commercial types that are more common-the American TRIGA and the Canadian Slowpoke. The TRIGA reactor is a popular multipurpose research reactor. About 50 of them are operating with power levels of 18 kW to 3 MW (fluxes of 7 X lot5to 3 X lot7d m 2 . s). The most common types are those of 250 kW and 1 MW. They are of the pool type, graphite reflected with uranium-zirconium hydride fuel, and 23SUenrichment of 10 to 70%. The Slowpoke reactor is a low-power (20 k W ) reactor designed specifically as a teaching aid, with additional purposes of activation analysis and production of small amounts of radioisotopes. The system is designed to operate remotely. It can be provided with up to five irradiation sites in the core with a flux of 1016 d m 2 s and five further tubes outside the reflector with half that flux. Heavy-water research reactors are tank types. They usually have enriched uranium fuel, heavy-water moderator and cooled, and heavy-water and graphite reflected. Due to the low cross-section for thermal neutrons, absorption by D and 0 , they are characterized by well-thermalized neutron flux (very little epithermal and fast neutrons fluxes except the core). Due to the lower moderation power of D compxed to H, the physical size of heavy water reactors is larger and hence they have a large available irradiation volume. Their power is usually bstween 10 and 26 MW (fluxes of up to 2 X loi8dm? - s).

2 3 5 ~ ~

a

1. Neutrons from Accelerators and Neutron Generators Charged particle accelerators can produce neutron fluxes by (d,n), (p,n), or (a,n) rcactions. Electrona&$dlerating machines can produce neutron fluxes via (y,n) reactions, where the y-flux is obtained by stopping the electrons in a high-Z material. The main reactions for producing neutron fluxes are D(~,II)~H T(d,n)4He, ~, and 9Be(d,~~)10B. The first two reactions are exothermic and require very little acceleration of the deuteron beams; this is the reason why they are used in the neutron generators, which are very small accelerators with accelerations of 150 to 500 keV. The disadvantage is that the or T2) adsorbed onto metal. If the target is heated, too much of the gas will target material is gas (D2 be desorbed from the metal target; this limits the current of the bombarding deuteron beam and consequently the neutron flux. In the case where a higher flux of neutrons is required, a target of Be is used; however, the deuterons should have an energy of a few meV. The most common use of the reaction T(d,n)4He is due to the low acceleration needed and Lhe higher cross-section of this reaction, as compared to D(d,t~)~He. Different high-voltage generators are used in various neutron generators: Cockroft-Walton, insulating core transformer (ICT), Van de Graaff, and electrostatic rotor machines. A deuteron beam is produced by various ion sources and accelerated to one to few hundred keV. The beam may also be a mixture of 50% tritium and 50% deuterium. Qpical beam intensities are of the order of a few milliamps, but intensities as high as 150 mA have been used. Neutrons of 14 MeV are produced from the interaction of the beam with a large surface-tritiated target. The target consists of a few hundred microns of titanium, zirconium, or palladium evaporated on a 30-mm diameter backing disk and saturated with tritium gas. The disk is water cooled and made of heat-conducting material, silver, or copper. Rotating targets are sometimes used to ensure heat dissipation. Commercially compact sealed-tube neutron generators are also available. The usual neutron generators have fluxes up to 5 X 10" rds; however, special generators with outputs of 5 X lOI3 n s-' were also constructed. 2. Radioactive Neutron Sources Radionuclide neutron sources are composed either of a radionuclide emitting a- or y-rays, together with an appropriate surrounding material, or from a radionuclide decaying by spontaneous fission. The y-emitter in photoneutron sources is surrounded by beryllium or deuterium (such as DzO) and neutrons are emitted due to the (y,n) reaction: y

+ 9 B e - + 2 a + n - 1.67 MeV

y

+ D + H + n - 2.23 MeV

Very few radionuclides with reasonable half-lives emit y-rays with energies that high. Due to this, together with the disadvantagc of the long range of y-rays, the use of that kind of neutron source is very limited. The most c o m m o n l ~used photoneutrons source is '"Sb-Be, which emits neutrons of 2 6 -T 1.5 eV. If4Sb is produced l!&g$actor irradiation of natural antimhy. IZ4Sbemits sevex'd y-rays'f2including 1.692-MeV photons (with intensity of 48%), and has half-life of 60.9 d. The neutron source is composed of two parts, the core (a sphere or a cylinder) nmadc of irradiated antimony metal and a shell of beryllium metal about 2 cm thick. The practical yield is about lo7 n/s per I Ci '"Sb. Most neutron sources use an a-interaction with YBe: 9Be

+ a -+ I2C + n + 5.91 MeV (I2C is in the 4.43-MeV excited stare)

The spectra of these neutrons spread up to 10 to 12 MeV. The common a-emitters used are Ra, 210Po, n9Pu, and 241Am.The main properties of the sources are summarized in Table 1. Table I Properties of Neutron Sources from 9Be(~,n)i2CReaction a emitter +

Neutron yield (n s-' Ci-I) Half-life

Approximate size (cm3Ci-') Heating ( m W Ci-') y-dose (rad h-' Ci-I)

210P~

2.5 X lo6 138.4 d 0.1 32 0.11 (4.43 MeV)

23gP~ 1.7 X lo6 24,360 yr 12 31 0.08 (4.43 MeV)

2.2 X lo6

458 yr 3 33 10 (60 eV), 0.1 (4.43 MeV)

1

fie po-Be and Am-Be sources are prepared by mixing fine powders of beryllium with polonium metal jar americium oxide. The Pu-Be source is an intermetalIic compound, Pu-Bel3. The mixture or -the intermetallic compound is doubly encapsulated, first in an inner capsule of tantalum and then in an outer capsule of stainless steel. In 80% of the (a,n) interactions, the I2Cnucleus is left in the 4.43MeV excited state, which decays with an emission of 4.43 MeV y-photons. The y-dose of these three = . - q ~ ~ ( ~I4C , n sources ) is lower than with the initially used source of Ra-Be due to the y-rays from the Ra source. Lower doses of y-nys are associated with the neutron source of spontaneous fission. The kource of spontaneous fission is "*Cf. .spontaneousfusion ZS2Cf two fission products 3.8n 200 MeV .,

tln=2.65 yr

+

+

The half-life for spontaneous fission is 85.5 years, and for alpha emission is 2.73 years, and the effective half-life is 2.65 years:

The "'Cf sources are very compact. The neutron spectrum is very similar to that of neutron-induced fission, with a mean energy of 2.348 MeV. The neutron yield is 2.3 1 X lo6n/s pg. In order to increase with 23SUthan to use larger amounts of 252Cf. the flux of neutrons, it is cheaper to surround the ZS2Cf These devices are called neutron mulripliers. 252Cf(1 tng) combined with 1.4 g 2)5U (93.4%) and polyethylene as moderator is equivalent to a neutron source containing 33 mg 252Cf.This device has thermal and fast neutron fluxes of 4 X lo8and 6 X 10" n/s . cm2.

-

111. GAMMA-DETECTORS In activation analysis, we need a detection system for the y-photons that will measure the total number of photons together with their energy distributions; i.e., we need to have a y-ray spectrometer. These measurements are done mainly by the use of two solid-phase detectors working o n different phenomena. A. SClNTlLLATlON DETECTORS Na!(TI) AND EGO A scintillation radioactivity detector consists of a scintillator or phosphoc optically coupled to a photomultiplier tube. The most common scintillator for y-ray measurements is a large crystal of NaI activated with 0.1 to 0.2% TI. The y-photon is reacting with the detector, ejecting electrons. These electrons produce excitation or ionization in the scintillator crystal. De-excitation of the scintillator occurs via fluorescence in about 0.2 p s by the TI+ activator (visible light). The small percentage of TI, the "activator", is added to shift the wavelength of the emitted light by the detector to longer wavelength for two reasons: ( I ) IRorder to reduce the self-absorption of the emitted de-excitation light by the NaI crystal; and (2) the shift from UV light to visible light increases the sensitivity of the photomultiplier to the emitted light. The most popular size for routine y-ray spectrum measurements is a 7.5-cm diameter, 7.5-crn high cylinder. It requires approximately 30 eV of energy depositeci in NaI crystal to produce one light photon. The light photon ejects an electron from the photocathode and this electron is accelerated toward the dynodes in the photomultiplier by electric voltage. In each dynode, the electron ejects more electrons (typically 3 to 4), mu!tiplying the electron current. Usually, there are 10 dynodes and each electron ejected from the photocathode is producing about lO%electrons. It takes, on the average, about 10 light photons to release one photoelectron at the photocathode of the multiplier. Thus, it takes approximately 300 eV of y-energy deposited in NaI to release one photoelectron. Since the number of photoelectrons is proportional to the y-energy, the number of final electrons in the pulse, the voltage of the electric pulse, is proportional to the energy of the y-photon. The pulse voltage obtained from the photomultiplier is quite high (50 to 1000 rnV) and only modest further amplification by a pulse amplifier is required. The electric pulses are sorted according to their energies by a multichannel pulse height analyzer, which usually sorts pulses of 0 to 10 V. Each energy range is fitted to one channel. Nowadays, most multichannel analyzers are ADCs on an electronic card fitted to a PC. The PC is used also to analyze t h e obtained spectrum. The spectrum of counts per channel is actually an energy spectrum of the ?-rays.

LOO

Figure 1

1 00

.

1

.

1 ZOO

charnel

ray spectrum of a radioactive sample (a) taken with a 3" x

1

L

1

1600

3" Nal(T1) detector and (b) taken

with a Ge(Li) detector. Another scintillation detector used i s a crystal of bismuth germanate (Bi4Ge3OI2; abbreviated BGO) due to its higher efficiency for absorption of y-rays (since it has a high Z-element). However, BGO crystals are more expensive and are less frequently used.

6. SOLID-STATE IONIZATION DETECTOR The simplest idea for measurement of radioactive decay is by the use of the main property of the emittcd particlcs or photons as ionization radiation. When an ionizing radiation is striking a nonconducting or semiconducting material, it forms in it electrons and holes or cations. The amount of electrons formed is proportional to the energy of the striking photon or charged particle. The amount of energy required to raise an electron from the valence band to the conduction band in a semiconductor is considerably smaller than the same energy in insulators, or the energy required to form an electron-ion pair in the liquid or gaseous phase. Thus, for the same amount of energy absorbed in the detector, more electrons are formed in a semiconductor material than in an insuIator. The larger number of electrons reduces the statistical fluctuation in the number of electrons and hence reduces the width of the radiation signal in the detector (measured as FWHM = full width at half maximum of the peak). Figure 1 shows the peaks obtained from the same radioactive sample by NaI(T1) (a scintillator) and a Ge detector (an ionization detector). The smaller FWHM leads to much higher resolution. The currents formed by the electrons produced in the ionization detector by the radiation is smaller than those formed in the scintillation detector due to the high multiplication factor in the photomultiplier. Hence, before measurement of the current from the ionization detector, it has to be amplified. In order to reduce losses in the pulse current and to reduce the rise time of the pulse, a preamplifier located as close as possible to the ionization detector is used for initial amplification of the detector output signal. Since the capacitance of the solid-state detector depends upon a high-voltage bias, a charge-sensitive preamplifier should be

-.

"7.

used. The voltage pulse produced at the output of the preamplifier is proportional to collected charge :Y-andindependent of detector capacitance. However, the output pulse from the preamplifier is too low ,for sorting by the multichannel analyzer (MCA). Further amplification is done by the main amplifier, serves to shape the pulse. In order to reduce the noise due to the leakage current of the -elecbonsin the conduction band agitated by thermal excitation, the semiconductor crystal is cooled to nitrogen temperature. Of all semiconductor materials. germanium is exclusively used for modem -- 1,y-ray spectrometry since only for Ge and Si can adequately pure material be prepared. Si is suitable only for X-ray measurements, due to its low atomic number, which reduces the interaction cross-section. .. n e impurities that are not tetravalent will produce either almost a free electron in the conduction band or a "hole", leading to increased leakage current (a noise), a current not due to ionizing radiation. In the past, even germanium could not be prepared pure enough to be used as a y-ray detector. However, its impurities could be small enough to be compensated by the lithium ion drifting method developed in 1960. The method was used to effectively compensate p-type (acceptor type containing an excess of trivalent impurities) grown crystals of germanium. The small lithium ions were pulled into the crystals by electric field and high temperature. Due to its very low ionization potential, lithium acts as donor impurities compensating the excess of holes. The lithium ions, having high mobility, were drifted under the influence of the local fields in such a way that the number of lithium donor atoms compensated everywhere in the crystal exactly the acceptors of the original ~naterials.These lithium-drifted detectors are called extrinsic detectors and are written as Ge(Li) (pronounced "jelly"). Lithium ions have high mobility in germanium at room temperature; thus, Ge(Li) detectors must always be kept cooled at liquid nitrogen temperature (7710 immediately after the desired compensation is obtained. In the mid 1370s, advances in germanium purification technology made available high-purity germanium that could be used for y-ray spectrometry detection without lithium drifting. These intrinsic germanium detectors are usually called HPGe (abbreviation for high-purity germanium). The outstanding feature of these detectors is that they do not have to be kept at liquid nitrogen temperature constantly. HPGe detectors must be cooled to 77K only during the measurement in order to reduce the leakage current due to thermal excitation of electrons to the conducting band. HPGe and Ge(Li) detectors are virtually identical from the point of view of measurement. However, due to the more convenient use of HPGe, they completely replace Ge(Li) detectors in contemporary y-ray spectroscopy. The main performance characteristic of a nuclear detector is its resolution, expressed as the full width of the peak at half its maximum (abbreviated RVI-IM). The narrower the peak (lower FWHM), the better is the ability of the detector to separate two close peaks, i.e., better resolution. The Ge detectors feature very high resolution as compared to NaT(Tl), as can be seen from the narrower peaks in Figure 1. At 1332 keV (this is one of the y-rays of T o and is used usuaIly to characterize the FWHM of a detector), the resolution of a good HPGe detector is about 1.8 keV, while the resolution of 3" X 3" NaI(T1) detector is about 60 keV. The resolution is better for a Ge detector; however, the efficiency (the fraction of y-photons hitting the detectors that appear in the photopeak of the spectrum) of Nd(T1) is larger. The efficiency of the Ge detectors is given usually relative to NaI(Tlj. The first Ge(Li) detectors were small and their efficiency relative to NaT(Tl) were less than 10%(for 1332 keV). However, nowadays, HPGe detectors with about 100-mm diameter are constructed and their relative efficiency to Na(T1) 3" X 3" are almost 60% at 1332 keV. The relative efficiency decreases with increasing energy due to the higher Z of iodine compared to germanium. For most applications, the resolution is more important than efficiency and Ge detectors are the more commonly used detectors. A schematic diagram of an HPGe/Ge(Li) detector and the measurement system.is given in Figure 2. ,

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-e

IV. THE SHAPE OF THE y-SPECTRUM In order to understand the observed shape of the y-spectrum, it is imperative to know the different processes of interaction of radiation with matter. A photon of light in the IR, visible, or UV range can be absorbed totally or not at all, but it cannot lose only part of its energy. The situation is difFerent for high-energy y-rays. Low-energy X-rays interact with matter mainly by the photoelectric process. In this process, the y-photon interacts with one electron of the material, losing all its energy to this electron. The transferred energy is higher than the binding energy of the electron, and thus the electron is ejected from the atom with kinetic e n e s y equal to this difference (hv+,,,,- binding energy of the electron). Since the range of high kinetic energy electrons is considerably shorter than that of y-rays, the ejected electrons will lose their kinetic energy inside the detector material; from the point of view of the

.

Figure 2 A schematic diagram of HPGe or Ge(Li) detection sys-

tem. (1) HPGe or Ge(Li) crystal, (2)end cap, (3)Field Effect Transistor preamplifier, (4) metal housing, (5) vacuum/pressure release, (6) liquid nitrogen filling tube, (7) cold finger to cool the crystal, (8) gas absorbant in order to keep vacuum, (9) liquid nitrogen, (10) vacuum heat insulation. detector, all the photon energy has been absorbed in it, resulting in the same energy absorbed for all photons (assuming that the photons are monoenergetic). The situation is different for higher energy yrays. The cross-section (probability of reaction = rate constant) for the photoelectric process decreases with increasing energy of the pb~ton.For higher energy photons, the main interaction with matter is via Compton scattering. In ge&rdiurn, the photoelectric effect is the dominant process for y-interaction up to about 200 keV; whereas:%&n 200 keV and up. the Compton scattering becomes more important. In the Compton scattering, the photon interacts with what might be called a "free electron" (mainly electrons from outer shells, whereas a photoelectric process occurs with inner shell electrons, mainly from the most inner one, the K shell). The photon is transmitting only part of its energy. The energy that the electron received is more than sufficient to eject it from the atom and it moves with the excess energy as kinetic energy, losing it by collisions in a very short-distance. The photon retains part of its -energy by changing its frequency since E = hv, and is scattered in a different direction. In the Compton process, the energy of the original photon is shared between two particles (the ejected electron and the scattered photon) and, consequently, there is a continuous distribution of energies of the scattered photon. In order to conserve both energy and momentum, the y-photon cannot lose all its energy (in the photoelectric effect the momentum conservation is compensated by the recoiling of the atom from which the electron was ejected, while in Compton scattering it is an interaction with the "free" electron). The maximum energy the photon can lose is given by the expression:

is the electron rest mass. moc2 is the rest mass where E,. is the energy of the initial photon and energy of an electron, which is equal to 0.51 1 MeV. The main difference between the photoelectric and Compton processes with respect to their responses in the detector material is that while in both processes the electrons lose all their energy in the detector, due to their short range, the scattered photon

might escape from the crystal without further interaction, thus leaving in the detector less energy than that of original photon hitting the detector. In y-ray spectrometry, we are measuring the full-energy peak, called the photopeok, as this energy will be the only one to appear if the only interaction process is the photoelectric absorption. The photopeak results either from the y-photons losing all their energy by a photoelectric absorption or by a Compton scattering followed by photoelectric absorption of the scattered lower energy y-photons in the detector (the photoelectric absorption can be after one scattering step or several scattering steps, all of them within the detector crystal). However, the scattered y-photons from the Compton process might escape from the detector crystal, leaving in the detector less energy than the full-energy peak. This escape of the scattered photon not only reduces the photopeak, but its main disadvantage is the formation of a background for lower energy peaks since part of the energy (that given to the electron and some of y photons absorbed ip the detector) is absorbed in the detector. For a monoenergetic source, the Compton scattering produces a conti~iuumbackground ranging from zero up to a maximum, called the Compton edge. This results from the fact that the minimum energy that the scattered photon can have is not zero, but rather is given by the equation:

For E,

>> mc2, E,,,ini,,,,,approaches a value of

*, 2

i.e., 256 keV. Thus, for high-energy y. the Cornpton

edge will be separated from the photopeak by about 256 keV. For a monoenergetic y-emitter radioactive source, the energy range between the photopeak and the Compton edge is almost free of counts. However, most real samples have many different y-energies and the Compton continuum stretches from zero up to the Compton edge of the highest y-energy. If the energy of the measured photon is above 1.022 MeV, the photon can react also in a third process-a process called pair production. In this process, the photon energy is transformed, under the influence of the field of a nucleus, into matter in the form of a pair electron-positron (transformation of energy into matter-antimatter pair). Since the rest mass of either electron or positron is 0.51 1 MeV, the threshold of this reaction is 1.022 MeV to conserve energy. The cross-section for pair production is very low, below 1.5 to 1.6 MeV. The excess energy is shared between the kinetic energies of the electron and the positron. Since the ranges of positrons andelectrons are very short, this excess energy (EL, - 1.022 MeV) will be deposited in the detector. When the positron loses all its kinetic energy, it reacts with an electron in an annihilation reaction to form two y-photons of 0.51 1 MeV each (transformation of matter-antimatter into electromagnetic energy). Each of these 51 1-keV photons can escape from the detector without any interaction, can lose part of its energy by Compton scattering, or can lose all its energy in the detector (either by photoelzctric absorption or by successive Compton scattering and photoelectric absorption). If one of the 51 1-keV photons deposits all its energy in the detector, while the other one totally escapes from the detector, the energy absorbed in the detector will be 5 11 keV less than the full-energy peak. With y-photons of above 1.6 MeV, one usually sees also a peak of E, - 5 11 keV, where E, is the photopeak energy. This peak is called single-escape peak, due to one 5 11keV photon escaping from the detector. Another peak in the spectrum is the double-escape peak, arising from the two 51 1-keV photons escaping from the detector. The energy of this peak is E, - 1.022 MeV. 24Nahas two y-lines at 2.75 and 1.39 MeV. One expects to have four lines in the y-ray spectrum at 2.75, 2.24 (single-escape), 1.73 (double-escape), and 1.39 MeV. Although 1.39 MeV is above the threshold of 1.022 MeV for pair-production, its cross-section for this process is quite low and pairproduction contributes very little to the intkraction of the 1.39 MeV photon with the detector. Hence, in most cases, one will not see the single and double escape peaks of this photon. Figure 3 gives the measured y-ray spectra of 24Na. The ratio of escape peaks to the photcpeak depends on the detector size (influencing the probability of the y-escape) and on the energy of tile y-photon (affecting the chance that the photon will interact by pair production). Many of the y-lines in PGNAA have very high energies, resulting i n the main interaction with the detector being pair-production and hence large single- and double-escape peaks.

Figure 4 Schematic diagram of detection system for pair spectroscopy and Compton's suppression: (1) an HPGe detector with liquid nitrogen cooling, and (2) Nal(TI) detectors with photomultipliers.

B. COINCIDENCE DOUBLE-ESCAPE COUNTING For high-energy y-rays, the cross-section (probability of reaction) of pair-production is higher than that for Compton scattering. Since, the chance of the two 511-keV y-photons produced in the annihilation of the positron to escape from the germanium is quite high, the counts of the double-escape peak are quite high. In early PGNAA, some people used smaller detectors to increase the escape of the two annihilating quanta, obtaining mainly only double-escape peaks. This choice decreases the efficiency of the photopeaks and single-escape peaks, leading to a simpler spectra in the high-energy range. However, the choice of a smaller Ge detector also reduced considerably the efficiency'for pair-production, leading to a smaller peak. A better snlution to obtain an only double-escape spectrum is the use of triple coincidence or 1.022-MeV coincidence counting. In triple coincidence, the large Ge detector is surrounded by two (or sometimes four) NaI or BGO detectors. These scintillation detectors are connected to single-channel analyzers, al!owing an output signal only if it due to 511-keV y-rays. These singlechannel analyzers are connected to a coincidence unit, together with the Ge detector. Only when two simultaneous pulses come from two different scintillators does the coincidence unit'allow the recording of the signaI from the Ge detector. The two simultaneous (within a fixed time range) 51 1-keV photons indicate that the event occurring in the Ce detector is a pair-production process, rejecting all Compton scattering events together with pair production events where one or two 5 11-keV photons were absorbed in the detector. Figure 4 gives a schematic description of a Ge detector surrounded with scintillators. The same system is used both for coincidence double-escape counting and Compton suppression anticoincidence counting, as can be seen in the electronic scheme in Figure 5. The system measured also the spectra of singles (photopeaks). Thus, using different ADCs (usually connected to the same PC), the three different spectra are measured simultaneously. In some systems, Lhc scvernl surrounding scintillators arc rcpl~lcctlby onc nllnr~ltlrscinlillntor ~rnd the triple coincidence is replaced by double coincidence, in which the absorption of 1.022 MeV in the annular detector is used to open the gate for recording the signals from the germanium detector. It is worse than the triple coincidence since the 1.022 MeV must not be due to two 0.51 1-MeV photons.

ADC- l 1

Ge C

Singles

1 Anti

stan

TAC - SCA A stop

b

Coicidence Unit

-

ADC-2 Compto~~

Suppression

Scintillation

Specuum

-

detector

-+ -

f

Coinadence Unit

ADC-3 L.

double

Figure 5 The electronic scheme for simultaneous measuring of the three different spectra: CFDConstant f;action discriminator used as a single-channel analyzer; OR-A logical unit for which one pulse input is sufficient to get output; TAG-SCA-Time-to-Amplitude Converter plus single-channel analyzer to limit the time interval between the two pulses. The anticoincidence unit is usually part of the ADC and not a separate unit. The detector boxes inciude their high voltage supplies, amplifier, and preamplifiers.

In some system$ the annul dence me:tsurements.

tillator is split into optically isolatdh secton to allow triple cainci$4

1. Sowerby, B. D., On-line nuclear tecllniques in the coal industry, Nucl. Geoplrys., 5. 491, 199 1. 2. Gozani, T., Physics of recent applicatior~sfor on line analysis of bulk minerals, in Capture Gomnm-Ruy Spectmscopy and R e l a r d Topics-1 984, Raman, S., Ed., kmeric:tn Institute of Physics, New Yorl:, 1985,82G.

Chapter 2

instruments and Shielding Chien Chung CONTENTS I. Introduction ................................................................................................................................. 13 11. .Prompt y -Ray Spectroscopy ....................................................................................................... 14 A. Semiconducting Detectors .......................................~............................................................ 15 B. Scintillation Detectors ...........................................................................................................16 111. Advanced Instrumentation ...................... . ................................................................................. 17 A. Anti-Compton and Pair Spectrometers ................................................................................19 B. Field Instruments and Spectroscopy .................................................................................... 23 1V. Shielding of the PGAA Facility .................................... ............................................................. 25 A. Requirements for Shielding .................................................................................................. 26 B. Biological and Detector Shields ........................................................................................... 29 V. Discussion ................................................................................................................................... 35 ............................................................................................................... 35 References ........................... .

As mentioned earlier in this book, the PGAA method demands that neutrons bombard the target and utilizes a spectrometric detector to count the emitting prompt y-rays for quantitative analysis; therefore, a y-ray spectrometer and radiation shields, together with the neutron beam and sample handling device, are the major parts of a PGAA facility. Since the PGAA facility is usually attached to a radiation control zone such as a nuclear research reactor, the researchers are inevitably required to perform the on-site sample changing and troubleshooting while the intense neutron beam as well as scattered prompt yrays are all around. Hence, pcrsonal radialion safety must bc observcd for PGAA operation. A y-ray spectrometric detecting system contains complex electronics in addition to the y-ray detector. These electronic signal processing units, and associated computer with analyzing software, should be placed in a counting room as part of the PGAA facility and preferably have temperature and humidity control. Since hundreds of prompt y-rays appear in the multichannel spectrum and many of them are overlapping one another, software with automatic data reduction and sophisticated photopeak analyzing ability are mandatory. Many software packages with a prompt y-ray library are commercially available for upgrading the current PGAA facility. A prominent part of the prompt y-ray spectrum is the Compton background. The background of the Compton continuum not only obscures the identification of minor photopeaks, but also increases the uncertainty of the position and intensity of observable photopeaks. Reduction of this Compton continuum can be achieved using the Compton-suppressed spectrometer with detector shield. Several types of detector shields have been used as anti-Compton annuli: sodium iodide detectors, plastic scintillators, and more recently bismuth germanate detectors, resulting in great improvement of the spectral analysis. Numerous reactor-based PGAA facilities have now adopted such systems to improve their spectrometric performance. The y-rays and, in particular, neutrons are ionization radiations, causing human tissue damage and organ dysfunction; prolonged exposure to radiation, in particular to the high-energy neutrons and yrays, may lead to instant injury or even fatality. An upper limit of accumulated radiation dose of 50 milli-Sivert (mSv, a radiation dose equivalent unit) each year is recommended for radiation workers.' A 1-mSvh dose rate is approximately equal to an exposure to thermal neutron flux of 26,000 nls . cm2 or to fast neutron flux typically of 670 n/s cm2, or to 8 MeV high-energy y-flux of 12,000 photonst cm2 s. In the previous chapter, a thermal neutron flux on the order of 107n cm-*s-' from nuclear reactor is delivered to the samples: m5ilental exposure to these neutrons for 8 min can cause the researchers to exceed the annual dose allowance, facing the risk of rad~ationinjuries. Therefore, in the

-

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HPGe PREAMP DETECTOR

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Figure 1 Basic electronic flow chart of (A) scintillation and (R) semiconducting detector systems used for PGAA facility.

PGAA facility, maximum effort shall be put to shield off the radiation and facility itself for biological protection.

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II. PROMPT y-HAY SPECTROSCOPY

'

'

To detect and analyze the prompt y-ray in PGAA experiments, a photon tlcrector is required. 'Thc choice of a particular detector type for PGAA application depends upon the y-cnergy rangc of Iitterest and the application's resolution apd efficiency requirements. 'i%e detector must have s:~Ificientnlateriai to absorb a large f~actionof tfiehy-ray energy. Thus, any gas-filled counter is not suitable for de~ecting full-energy y-rays because the probability of absorbing all the y-ray energy is too low. Further, the higher y-ray energies, as frequently encountered in PGAA, are more effectively absorbed by highcr Z materials. Other considerations are count rate capability, resolution, and if timing applications are involved, pulse rise time. The kinds of detectors commonly used in PGAA can be categorized as scintillation detectors and semiconducting detectors. Scintillation detectors are used in conjunction with a photomultiplier tube (PMT) to convert the scintillation light pulse into an electric pulse. Solid crystal scintillators such as sodium iodide or NnI(TI) are commonly used in PGAA. Semiconducting detectors, made from single crystals of very pure germanium, are the highest performance detector type. The superior resolution of these detectors has revolutionized data gathering for prompt y-ray measurements. Mmy configurations of scintillation detectors are cornmcrcially available, ranging from small bismuth germanate (BGC)) to large NaI(T1) with multiple phototubes, a typical electronic flow chart of a scintillating detecting system is shown in Figure 1A. On the other hand, the semiconducting detector, which exhibits the highest resolution of all y-detectors, virtually requires the use of a multichannel analyzing system for spectrum analysis. A typical semiconducting y-ray spectroscopic syslem consists of the germanium detector, high-voltage power supply, preamplifier (PA), amplifier, analog-to-digital converter (ADC), and multichannel analyzer (MCA), as illustrated in Figure lB. Details of these photon detection systems used for PGAA experiments are given below.

Table 1 Photopeak Energy Resolution for Various Detectors Used in PGAA Experiment Resolution: FWHM (keV) at Detector type and size (Diameter x Height)

HPGe (2" X 2.3") NaI (2" X 2") BGO (2" x 2")

122 keV

662 keV

1332 keV

1.2

1.8 75 125

0.8 23

53

40

85

10829 keV

6.0 214 450

A. SEMICONDUCTING DETECTQRS A semiconductor is a material that can act as an insulator or as a conductor. It is fabricated from either elemental or compound single-crystal materials having a band gap in the range of approximately 1 to 5 eV. The Group IV elements, in particular germanium, are by far the most widely used semiconductors. Semiconducting detectors have a P-I-N diode structure in which the intrinsic (I) region is created by depletion of charge camers when a reverse bias is applied across the diode. When photons interact within the depletion region, charge carriers (holes and electrons) are freed and swept to their respective collecting electrode by the electric field. The resultant charge is integrated by a charge-sensitive PA and converted to a voltage pulse with an amplitude proportional to the original photon energy. The band gap signifies the temperature sensitivity of the materials and the practical ways in which these materials can be used as detectors. As a practical matter, high-purity germanium (HPGe) detectors must be cooled in order to reduce the thermal noise to an acceptable level. The most common medium for detector cooling is liquid nitrogen (LN,); however, recent advances in electrical cooling systems have made an electrically refrigerated cryostat a viable alternative for many field applications. In LN2cooled detectors, the detector element is housed in a clean vacuum chamber attached to or inserted in a LN2 dewar. The detector is in thermal contact with the liquid nitrogen, which cools it to around 77K. At this temperature, reverse leakage currents are low enough to be ignored. Performance of any photon detection system can be rated by its resolution of spectral photopeak and efficiency to detect the full-energy y-ray. Semiconducting detectors provide greatly improved energy resolution over other types of radiation detectors for many reasons. Fundamentally, the resolution advantage can be attributed to the small amount of energy required to produce a charge carrier and the consequent large "output signal" relative to other detector types for the same incident photon energy. At 3 eV per electron-hole pair, the number of charge carriers produced in Ge is about 1 and 2 orders of magnitude higher than in gas counters and scintillation detectors, respectively. The charge multiplication that takes place in the PMT associated with scintillation detectors, resulting in large signals, does nothing to improve the f~nd~amental statistics of charge production. The photopeak energy resolution, in terms of full-width-at-half-maximum (FWHM) for several prompt y-ray energies, are listed in Table 1 for various types of detectors for comparison. The energy resolution of the y-ray spectrum taken from an HPGe detector is explicitly superior to those taken from scintillating detectors with similar siye, with reduction factors on FWWM ranging from 30 to 75 at various photon energies. In Figure 2A, the spectrum of the prompt y-rays emitted from the Dy sample in the PGAA facility attached to the I-MW Tsing Hua Open-pool Reactor (THOR) in Taiwan is illustrated; the neighboring photopedcs are clearIy identifiable using a high-resolution HPGe detector, while the multiplet photopeaks, such as the 466-keV of Dy and 472-keV promptay-rays from Na of the NaI(T1) shield, are easily resolvable. On the other hand, the detector efficiency to absorb the full energy of the incoming prompt y-ray is closely related to its atomic number and density of the detector. At low energies, detector efficiency is a function of cross-sectional area and window thickness while at high energies, total active detector volume more or less determines counting efficiency. Coaxial HPGe detectors are specified in terms of their relative full-energy peak efficiency compared to that of a 3-in diameter by 3-in long (3" X 3") NaI(T1) scintillation detector at a detector-to-source distance of 25 cm. Detectors of greater than 100% relative efficiency have been fabricated from germanium crystals ranging up to about 75 mm in diameter. Approximately 2 kg germanium are required for such a detector. Typical efficiencies, in terms of counts per photon emitted from the source is shown in Figure 3, where the efticiency curves for the MPGe detector, as well as those for 13GO and NaI(T1) scintillation detectors with similar detector size, are

also displayed.

CHANNEL No., keV Figure 2 Excerpt of (A) normal and (6)Compton-suppressed prompt y-ray spectra measured by a highresolution HPGe detector attached to the P G W H O R facility.

8. SCINTILLATION DETECTORS The y-ray interacting with a scintillator produces a pulse of light that is converted to an electric pulse by a PMT. The PMT consists of a photocathode, a focusing electrode, and 10 or more dynodes that multiply the number of electrons striking them several times each. The anode and dynodes are biased by a chain of resistors typically located in a plug-on tube base assembly. Complete assemblies including scintillator and PMT are commercially available. The properties of a scintillator material required for good detectors dre transparency, availability in large size, and large light output proportional to y-ray energy. Relatively few materials have such good properties for detectors. Thallium-activated NaI and BGO crystals are commonly used. The high Z of iodine in NaI(T1) gives good efficiency for y-ray detection. A small amount of TI is added in order to activate the crystal, so that the designation is NaI(T1) for the crystal. The best resolution achievable is about 8.0% for the 662-keV y-ray from I3'Cs for a 2" X 2" crystal, and is slightly better for smaller sizes. Although the spectral rc:sohtion of the NaI(T1) detector is inferior to that of the HPGe detector, as indicated in Table 1, its detecting efficiency, particularly in the high-energy region, is several times better than that of the high-resolution HPGe detector. The light decay time constant in NaI(T1) is about 0.25 ks. Typical charge-sensitive PAS translate this into an output pulse rise time of about 0.5 p-s. The NaI(T1) detector is the dominant material for y-detection, used as either spectrometer or detector

-

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Figure 3 Absolute efficiency curves of the prompt y-ray detecting systems using the BGO, Nal, and HPGe

detectors with size indicated; the curve for the pair spectrometer using the HPGe and a 9 X 1 0 Nal(TI) shield is also plotted.

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shield for Compton-suppressed and pair spectroscopic systems; this is due to its relatively economical cost in terms of the benefits for both spectral resolution and detecting efficiency. The bismuth germanate (Bi4Ge3O,J detector, commonly abbreviated as BGO, became commercially available in the late 1970s. It has been utilized as a y-ray counter or spectrometer in a rapidly growing number of applications. A major advantage of BGO is its high density (7.13 g/cm3) and even larger effective atomic number, resulting in a high detecting efficiency for high-energy y-rays. Despite its lower Iight output and poor spectroscopic resolution relative to the more popular NaI(T1) scintillator, the BGO detector has been widely applied to either the main detector or the guard scintillator in the anti-Compton spe~trometer.~ For y-ray energy above a few MeV in the medium-energy region, observation using NaI(T1) and HPGe detectors suffered from inferior detecting efficiency and insufficient sensitivity for high-energy photons. The detecting efficiency of the BGO detector is at least 10 times higher than that of the Nal(T1) scintillator with identical detector size for photon energy above 5 MeV, as explicitly observable in Figure 3, further encouraging its application in high-energy y-ray experiments. In Figure 4, the prompt y-ray spectra taken by the BGO, NaI(TI), and HPGe detectors with similar size in an in vivo PGAA scan using the 0.1-W Tsing Hua Mobile Educational Reactor (THMER) in Taiwan are shown.3 Although the high-resolution HPGe detector yields excellent quality of the prompt photon spectrum, in particular for the 10.83-MeV high-energy prompt y-ray emitted from the N(n,r) reaction in the sample, it takes, however, a long counting period of 50,000 s to collect enough counts. On the other hand, only 1800 s is needed by use of the BGO detector to acquire an identifiable 10.83MeV prompt photopeak due to its superior counting efficiency-almost 100 times higher than that of the HPGe detector, as illustrated in Figure 3. With the same counting period of 1800 s, the NaI(T1) detector is impractical for on-line measurement of nitrogen prompt y-rays because the accidental sum in the 9- to 11-MeV region, as shown in Figure 48,obscures any qualitative andquantitative identification of the 10.83-MeV prompt photopeak. Hence, the scintillating detectors, in particular the NaI(T1) crystal for medium-energy and BGO for high-energy prompt y-rays, are useful in a PGAA experiment provided that the counting efficiency on sample is preferentially demanded.

! I ! . ADVANCED INSTRUMENTATION y-Ray spectroscopy using eithzr high-rcsoltttion serniconcluc~ingdetectors or high-efficiency scintillating detectors has been widely app!ieri in PCA A experiments. Since a high-quality prompt spectrum with

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Figure 4 Prompt y-ray spectra taken by the (A) BGO, (B) Nal(TI), and (C) HPGe detector in IVPG4A scan using THMER facility. (Reprinted with permission from Appl. Radiat. Isot. 547,42(6), Lea, C.J., Chao. J. H., and Chung, C., High energy gamma ray spectrometer using BGO detector. Copyright 1991; and Appl. Radiat. /sot 44(6), Chung, C., Wei, Y Y., and Chen, Y. Y, Determination of whole-body nitrogen and radiation assessment using IVPGAA techpique, 941, Copyright ?993,Pergarnun Journak Ud.)

minimal neutron damage to the detector is preferable on the one hand, and a complex fail-save alarming device is required to link up the real-time analytical result on the other, advanced instrumentation in pGAA experiment is in great demand. In this section, advanced prompt y-ray spectroscopy such as anti-Compton and pair spectrometry, as well as those used in field applications, are briefly introduced below and their distinguishing features are discussed.

A. ANTI-COMPTON AND PAlR SPECTROMETERS A prominent part of the prompt y-ray spectrum is the Compton background arising from the incomplete deposit of original photon energy owing to the escape of incoherently Compton-scattered y-rays from the detector. The background of the Compton continuum is not only obscuring the identification of minor photopeaks, but is increasing the uncertainty of the position and intensity of observable photopeaks. Reduction of this Compton continuum can be achieved by surrounding the HPGe detector with a large detector shield that is used in an anticoincidence mode. Scattered photons escaping from the HPGe detector and leaving part of the original photon energy behind are eventually captured by the surrounding detector shield. When signals generated from both detectors occur simultaneously, storage of the HPGe event is blocked while full-energy photons are stored; thus, the Compton continuum is suppressed accordingly. In the past 30 years, several types of detector shields have been used as anti-Compton sinnuli: NaI(T1) detectors, plastic scintillators, and more recently.BG0 detectors. These have resulted in great improvements of the spectral analysis. In Table 2, some representative anti-Compton prompt y-ray spectrometers using NaI(T1) shields are listed." The suppression factor of the anti-Compton spectrometer, usually quoted as the major feature of a Compton-suppressed y-ray spectrum, is the ratio of the height of the Compton continuum without suppression to the height with suppression; in Table 2, the Compton suppression factor for low-energy y-rays is around 5 to 8 and greatly improves the quality of the lowenergy part of the y-ray spectrum. The anti-Comptodpair spectrometer used at the P G M H O R facility is described here as a typical advanced instrument for PGAA expe~iments.~ The main y-ray detectors are an HPGe detector and an annular NaI(TI) detector shield to perform Compton suppression and a pair of 51 I-keV y-ray detectors. The 145-cm3 n-type HPGe detector has 30% relative efficiency, a resolution of 2-keV FWHM at 1332 keV, and a peak-to-Compton ratio of 46: 1 at the said photope'ak energy. The detector shield surrounding the primary HPGe detector is an annulus 9" X 10" NaI(T1) detector. A 3"-diameter lateral hole can

Table 2 Dimensions a n d Performance of Representative Anti-Compton Prompt y-ray Spectrometers Using Nal(T1) As a Shield Dimensions of Mal(TI)

vpe

Efficiencya

Diarnaier (cm)

Thickness (cm)

Well size (cm3)

Compton suppression factor" Well typeb

%o

13'Cs

Ge (Li) Ge (Li) Ge (Li) HPGe HPGe Ge (Li) Ge (Li) Ge (Li) Ge (Li) Ge (Li) Ge (Li) HPGe Efficiency relative to that of 3" X 3" Nd(T1) detector. L = asymmetric arrangement of L-shaped well; T = symmetric arrangement of through-side-hole. 'Compton suppression factor taken around 80% of the full-energypeak just below the Compton edge of the photopeak. (Reprinted with permission from Nrtcl. Inslr. and Meth. A243, Chung, C. and Yuan, L.J., Performance of a HPGeNdII(T1) Campton suppression spectrometer in high-level radioenvironmerllal studies, 102, Copyright 1986, NorthHolland Physics Publ~shing.) a

accommodate the 38-cm long HPGe detector can. Eight 3" PMTs, four on each optically isolated half, view the NaI(T1) detector. The sample ;o; PGAA iri-adiaiion can be placed as close as 25 cm to the HPGe detector head if the interference from scattering neutrons is less intensive. The electronic block diagram h e d for this spectrometer is shown in Figure 5. The basic electronic set-up is for the two optional data accumulation modes. In the Compton-suppressed mode, y-ray events collected in thc HPGc detcctor (which has coincidcnt events in annular NaI(T1) detcctor within 0.1 ps) are rejected at ADCl input. For pair simuttaneously in both halves of the NaI(T1) crystal is treated as an allowable gate for an HPGe pulse at ADC2. Digitized signals for either mode and single spectrum are further processed in data acquisition of a multiparaineter system in which four 409b or two 8192multichannel spectra, each with different effects, can be obtained simultaneously. The useful detecting y-ray range of the data accumulation modes covers the energy from 0.1 to 11 MeV: the Compton suppression data are accumulated within the range of 0.1 to 4 MeV, while the pair spectra are collected from 3 to 11 MeV. An off-line spectrum analytical code based on the software package of modified SAMPO is available. The collected spectra can be transferred to either a PDP 11/34 computer or a personal computer for further analysis. The effect of Compton suppression, judging at 662 keV of the '37Cspho:ope&s peak-to-Compton ratio, is improved by a factor of 5.2 as indicated in Table 2; at 2754 keV of the "Na photopeak, the peak-to-Compton ratio is further improved by a factor of 7 under anti-Cornpton operation. The improvement effect of such Compton suppression is shown in Figure 6. At complex radiocnvironmenta1 tcst. the effect of Compton suppression not only yields a much better ratio of the Compton continuun~ between unsuppressed and suppressed spectra, but also eliminates most contaminating and interfering y-rays. The effect of Compton suppression in a PGAA experiment is demonstrated more dramatically in Figurc 7, which shows y-ray spectra taken duriilg the full-power operation of the THOR facility. In additiun to the background photopeaks from natural radioriuclidcs and isotope productic,rl, ihe experimen- . tal reactor floor contains scattering neutrons originating from the PGAA facility itself. Thc anti-Compton operatic?. yields a strikingly'good ratio of the Compton continuum among normal, unsuppressed, and suppressed spectid (1 16:11:1). The reductions of the continuum background in y-ray spectra for such high-level counting were evaluated at varioas photon energy ranges and the results are shown in Figure 8. For high-level counting, the average reduction of background is improved by a factor o r 9 lo 12 on all energy ranges, as indicat4,Jn the figure. The results indicate that th%&tein is adequately sensitive to many $menis of inte&. The detectin$: sensitivity can be divided into kroups. The most sensitive elements ire B, Gd, Cd, Srn, and He. f$50 mg boron. For most samples studied, 5 to 15 rnin counting was sufficient. However, for materials with lower concentration of B, each sample was measured for several hours. The largest interference for the 477-keV line of B comes from the 472-keV line of Na. This interference is due to the wide peak of B caused by Doppler broadening. Samples with NalB ratios of lo3can be measured accurately; however, at Na/B 1 OS, the precision of B measurements was relatively poor. Repetitive analyses show the method to have a precision of 5%. They found that although Cd can be measured by PGNAA in these materials, the

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measurement is not sufficiently sensitive to provide a practical alternative for atomic absorption analysis except in cases where nondestructive methods are required. Heurtebise and L u b k o w i t ~used ~ ~ a collimated neutron beam from a nuclear reactor and impinged it on 1- to 5-g alloy samples. A flux of 4.75 X lo7 dcm2s, thermal neutrons (Cd ratio, 30.5) were used to measure the contents of several metals in various alloys by 30-min counting. In one steel sample, they measured Fe, Ni, Mn, Cr, and Al. In another steel sample, they measured Fe, Ni, and Co. In synthetic samples, they measured Ni, Cu, Co, Fe, and Ti (one mixture) or Zn, V, and Mn (another mixture) with errors of usually less than 4%. Only for Ni did they find a 9% error from the true composition. Heurtebise et aLn used the same PGNAA system to analyze the metal contents of 12 hydrodesulfurization catalysts. Co and Mo required 10-min counting for sufficient statistics. If Ni was present, they measured an additional 30 min per sample to obtain good counting statistics for the Ni. Titanium oxide was used as an internal standard. Relative standard deviations of 5% were typical for concentrations of 2 to 15%. Heurtebise et aLn applied PGNAA to determine the water content (by measuring the 2.232 MeV 7-line due to hydrogen) simultaneously with the determination of Co, Mo, and Ni in hydrodesulfurization catalysts. They used 0.1 to 1.0 g catalyst, pulverized and mixed with 50 to 100 mg Ti02; in order to use Ti as an internal standard. Samples were irradiated for 10 min with a thermal flux of 4.75 X lo7 n/cm2 s using 96 cm3 Ge(Li) detector at 23 cm from the sample; 10 min was found sufficient to obtain good statistics. This method was found to be the easiest and most rapid way to measure moisture content in the catalysts. It should be mentioned that this method can be used only if the sole compound containing hydrogen is H20.Large contamination with organic materials will be translated into exaggerated H@ content. To check the reproducibility of the method, they analyzed one catalyst eight times and obtained a relative standard deviation of 3.6% for 12.6% water content. Jurney et aL7 used PGNAA with a nuclear reactor to measure sulfur in complex materials. Previous studies for measuring sulfur were performed with a 2"Cf source. They determined sulfur in nine NBS standard reference materials and six standard coals. Comparing their results to measurements done by other methods, they obtained agreement within 1.0 -L 0.04. The standard deviation of repeating results are usually better than 5%. Cakium and potassium are the most likely sources of direct interference due to their close y-lines (S, 841 keV; K, 843 keV, Ca, 837 keV). However, unless the ratios are very small in S, the peaks can be separated. The main background is due to Compton from 2223 keV of H in the polyethylene container of the sample, which causes a detection limit (twice the'standard deviation of the background) of 100-pg S in 3.5-h irradiation and 3 0 pg in 20-min irradiation. As a standard, they used 100 mg elemental S, assuming that the main absorbing material is the polyethylene capsule that is identical for sample and standard. Gladney et aI? used the same system, measuring simulhneously nine elements of major and minor abundances: Si, Al, Fe, Na, K, Ca, Ti, Mg, and P. A1 was measured both by the prompt y-rays and the delayed y-rays. For the delayed y-rays, they started the countings 15 min after the start of irradiation. For each sample, they irradiated 3 h. All samples give results that agree with other analytical methods to better than 5% for Si and Ti and between 5 and 10% for A), Fe, Na, K,and Ca. This worse agreement is due to a single result that falls off the correlation of the others and might be due to larger interferences. Neglecting this outlier, the correlation is better than 5% for all these elements. The correlation for Na, Mg, and P is worse, probably due to the concentration being close to the detection limit. With the exception of Si, Fe, and Al, the results obtained by measuring the double-escape peak in coincidence with 1.02 MeV in the annular NaI(T1) were inferior to those obtained by measuring photopeaks with anticoincidence mode. Gladney and co-workers9 also measured nitrogen, carbon, and hydrogen simultaneously in 1-g samples. For N, they used the 5269- and 5298-keV lines, which were found to be more sensitive than the other lines. Since these lines lie very close together with no discernible interferences, they were summed to yield lower detection limits. For carbon, they used the 4945-keV line since the 3684-keV line has interferences from N. Hydrogen has only one y-line at 2223 keV. Irradiating for 1 h, the following detection limits were obtained: N, 500 ppm; C, 10%; and H, 5 ppm. Using 14 samples of environmental standards the correlation between the measured value and the previously known value was 1.02 + 0.08 for N, 1.02 4 0.04 For C, and 0.97 r+_ 0.07 for H. The same Los qlamos Group (GIadney and co-workers) used its system to measure the concentration of boron in meteorite^.'^ Islam et aL3' measured the content of S, Fe, and Si in n lunm rock sample with a 1-MWswimming Pool reactor. The system was of the internal type, irradiating in the reactor core with thermal flux of

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3 X 1012n/cm2 s and with a Ge(Li) detector far from the sample. Only double-escape peaks, using Ge(Li)-NaI(T1) triple coincidence pair-prr,3uction, were measured. The container capsules were trom carbon or beryllium, due to their low (n,y) cross-section. Very long runs of 50 to 260 h were performed. The 5420-keV line of sulfur is interfered with by an iron line, and correction should be done using other Fe lines and the ratio between Fe !ines. Using the previously measured values of Si and Fe content of this rock, the S content was measursd. Proteins were measured by measuring the N contents of samples (usually grains and other food) and using an asumed constant N fraction in proteins. The usual conversion factor is 6.75 (1 g nitrogen = 6.75 g protein). This is one of the main uses of PGNAA neutron reactors nowadays. T i w d 2 used the 10.82-MeV capture y-rays from the I4N (n,y)I5N reaction to measure N content, and hence protein content using an Am-Be neutron source; whereas, Andras et al.j3 used ILL nuclear reactor with a neutron guide. This was the first experiment to study the protein contents of single seeds, which is very important for genetic studies and for finding the right seeds for larger crops. A carefully shielded measuring chamber was connected to the exit window of a neutron guide where the flux is 8 X 10%/cn12 * s. They put the seeds on a moving plastic strip of 100 m, with 10-cm spacings behv ,en each seed. A 5" X 3" NaI detector was placed 20 cm from the sample. I4N was determined by the 10.82-MeV 7-line without any interference, and H was determined by the 2.23-MeV y-line. Due to the small mass of each seed, the contribution from the atmospheric nitrogen should be eliminated either by evacuating the measuring chamber or filling it with helium gas. Background for 9.5 to 11.3 MeV used for the nitrogen channel was measured with a pure C-H-0 compound such as glucose. They used 3 0 - s irradiation for each seed. The ratio between the nitrogen and the hydrogen (2.1 to 2.3 MeV) C O U ~ I L ~ K : ~ was used for the calculation of the nitrogen (H was the internal standard and compared to glucose). Kobayashi and Kandd4 used the Kyoto University Reactor (KUR) with a neutron guide lube LC) measure 1°B concentrations (ppm range) in tissues. This measurement is important for measuring 11 concentrations in tissues after injection of boron for the purpose of tumor neutron therapy via the I O U (n,a) 7Li (boron neutron capture therapy). 'fie detection limit of the system is 0.1 to 0.5 pprn 'OU.In practical measurements, 10 ppm O ' B can be measured in 1-g samples within 10% accuracy in less tiian 30 s. Hydrogen in the tissue is used as internal standard. Glascock et al.35used reactor-based PGNAA to measure Pb, Zn, and Sn, together with Cu in copperbased metallic bronzes and coins. All four elements are measured nondestructively in one ineasuremeni. Fairchild et al.36measured boron in patients pretreated by injection of boron, for boron neutron capture therapy (BNCT), using PGNAA with both the high-flux beam reactor and the medical research reactor to provide "on line" boron analysis for BNCT. Spychala et ale3' used a PGNAA facility with internaltarget geometry that was installed at a tangential tube of the research reactor at the GKSS research center in Germany. Tiley measured only the double-escape peaks using only triple coincidence rneasure~nents.A central HPGe detector is surrounded in nearly 4 n geometry by a large NaI(T1) scintillator split into four optically isolated sectors to allow triple coincidence measurement (it is called a pair spectrometer). Concentrations of the major and the minor characteristic elements of the sediments Na, Al, Si, K, Ca, Ti, Mn, Fe, Zn, C1, and S are determined simultaneously, as well as the trace conslituents Cr, NI,Cu, Cd, and Hg. The major and minor elements can characterize the sediment, and CJ and Hg w e irace elements that belong to the key elements in environmental research and protection. Ward et aL3*studied 35 elements in coals (standards and various samples) by DGNAA and added to it the measurement of boron by PGNAA. The y-line used for boron is the 478-keV y-line from "13 Y (n,y) 7Li* _) 7Li. Due to the recoiling of the 7Li, the 478-keV line is Doppler-broadened, and due to the broadening it has interferences from the 472-keV line of Na. A graphical interpolation correction was needed to correct for the 472-keV Na line. They measured only single spectra since they need only low-energy measurement. They found it was sufficient to use a "bare" HPGe detector without Compton suppression. Matsumoto and A i ~ a w aused ~ ~ a reactor-based PGNAA system to measure I0B concentration for BNCT. They used the thermal column of a Triga I1 (100-kW) reactor. A silicon single crystal has been used as a filter inserted into a horizontal beam hole to extract a relatively pure thermal neutron beam. The beam area was about 7 cm2 (defined by polyethylene and lead collimators). The neutron fluence ~ ~ the 2.23-MeV line of H for internal standard. is 5 X lo5 n/cm2 s. Kobayashi and K a r ~ d aused This is a good choice for large samples with uniformly distributed hydrogen, but unsuitable for a

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onhomogenized sample or small samples. Matsumoto and A i ~ a w used a ~ ~ a small silicon detector with 6 ~ i Fto measure the neutron h e n c e , rather than using an internal standard due to the small samples. ~ o g u et s a1.@built a prompt y-neutron activation analysis facility at the 5-MW MITR-I1 (MassachuPSsctts Institute of Technology Research Reactor), to support the neutron capture therapy program by ;devising a sensitive method for nondestructive measurement of B, independent of the chemical binding .. Eand surroundings. This facility is unique in that it uses a diffracted neutron beam. This facility has :jower thermal neutron flux than will be obtained using the direct beam; however, it has a considerably Flower fraction of fast neutrons and photons. This system has lower flux than those facilities using :neutron guides, which also have a low fraction of fast neutrons and photons; however, the building of a neutron guide facility is very expensive, existing only in very few places. The construction of the kdiffracted neutron beam is relatively inexpensive. The use of the relative high flux of thc PGNAA facilities is used mainly for measutement of carbon, hydrogen, boron, nitrogen, sodium, chlorine, potassium, sulfur, calcium, and cadmium in biological and gfood sarnple~P'.~~ - ~ o r i s o vet studied the prompt y-ray emission from a living tissues simulator in order to calculate ,the maximum neutron dose equivalents for in vivo analysis by PGNAA using a nuclear reactor. The used was a 25-crn diameter cylindrical tank filied 20-cm deep with an aqueous solution of Lufea (3% N). Various filters (Si, Gd,Fe, Al, S, and B) were used. Their recommendations were: for in ',viva analysis of human tissues at a depth of 2 to 5 cm, it is advisable to use neutrons of 20 to 40 keV $mean energy with a beam area of at least 45 crn2.The analysis sensitivity per unit neutron dose equivalent '.can be more than 1 order of magnitude compared to a 238P~-Be neutron source. 2. Riley and L i n d s t o d measured boron in borosilicate glasses by PGNAA using a boric acid sample !as the comparator. No correction was necesmy for overlap from the sodium peak at 472.4 keV since iB was found to be 3500 times more sensitive than Na (per equal weights). Due to the high cross-section for neutron capture by boron, high concentrations of boron can lead to appreciable neutron self-shielding ( l f e thickness of 2.4 mg B/cm2). Consequently, the amount of the boron was limited to I mg, providing an optimal balance between sufficient count rate for acceptable counting statistics within a convenient Jrradiatiodcounting time and minimal error from the effects of high count rate. Approximate calculations of the self-shielding show that 1 rng I3 Ieads to a 3.1% decrease in the neutron flux. This e p r was made constant for all samples by making all samples of the same-size pellets and 1 mg B, via dilution .with boron-free graphite. However, the error of the self-shielding is not large even if not taken care of. Changing the mass of boron in the pellets from 0.5 to 5% causes only 7% deviation from linearity. Kim and B W measured the boron content in reactor-grade aluminum by PGNAA. The experimental detection limit was found to be 5.7 ppm with 10% uncertainty using a beam of neutrons with 2.54-cm diameter and flux of 1.2 X lo6 rr/crn2 s (Cd ratio for gold, 13). For most reactor-grade materials, the boron content is much lower than this detection limit. In order to lower the detection limit, boron was preconcentrated using a cation exchange column. The eluate, containing boron, was evaporated to reduce the volume, after addition of mannitol to prevent the loss of boron by evaporation. The preconcentration technique allows the measurement of 0.57 ppm for 30-g A1 samples. Ward46used a PGNAA apparatus situated at the 0"-180" through-tube of the Impend College Consort 11 reactor with a thermal neutron flux at the target position of approximately 2 X lo6 n/cm2 s to measure the boron content of biological material. The target is about 23 cm from the Ge(Li) detector surrounded by Nal crystal. The Cornpton suppression reduces the Compton continuum in the region of the boron peak by a factor of 5.8. 50 ng boron can be detected with a lo4-s period' of measurement. Corrections for the interference of the' Na peak of 472 keV to the Doppler-broadened 478 keV peak of B was done by graphical interpolation between the interference-free portions of the boron peak. Boron contents were measured for bone and tooth samples of healthy and rheumatoid arthritic individuals. James et a1.47evaluated the use of PGNAA for determination of an intestinal rare earth marker, samarium, in comparison with TNAA. For PGNAA, 400-mg samples were measured at MURR for 30 min each. PGNAA has significant advantages with respect to its higher cross-section, leading to shorter turnaround time. Samples were measured both by PGNNAA and INAA and in only one sample did results from the two methods differ by more than the experimental error. Glascock et al.48discussed the use of reactor PGNAA for geochemical analysis. All of the major constituents (Si, Na, Al, Fe, K,Mg, S, iind Ca) usually measured by geochemists except oxygen and PhoVhonrs can be measured by PGNAA, Besides, PGNAA ciln be used to measure R and three REEs (Nd, Stn, and Gd).

M i ~ h a e l i compared s~~ the analytical d a b obtained by PGNAA, INAA, and ICP (inductively coupled plasma) for a sediment sample from the Elbe River. A total of 15 elements were measured, though PGNAA was not used for Mg. For most elements, the agreement between the methods wai quite good, except for Cr and Cd, for which ICP gives 40 to 50% higher results. Many studies used a nuclear reactor both for INAA and PGNAA, measuring most elements by INAA and complementing that by measuring others via PGNAA: predominantly those that could not be measured by INAA, but not only those. Germmi et d.'O measured I-g pellets of coal by irradiation in [he NIST reactor for 12 to 20 h. They measured H, B, C, N, Si, S, Cd, and Gd, which could not be measured by INAA in the same reactor; they obtained also the concentrations for Na, Al, C1, K, Ca, Ti, Mn, Fe, Nd, and Sm, which were also measured by INAA. Graham et aL5'used both INAA and PGNAA to measure the elemental concentrations in NBS standard reference materials using the MURR reactor. For PGNAA, they irradiated 1-g samples (geological samples) for 12 h and measured B, Na, Mg, Al, Si, C1, K, Ca, Ti, Mn, Fe, Sm, and Gd. Vogt and SchlegelSzdetermined 23 elements in ash by INAA and 10 by PGNAA; 6 of the 10 (Al, K, Ca, Ti, Fe, and Sm) were determined by both methods, but four elements (B, Si, Cd, and C;d) could be measured only with PGNAA. The interference of Na 472-keV peak with the 478-keV boron peak was subtracted by measuring the 90.9-keV peak of Na using the constant ratio of the two pc&s of Na. Simons and LandsbergeP measured 30 elements in reference material by nondestructive neutron activation methods. Cd and S were determined by PGNAA using 2 h for measurement. Ccl was dctenined ~~ 35 elements in by the 558.2-keV line and S by the 840.4-keV line. Ward and c o - w ~ r k e r smeasured human placenta by INAA (mainly instrumental) using three irradiations and four countings; they also measured boron by PGNAA. The detection limit was 0.01 ppm for 1- to 2-g samples measured for i04 s. A similar study was performed by Ward and Masods on brain tissue from Alzheimer's disease patients and control individuals: 30 elements were determined by INAA. Si was determi~tedby fast neutron activation analysis, and B was determined by PGNAA. For lo4-s measurement, a detection limit of SO ppb was found. Gokmen et aLS6measured trace element content in blood by both INAA and PGNAA. Using PGNAA, they measured H, C, N, 0, S, C1, and K. From those, only C1 and K were measured also by INAA. The C1 results obtained by PGNAA were lower than those obtained by MAA. The authors said that this often happens for matrices containing high levels of hydrogen, but no further explanatiorr fur Lhis effect was given. Chung2 used a mobile small reactor to measure Cd in vivo and in vitr-o. Absolute measurement of PGNAA can be done either by comparison to standards for each element or by one standard for all elements. For elemental standards, the mass is calculated using the equatron

where m stands for the mass of the measured element and C stands for the counts in the y-ray spectrum's peak due to this element. PGNAA often requires long irradiations and handing many standards in multielement analysis is time consumicg. One way to overcome this long irradiation time of many standards is to use multielement standads. This can be done by thoroughly mixing different compounds, or using standard reference materials or certified reference materials prepared by various agencies. These reference materials are not always available and difficulties may arise when one has to anaiyze a new type of material that has a different elemental composition than that of the reference material. Thus, it.is preferable to use only one elemental standard material to measure the neutron fluence, calculating from it the masses of all observed elernents using the available information on cross-section, abundances, y-line intensities, and detector efficiency. Using only one or two standards is called thc "absolute method" and it requires accurate knowledge of a11 nuclear and system parameters involved, besides having reproducible experimental csnditions. Khrbish and SpyrouJ;'discussed the possibility of using the absolute method for PGNAA. They compared the ratio of sensitivities for various elements to the sensitivity of boron in three different set-ups (theirss7and those of References 13 and 19). Having similar ratios of sensitivities will indicate thz possibility of using an absolute method; whereas, different ratios of sensitivities for two elements in the three set-ups show that the absolute method cannot be used. They found that S, C1 (by 517-keV line), K, Ca, Cd, and Au can be determined by the absolute method, but not C1 (by the 788-keV and 1164-keV lines), Fe, and Hg. There is much better agreement between Reference 13 and Reference 19 data than with those of Reference 57. This might be due to the better thermalization of the neutrons in the set-ups of References 13 and 19, as can be judged by

the cadmium ratios (CR). (Reference 13, CR = 42.0; Reference 19, C R = 55.0; Reference 57, C R = 9.0.) Thus, there is very good agreement for C1 (1164-keV line) between References 1 3 and 19, but a very large discrepancy with Reference 57. T h e same applies also for Fe. These results indicate that the absolute method can b e used only for a restricted range of cadmium ratios.

REFERENCES 1. Isenhour, T. L. and Morrison, G. H., Modulation technique for neutron capture gamma ray measurement in activation analysis. Anal. Chem. 38, 262, 1966. 2. Chung, C., Activation analysis with a mobile reactor in Activation Analysis, Vol. 2, Alfasi, Z. B., Ed., CRC Press, Boca Raton, FL, 1990, pp. 2: Chen, W. K. and Cl~ung,C. In vivo and in v i m medical diagnoses of toxic cadmium in rats, J. Radioanul. N~icl.Chem. 133, 349, 1989. 3. Hammermesh, B. and Hnmmel, V., Neutron capture y-ray spectra from elements Z = 17-30 and Z = 45-57, Phys. Rev. 88, 916. 1952. 4. Lombard, S. M., Isenhour, T. L., Weintz, P. H., Woodruff, G. E., and Wilson, W. E., Neutron-capture gamma-ray activation ana!ysis. Design of apparatus for trace analysis. Int. J. Appl. Radiat. Isor. 19, 15, 1968. 5. Henkelmann, R., and Born, M. J., Analytical use of neutron-capture gamma-rays, J. Radioanal. Chem. 16, 473, 1973. 6. Gladney, E. S., Jumey, E. T.,and Crarliss, D. B., Nondestructive determination of boron and cadmium in environmental materials by thermal neutron-prompt y-ray spectrometry, Anal. Chem. 48, 2139, 1976. 7. Jurney, E. T., Curtiss, D. R., and Glniiney, E. S., Determination of sulfur in environmental materials by thennal neutron capture prompt gamma ray spectrometry. Anal. Chem. 49, 1741, 1977. 8. Gladney, E. S., Curtiss, D. B., and Jumey, E. T., Multie!en~ent analysis of major and minor elements by thermal neutron induced capture gamma-ray spectrometry, J. Radioanal. Chem. 46, 299, 1978. E. T., Simultaneous determination of nitrogen, carbon and 9. Gladney, E. S., Curtiss, I). B., and J~~rney, hydrogen by thermal neutron prompt y-ray spectrometry, Anal. Chim. Acta, 110, 339, 1979. 10. Henkelmann, R., Analytische Verwertung der prompten gammastrahlung nach dem neutroneneinfang, Radiochim. Acta, 15, 169, 197 1. 11. Failey, M. P., Anderson, D. L., Zoller, W. II., Gordon, 6. E., and Lindstrom, R. M., Neutron-capture prompt gamma ray activation analysis for multielement determination in complex samples, Anal. Chem. 51, 2209, 1979. 12. Rustad, B. M., Als-Nielsen, J., Bahnsen, A., Christensen, C. J., and Nielsen, A., Single-crystal filters for attenuating epithermal neutrons and gamma rays in reactor beams, Rev. Sci. Instrnm. 36, 48, 1965. 13. Hanna, A. G., Brugger, R. M., and Glascock, M. D., The prompt gamma neutron activation analysis facility at MURR, Nucl. Instr. Meth. 188, 6 19, 1981. 14. Greenwood, R. C., Elemental analysis using the neutron capture gamma-ray technique with a Ge(Li) detector, Trans. Am. Nucl. Soc. 10, 28, 1967. 15. Orphan, V. J. and Rasmussen, N. C., A Ge(Li) detector for studying neutron capture gamma rays, Nucl. Instr. Meth. 48. 282, 1967. 16. Lombard, S. M. and Isenhour, 7'. L., Neutron capture gamma-ray activation analysis using lithium drifted gennanium semiconductor detectors, A n d . Chem. 40. 1990, 1968. 17. Comar, D., Crouzel, C., Chasteland, M., Riviere, R., and Kellershohn, C., The use of neutron-capture gamma radiation for the analysis of biological samples, Nucl. Appl. 6, 344, 1969. 18. Yonezawa, C., Haji Wood, A. I4 MeV) for exciting. nuclides in the way of inelastic scattering, having the same effect as a neutron generator. On the other hand, the thermalized neutrons in the sample matrix may undergo capture reactions followed by emission of capture photons, which simultaneously appear with the inelastic prompt y-rays in the spectrum.

C. DETECTORS Both scintillation and semiconducting detectors have been employed as a counter or a spectronieter for ISPGAA. An ideal detector should fulfill the following properties: High detecting efficiency High count rate Good energy resolution Resistance to neutron damage Free from cryogen Ruggedness and nonhydroscopic Independent of temperature variatibn Among these detectors, the NaI(T1) scintillator with energy resolution of 5 to 10% has been used extensively in ISPGAA applications. Its important advantages of moderate scintillation efficiency and mild resistance to neutrons make it viable in hostile environments. Nevertheless, a variety of scintilhtors other than the NaI(T1) detector have been introduced as altel-natives for irr situ measurements in different conditions. The bismuth germanate (Bi4Ge3OI2) detector; comrnoniy abbreviated BGO, has high detection efficiency and nonhydroscopic properties, enabling it to perform a rapid survey in hostile environment^.^ Although the energy resolution of the BGO detector is much broader than NaI(Tl), the high-energy detecting efficiency of E, > 5 MeV makes it an ideal counter for high-energy y-ray detection in ISPGAA. The cadmium tungstate (CdW04) detector shows some promise for ISPGAA applications due to its high detection efficiency and reasonable light output, but its slow response limits its use to low count rate measurement^.^ The recently introduced barium fluoride (BaF2) scintillator has a fast response component of less than 1 ns, indicating its potential for high count rate m e a s ~ r e m e n t The . ~ ~ ~intensity ~ of the emission wavelength at 220 nm for the BaF, detector is independent of temperature variation although it is weak for coupling to a PMT.8 The unique property of the cerium-doped gadolinium orthosiIicate (GSO) detector can achieve a higher counting efficiency by varying cerium concentration without cooling as other scintillators do in nuclear well-logging measurements." The detection efficiency of inorganic detectors decreases with increasing temperature due to variations of the light output response time constant and the light emission spectrum,1° as shown in Figure 4. Such degradation and spectrum shift should be well understood prior to practical use in the field. For example, the emission spectrum of a scintillating material is more critical for matching a photocathode response spectra, since the greater restriction on photocathode materials is imposed in the logging industry. In addition to y-detectors, a variety of associated electronics capable of operating at high-temperature (up to 175 OC) borehole environments were developed." The energy resolution for detecting a y-peak is degraded with increasing temperature of the scintillation detector, owing to the reductims both in the light output of the scintillator crystal and in the photocathode sensitivity of the PMT. The adaptability of PMT optically coupled to the scintillator to the borehole environments has been investigated. A ceramic PMT was developed and proved to have excellent performance at temperatures up to 200 OC,a long operating life, and extreme mechanical ruggedness.13 With the enhancements in detector assembly packaging, the impacts of thermal, mechanical vibration, and magnetic field on the system can be minimized. In comparison with inorganic scintillation detectors, the semiconducting detectors exhibit an excellent energy rssolution that enables detailed elemental analysis in situ. However, disadvantages of the semiconducting detector listed below limit its usefulness in a borehole environment:

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Figure 4 The emission spectrum of BGO shifts approximately 11 nm between -20 and +85 "C. (Reprinted with permission from lEEE Nucl. Sci.35, Melcher, C. L. and Schweitzer, J. S., Gamma-ray detector properties for hostile environments, 877, Copyright 1988,The Institute of Electrical and Electronics Engineers, Inc.)

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760

Wavelength (nm)

Operation at liquid nitrogen temperature Prone to neutron damage Inferior detection efficiency Sensitive to temperature and moisture variation Liquid nitrogen, used as a cryogen for semiconducting detectors, suffers from the problem that the high-pressure conditions of boreholes result in an increase in temperature of liquid nitrogen, which can cause spectrometric drift, generate noise, and deteriorate spectral resolution. To countermeasure such inferiority, melting cryogens with large latent heats were proposed to replace liquid nitrogen as potential alternative^.'^ Liquid propane and fluorocarbon refrigerants, having properties of high boiling points and inert characteristics, were recommended as substitutes for operation in a borehole, outer space, and planetary surfaces.1616 A pressure vessel filled with propane was designkd by Boynton for such application, free from impurities for refill equipment,15 as shown in Figure 5.

El!. 81L FORMATION IN SlTU ANALYSIS The ISPGAA technique has been introduced to petroleum industries for exploration and evaluation of oil reserves for more than two decades. Early ISPGAA restricted its utilization to the determination of the presence of carbon contents, bulk properties of oil formations, and Today, apart from knowing where and how much oil is in the formation, realizing how to produce oil efficiently from current reservoirs is urgent!y demanded from an economical point of view. Hence, an understanding of the minerals in rock fornlation by means of ISPGAA multielemental analysis is of great In contrast to traditional electric, acoustic, and radioactivity logging methods, ISPGAA provides much more direct information regarding borehole environments by means of elemental analysis, which is derived from the measurement of characteristic prompt y-rays of concerned elements. More importantly, this method can be carried out in the cased boreholes with rapid readout of results while most of the traditional methods cannot. However, problems of interpretation arise when the desired measurement is perturbed by changes in other formation or sample chnntctcristics. The disturbances of the f~rmation,?~ which can invalidate a laboratory calibration of the probe, are:

Variations of formation constituents Density changes

Figure 5 Sketch of typical borehole probe using canister cryostat. (Reprinted with permission from Nucl. Instr. Meth. 123, Boynton, G. R., Canister cryogenic

system for cooling germanium semiconductor detectors in borehole and marine probes, 602, Copyright 1975, Elsevier Science Publishers B. V.) Moisture content Occurrence of elements of high neutron cross-section Interaction of measuring tool compound with neutrons Temperature changes The disturbances listed may perturb tool calibration by influencing the spatial and energy distributions of the neutron field, leading to important differences in interaction cross-sections and average probability of induced prompt y-rays reaching the detector. In order to simulate the actual statistical nature of the interaction processes, Monte Carlo techniques were developed for tracking the neutron and y-ray transport through matter, providing interpolation of results for the logging conditions difficult to simulate e~perirnentally.~~

Water -Saturated

O:.!i

Figure 6 Comparison of a 30-porosityunit sandstone formation oil- and watersaturated. (Reprinted with permission from J. Pet. Tech. July, Scott, H. D., New developments in remote analysis of rock formations, 712, Copyright 1986,Society of Petroleum Engineers.)

sat_u_ro_t_e_d-_d

Energy (MeV)

A. BULK PROPERTES QF ROCK FORMATIOM The economic effectiveness of an oil reservoir depends strongly upon its physical and chemical properties. The organic-rich source rock to generate oil and/or gas, consisting of a variety of hydrocarbons, is an essential subject for probing. The reservoir must have porosity to contain oil andlor gas and permeabiIity to permit fluid flow. Until the advent of high-resolution semiconductor detectors for in situ application, the NaI(T1) scintillator was the only c!~oice for y-ray spectroscopic analysis. Several elements can be quantitatively identified in the prompt y-ray spectrum with its mild resolution. Some essential parameters leading to the confirmation of the quality of oil reservoir, such as hydrocarbon content, can also be determined to some extent. The earliest application of the ISPGAA technique in oil well logging was to differentiate oil from water in formations having low salinities, offering information for evaluation of.thedegree of hydrocarbon saturation and subsequently the size of the reservoir. The technique is referred to as "carbon/oxygen l~gging";'~.~' the measured ratio of carbon to oxygen indicates the fraction of pore space occupied by oil and water. These two elements can be determined readily by measuring their inelastic prompt yrays: 4.43 MeV and 6.13 MeV emitted from I2C(n,n'y) and 'hO(n,n'y) reactions, respectively. An NaI(T1) detector coupled to a neutron generator operated in pulsed mode is sufficiently adequate for such m e a s ~ r e m e n t . ~The * ' ~prompt ~ y-ray spectra, as illustrated in Figure 6, show differences of the spectra coIlected in oil and water tanks in a laboratory test for the determination of optimal response of an ISPGAA system; the carbon-to-oxygen ratios for the oil and water tanks are 7.6 and 1.2, respe~tively.'~ The measurement of C/O logging can be significantly altered if the salinity of borehole formation is higher than 0.1%. As salinity increases, the y-ray response from chlorine (notably 6.1 1 MeV) also increases; this has the effect of reducing the actual C/O ratioI9 since chlorine capture y-rays are of the same energy as those from oxygen. On the contrary, the chlorine usually presents in the form of salt water and its prompt y-rays dominate the y-ray spectra; some investigators took early advantage of this to develop chlorine logs to measure the absence of saltwater underground and to infer the presence of hydrocarbons."*26These logs have largely been replaced by pulsed neutron capture logs, which are also dominated by chlorine response. The measurement of C/O logging is subject explicitly to the lithology of the formations surrounding boreholes. The carbon-dominated rock, for example the limestone, presents a false high C/O ratio to make the sand appear potentially oil productive. This can be corrected by knowing the content of major constituents or elements of formations. Thermal neutron capture prompt y-rays of 3.54 MeV in Si(n,y) and 1.94 MeV in Ca(n.7) reactions can he measured simultaneously to indicate the 1ithoIogyof formation. The ratios, denoted as SiKa or-Si/(Si -1- Ca), can be used as the lithology indicator^.'^.^^ A more explicit explanation of the environment of boreholes using both C/O and Si/Ca logging are illustrated in Figure 7, in which the rich oil sands correspond to high C/O and Si/Ca ratios, while limy sand or carbonate presents a low SiICa ratio with relatively high C/O ratios.19 The pore of formations is occupied by either water andlor oil; both contain hydrogen and can be measured using the capture prompt y-mys of 2.22 MeV emitted from the H(n,y) reaction. The ratio

S i / C a RATIO

Figure 7 Predicting formation productivity in limey/shaly sands from C/O and Si/Ca ratios. (Reprinted with permission from J. Pet. Tech. Sept., Lock, G. A. and Hoyer, W. A., Carbon-oxygen (C/O) log: Use and interpretation, 1047, Copyright 1974, Society of Petroleum Engineers.)

+

R(@) = W(Si Ca) was proposed as a porosity i n d i c a t ~ rin , ~ which ~ silicon and calcium are assumed to be the major constituents of rocks. The influence of porosity on the determination of the C/O ratio is predicted for limestone (CaC03), dolomite [CaMg(C03)2],and sandstone (SiOz) fomlations by theorectical calculation,24as illustrated in Figure 8. In combination with the C/O logging, the oil quality as well as quantity in formation are thus evaluated.

6. MULTIELEMENTAL ANALYSIS The detailed elemental analysis of formation constituents permits an understanding of the mineralogy of rock rather than only its lithology. This appiication expands rapidly, although it plays an insignificarrt role for oil exploration alone. It has pointed out the importance of elemental analysis of formation in interpretation of rock chemical compositions and properties relating to the oil produ~tion,~'-~~ as listed in Table 1. In resronse to this, an in situ logging is divided into three groups: direct, indirect, and inferred. Direct measurement is related to raw data of elemental concentrations for the fluids and minerals that comprise a formation. Indirect application, on the other hand, utilizes the output of the direct application, such as mineralogy, to estimate formation properties such as total clay content or cation exchange capacity (CEC). Finally, the inferred application uses the direct and indirect outputs to deduce or infer information about the history of the rock formation. The pernleability, anZiijx5iiant factor in determining oil productivity of a rock, is strongly affected by minerology, or clay type^.^'.^^ Clay minerals have a critical role in the production of hydrocarbons. For example, clays typically fill the space between sandstone grains, thus reducing the oil production, or oil flow from the formation ATOM lC RATIO

0a8-

0s7:

0.6

/-

DOLOM IT€ Ca M9 KO312

-LIMESTONE DOLOMITE

0.4-

0.2 -

Figure 8 Calculated dependence of atomic carbon-oxygen

-

+ are in fractional units. (Reprinted with permission from IEEE

0.1

3 MeV) capture y-rays emitted from major elements of Si, Al, Fe, and Ti. In addition, the source-to-NaI(T1) detector distance of the probe explicitly determines spacial resolution of the borehole; reducing source-to-detector distance yields better delineation of coal piles and interseam sedimentary' strata. Rather than coal location exploration, the coal quality becomes important for economical reasons. ISPGAA techniques developed for coal ash delineation have been used successfully in Australia. The commercially available SIROLOG borehole logging as shown in Figure 11, was designed and developed in the CSIRO group, where two types of scintillators (NaI(T1) and BGO) were used to measure prompt y-rays from coal using a 'Cf neutron source. The parameter R, derived from the prompt y-ray spectrum, is defined as the ratio of the counts recorded in energy windows 2.6-5.2 to 2.0-2.4 MeV, representing the ash-composite elements (Si, Al) to hydrogen, respectively. This normalization is used to correct the neutron flux variations in different physical borehole conditions. Therefore, R can be directly related to ash content of coal from the chemical assay results. Howevx, the correlation between R and ash content varies at boreholes with different water contents, where neutron flux distribution depends on the therrnalizing effect of hydrogen in the ~ a t e r . ~ For ' . ~ ~dry and water-filled boreholes, corresponding calibration equations were derived using regression analysis, as listed

Ash(%) = - 18.96 + 74.499 R; for dry boreholes Ash(%) =

- 15.64 4- 79.2335 R;

for water-filled boreholes,

(1) (2)

Another method for correcting neutron flux perturbation in borehole measurements uses a boron-painted BGO detector to record boron capture y-counts at 478-keV peaks, which serve as a measurement of

Channel

Figure 9 Sample measured pulse height spectra for the wet sample. (a) The first time interval 0 to 100 FS, and (b) the second time interval 100 to 250 ps. (Reprinted with permission from Nucl. Instr. Meth. A219, Evans, L. G. et al., Inter-pulse high-resolution gamma-ray spectra using a 14 MeV pulse neutron generator, 239, Copyright 1984, Elsevier Science Publishers B. V.)

Figure 10 Geophysical profiles given by

o

200

400

600

800

counls/~cc

neutron capture for three source-to-detector distances: (a) 15 cm, (b) 23 cm, and (c) 33 cm. Coal beds are identified by the dark bands on the right side of the figure. (Reprinted with permission fr ~m Geophysics, 51, Charbucinski, J. et al., Prompt neutronaamma loaaina -- - for coal ash in water-filled goreholes, 1113, Copyright 1986, Society of Exploration Geophysicists.)

neutron flux intensity;49the ratio of the count rate in energy windows 2.6-5.2 to 0.38-0.60 MeV can also relate to ash content of coal. This method ;E successfully applied to coal seams with high variation in iron content, obtaining more accurate results than the y-y technique for coal ash determination. As mentioned in the previous section, scintillation detectors are prone to temperature change; gain stabilization at the 2.22-MeV capture prompt y-ray peak of hydrogen is required and usually performed automatically. This is particularly important for the BGO detector, due to its high dependence on temperature change.

B. MULTIELEMENTAL ANALYSIS USING HIGH-RESOLUTION DETECTORS The borehoie logging for determination of i d quality can be accomplished using high-reso1utio1.1 germanium detectors.5b55The composition of the coal can be derived by measuring the constituent

SIROMCP.

WINCH CONTROLLER

Sinplr board eomvu#r

HOST COMPUTER

.

c

Floppy disc Hard dlsc

Winch- MCA interface

SipnaL Anolog to digital convertor (ADC) board

1 pv SUP

Spactrat memory board

rings

El Key baard

Imprdonco matching Input/otr?put

I I / O ) board

P a r e r supply

E

Radiooetivo source //////I

Figure 11 Schematic of SlROLQG borehole logging system. (Reprinted with permission from Nucl. Geophys. 2, Charbucinski, F? L. et al., Quantitative nuclear borehole logging based on neutron excited gamma-reactions, 139, Copyright 7908, Pergamon Press, Ltd.) elements. The performance of this analysis is influenced by the neutron source employed in the probe; two types of neutron sources, the isotopic 2"Cf and 241Am-Re,and neutron generators, have been studied to establish their characteristics and feasibilities.

I . Analysis Using lsotopic Neutron Sources The in situ borehole logging of coal in terms of multielemental analysis was studied by Clayton et al. This analysis was conducted in using a 5-pg 252Cfneutron source coupled to an 11% HPGe dete~tor.~' a mock-up borehole in which five representative coal samples with ash content from 7 to 30% were measured by the ISPGAA technique. The relative concenlrations of 1I. major elements were determined in situ: Al, C, C1, Fe, H, K, N, Na, S, Si, 'and Ti. The bulk analysis of elemental concentrations has been compared to the laboratory chemical analysis, resulting in most major elements in coal being determined by this probe within 10% accuracy for a Zmin rapid field scan. However, this technique posed limitations in detection of carbon and oxygen due to their small thermal neutron capture crosssections, while the need for higher energy neutron sources to raise the reaction probability is recommended. In addition, high count rate electronics is necessary to obtain results of acceptable accurary in a short scanning time of a few minutes, a typical in sit11 borehole logging period. On the other hand, the 24'Am-Bcneutron source, providing a higher average neutron energy of 4 MeV, makes the determination of carbon and oxygen possible by means of inelastic scattering reactions of C(n,nly) and O(n,nfy) reactions wit!^ emission of 4.44- and 6.13-MeV prompt y-rays, respecti~ely.~'~' Figure 12 shows the spectra measured using 252Cfand 2J'Am-Be sources; it is explicit that the "'AmBe source is superior for measurements of carbon and oxygen alone. Using a "'Am-Be neutron source coupled to an NPGe detector, the measurement of sulphur content in coal was conducted by Underwood; the sulphur concentration can be determined down to 1% by weight, but the accuracy is intluenced by the chlorine content in the horehole e n v i r ~ n m e n t A ~ ~neutron ~~' transport code ANISN wns used to dencribe the influence of chlorine concentrations on the sulphur peak area of 2380 key

Energy

- keV

Energy

- keV

Figure 12 Prompt y-spectrum from (a) 252Cf, and (b) 241Am-Beneutrons interaction with coal. (Reprinted with permission from Int. J. Appl. Radiat. /sot., 34, Clayton, C. G. and Wormald, M. R., Coal analysis by nuclear methods, 5, Copyright 1983, Pergamon Press, Ltd.)

2. Analysis Using Neutron Generators Using a 14-MeV neutron generator probe coupled to a NaI(T1) detector or an HPGe detector has been investigated and proven capable of offering reliable results with respect to carbon and oxygen determination; this cannot be accomplished using low-energy isotopic neutron sources. The neutron generator can be operated in pulsed mode to fulfill an ultimate analysis of coal, in which both thermal capture and fast inelastic spectra are recorded by means of a time-segregated gating technique. The U.S. Geological Survey and U.S. Bureau of Mines have worked together since 1982 on a project to demonstrate the uses of a 14-MeV neutron generator coupled to a germanium detector for borehole logging.s6 For collecting more y-signals for analysis, timing window techniques were used in their in situ set-up. A practical method of obtaining an ultimate Ganalysisunder field conditions is to utilize the generator at pulse repetition rates of about 100 pulses/s and recording two spectra: the decay window for determination of oxygen and the prompt thermal neutron capture window for other elements.

V. ENVIRONMENTAL WATER BODY IN SITU SURVEY There is growing concern about the environmental poIlution problem, particularly in industrialized countries. The causes of water pollution can be the consequence of waste discharge from factories, mills, and refineries; immediare action should be taken before the toxic chemicals expand into the ecosystem. For this reason, a nuclear technique based upon ISPGAA was developed; it certainly offers potential for in situ, rapid, and continuous survey of such water pollutants. For in situ measurement in effluent body, water is the on!y major constituent, and serves as a neutron moderator as well as a neutron shield free from temperature variation. Thermalized neutrons are confined to a certain volume immediately surrounding the detecting probe where capture reactions are dominant and the prompt y-rays are used for the determination of pollutants. 'This is in contrast to the in situ borehole formation logging, where the determination of the absolute concentration of the essential element in various geological formations encountered some difficulties. For example, spatial and energy variations of the neutron fltlx in an underground matrix with a strong neutron absorber and dense neutron moderator distributed heterogeneously; adaptation of instruments to high-temperature conditions; and the equipment damage due to hostile environments and neutron radiation. The underwater probe used for in situ measurement of elemental concentrations and pollutants of environmental water bodies was first proposed and constructed by Chung and T~eng.~' There are two types of underwater ISPGAA probes developed with configurations referred to as backscatter and transmission geomettie~,5~~l as illustrated in Figure 13. In the hackscatter ISPGAA system, all equipment is enclosed in an aluminum probe and the 252Cfneutron source, together with the HPGe detector, are

Figure 13 Geometric arrangements for (A) backscatter and (B) transmission in situ PGAA assemblies. (a) HPGe detector, (b) %f neutron source, (c) neutronly-shield, and (d) aluminum probe. (Reprinted with permission from Nucl. Geophys. 7, Chao, J. H. and Chung, C., In situ elemental measurements in an environmental water body by prompt gamma-ray spectromatry, Copyright 1893, Pergamon Press, Ltd.)

Operating depth

d

positioned on opposite sides of a lead block. The lead, and the neutron shield surrounding the detector, rcduce the intcnsity of [he fission y-ray directly from the neutron source and the neutron flux intensity at the detector crystal, therefore suppressing the interfering y-rays in the collected spectra. As for the transmission system, the neutron source is submerged in water, whereas the HPGe detector is placed above the water-line; the water in between serves as both the sample being analyzed and y-raylneutron shield to remove neutrons as well as low-energy y-rays, but allows high-energy prompt y-rays to penetrate and reach the detector above the water surface. The basic concept for designing the ISPGAA probe is similar to that of logging sondes employed in surveys of geological formation. The submerged probe consists mainly of a W f neutron source, a portable HPGe detector, and neutron and y-shields in between, as illustrated in Figure 13. Signal and prompt y-rays collected by the HPGe detector were transmitted and analyzed in the battery-powered, portable CANBERRA-10 PLUS MCA with built-in amplifier, high-voltage power supply, and rechargeable batteries; all are placed above water-line.

A. PERFORMANCE TEST The performance of the submerged ISPGAA probe was evaluated in a 5.7-ton tank. The tank was filled with specific chlorine-concentrated water by adding sodium chloride, and the background measurements were performed in chlorine-free deionized water. Optimum operating depth, neutron flux distribution, HPGe detector eff~ciency,effective detecting range, and rninirnum detectable concentration (MDC) of pollutants were determined in this laboratory test.57*58 Numerous chlorine prompt y-rays with considerable intensity are appropriate for determining the efficiency of the ISPGAA probe. The detecting efficiency of the backscatter detection system is illustrated in Figure 14, as compared to a point source in front of the HPGe detector. In the transmission system, the detecting efficiency is relatively high for high-energy y-rays when using a strong neutron source operated at deeper position, where high-energy y-rays can penetrate the thick water layer and reach the HPGe detector. Figure 15 illustrates a chlorine capture y-spectrum collected in such a test.61 On the other hand, the measurement in demineralized water explains the possible origins of spectral background; the major background radiations for an underwater ISPGAA probe contah6'

L

POlnT SOURCE

-

1d2

>

-

U

C

-ill

2U U

W

16~-

-

n

VOLUME SOURCE

0

IU U!

I-

W

0

.

I ..... I 1

lo2

,.....I

GAMMA-RAV

1

a

I

.I-

10 3 ENERGY, K

la* V

Figure 14 Detector efficiency of the HPGe detector for both volume source in ISPGAA operation and point source with 6 cm sourceto-detector distance in normal application. (Reprinted with permission from Nucl. Instr. Meth. A267, Chung, C. and Tseng, T. C., In situ prompt gamma-ray activation analysis of water pollutants using a shallow 252Cf-HPGe probe, 227, Copyright 1988, Elsevier Science Publishers B. V.)

.

0

-

Double escape peak

Gamma-ray energy, M e V

Figure 15 Chlorine prompt capture ?-ray spectra collected in water containing 1920 ppm CI using a : s; dead transmission system with 44 pg 2sCf at an operating depth of 70 cm; counting time (1,)36,000 time (DT): 8.5%. (Reprinted with permission from Nucl. Geophys. 7, Chao, J. H. and Chung, C., In sifu elemental measurements in an environmental water body by prompt gamma-ray spectrometry, 471, Copyright 1993, Pergamon Press, Lfd.)

1. Neutron-induced y-rays from neutron interactions with water. The most prominent peaks at 2223 and 6129 keV result from the reactions 'H(n,y) and 160(n,n'y), or from I6N, produced through the '60(n,p)'6Nreaction with a half-life of 7.1 s. 2. Neutron-induced y-rays-from the HPGe detector. Neutrons that escape from the water can interact with the detector, resulting, for example, in y-rays of 596 and 691 keV from Ge(n,y) and Ge(n,nfy) reactions.62 3. Neutron-induced y-rays from the probe material. The major y-rays are 7724 keV from Al(n,y) and 7368 keV from the Pb(n,y) reactions. Following a long detection period, the decay y-rays of 1779 keV from 28A1are clearly observed in the low-energy part of the spectrum. For the transmission system, the optimum neutron operating depth can be determined according to the ratio of sensitivity to background ( S B ) or MDC of chlorine measured at the 6111-keV prompt yr a y ~ , 5 ~which , ~ ' served as index photopeaks for salinity determinati~n.~~ Figure 16 displays the sensitivity, spectral background, and the derived SIB as a function of neutron operating depth." The sampling range of the probe is a prime concern; it is dictated by the intensity and distribution of neutron flux in water around the probes. The neutron flux in water is measured by the indium foil activation technique, reflecting the probability of the emitted prompt y-rays in water around the probe. The actual number of prompt y-rays detected by the probe is a function of both neutron flux and water attenuation, which in turn vary with distance to the detector. The effective detecting volumes of the probes for the backscatter and transmission geometries were estimated to be 0.14 and 1.8 m3, respectively, using the accumulation of total counts contributed from water body at various distances from the HPGe detector. The sensitivities of some index elements of environmental concern are evaluated by comparison with chlorine, as below:

L"PCf source depth 1 cm

Figure 16 Variations of (A) detection sensitivity at 61 11 keV, (B) spectral background in 5 to 9 MeV range, and (C) SIB ratio with a neutron source at the operating depth for the transmission detection system with a 252Cf neutron source activity of: (a) 1.3, (b) 8.0, (c) 44, and (d) 260 ~ g(Reprinted . with permission from Nucl. Geophys. 7, Chao, J. H. and Chung, C., In situ elemental measurements in an environmental water body by prompt gammaray spectrometry, 473, Copyriy ht 1993, Pergamon Press, Ltd.)

where S(x) = the sensitivity of element x, cps/ppm S(C1)= the sensitivity of chlorine, cpstppm = detecting efficiency at prompt photopeak E I = prompt y-ray intensity A = atomic weight The MDC values of metallic elements in pollutants are evaluated in the same manner as that of a submerged probe for various-intensity neutron sources at the corresponding optimized source depth; some results are shown in Figure 17.58

B. FIELD SURVEY USING THE ISPGAA PROBE Unlike traditional techniques for in situ determination of salinity by measuring the properties of sampled salt water, the ISPGAA technique measures salinity in terms of chlorine content by detecing its prompt y-rays emitted from the Cl(n,y) reaction surrounding a neutron source. The salinity, defined as total dissolved salt in water in units of parts per th~usand(%o), can be simply derived by a linear relation as 1.80665 X Cl%o, with C1 representing chlorine content or ~ h l o r i n i t y In . ~ ~every form of water body, the chlorine content in water is proportional to the total dissolved mineral content, or salinity. For those elements other than chlorine dissolved in seawater (such as bromine, iodine, and potassium) with low neutron capture cross-sections, they yield negligible contribution compared to those with chlorine. Other elements contribute little to the prompt y-ray spectrum.

Figure 17 The MDC of elements as a function of 252Cfsource intensity at the corresponding optimum source depths determined in a 10-h measurement with a water surface to detector distance of 60 cm. Prompt y-rays (keV) of elements are also indicated. (Reprinted with permission from Nucl. Instr. Meth. A299, Chao, J. H. and Chung, C., Optimization of in situ prompt gamma-ray analysis of lake water using a HPGe-252Cfprobe, 654, Copyright 1990, Elsevier Science Publishers B. V.) The in situ field survey of river salinity has been performed in the Tamsui River in northern Taiwan. where a transmission probe equipped with a 44-pg ZS2Cf neutron source was used.55The measured salinity variation during tidal cycles and salinity distribution across the river were performed. A backscatter probe for lake pollutant surveys in fresh or low-salinity water bodies was also carried out in a campus lake.'O The chlorine concentrations in lake water were determined to range from 20 to 86 ppm.

C. RADIATION SAFETY CONCERNS A bare 252Cfneutron source is hazardous to humans due to its tremendous neutron dose rendered. Exposed to a 44-pg 252Cfsource in air at a distance 1 m away from the source, one suffers both neutron and y-doses at an equivalent rate of 1 mSv/h, which is far beyond the limit imposed to a radiation worker, 0.025 mSv/h. The 25?Cfneutron source is submerged in water during operation of the ISPGAA probe. In this condition, water acts as neutron moderator, thermal neutron absorbor, as well as a y-ray shield, reducing significantly neutron radiation and prompt y-rays near the water surface where radiation workers are stationed. On the other hnncl, the induced prompt y-rays of 2223 keV in H(n,y), and 61 !1, 7414, 8578 keV in CI(n,y) reactions become the main source of the y-dose. The distribution of dose rate on board, including neutron and y-ray contributions, is measured around 1 to 4 p S v h at the operating p o ~ i t i o n , ~ ~ w his i cwell h within the safety level recommended by the ICRP report." In addition to induced prompt y-rays, the thermalized neutrons in water inevitably produce activated nuclides, which emit decay P- and y-rays. !n seawater, the thermal neutrons are mainly absorbed by hydrogen, chlorine, and sodium; the residual activities induced by the 44-p,g 252Cfneutron source in seawater with salinity of 35% are calc~latedusing the

Table 3

Main Activated Products in Seawater lnduced by the 44-pg "*Cf Neutron Source

Nuclear reaction "Cl(n,y)38C1 "N~(II,~)'~N~ B'Br(n,y)82Br 35CI(n,y)MC1

Tin 37.3 min 15.0 h 35.3 h 3

X

IVyr

wtto

Abundance,

barns

%

0.428

24.23

0.53 2.7

100 49.3 1

43

75.77

Induced activity rate, Bq/s 35 8

4.3 x lo4 2.6 X lo-'

Reprinted with permission from Nucf. Geophys. 7, Chao, J. H. and Chung, C., In situ elemental measurements in an environmental water body by prompt gamma-ray spectrometry, 477, Copyright 1993, Pergamon Press, Ltd.

where

A = the saturated activity in Bq N = the number of nuclides per unit volume 4, = thermal neutron flux at distance (r) from source Table 3 lists major radioactive nuclides possibly generated in seawater as an ISPGAA system operates infinitely long.6' The production rate of each nuclide is estimated using Equation 4. Assuming that'the flowing rate of seawater is 1 m3/s and total activities induced convect uniformly within the effective sampling volume of 1.8 m3, the radioactive concentration would be less than 50 Bq/m3, which is much lower than the standard level of drinking water (37,000 Bq/m3), as well as the concentration of % in seawater (1 1,000 Bq/m3), as reported elsewhere.65

VI. PLANETARY IN S1TU EXPLORATION Chemical analysis of planetary surfaces is necessary for understanding the origin and evolutionary process of our solar system. Information on distribution of both major and trace elements on the planetary surface is essentizl for predicting the history of extraterrestrial activity as well as the existence of constituent materials of a planet. In 1964, NASA organized a team of investigators to develop the most reasonable techniques to measure the elemental compositions, hydr~gzncontent, and bulk density in the planetary surface.& Preliminary reports emphasized the use of an accelerator-type neutron source to implement detection of prompt and decay y-rays, as well as neutron "die away" measurements. Figure 18a envisions a simplified diagram of the apparatus for prompt y-ray detection on the lunar surface; the detection system can be arranged either parallel or perpendicular to the surface to be analyzed. A preliminary experiment of measuring inelastic prompt y-rays using a 2" X 2" NaI detector was performed on an artificial granite surface; y-ray spectra collected for horizontal and vertical probe orientations are shown in Figure 18b for comparison. Trombka et al. proposed the use of a 252Cfneutron source due to its small size, stable flux, and complete lack of power supply, developing a technique based on the measurement of capture prompt y-ray~.~'The detecting system is illustrated in Figure 19a, in which a Ge(Li) or NaI('l'1) detector was used to measure the high-energy capture prompt y-rays, capable of penetrating the soil and shield. The spectra collected in a mock-up granite surface are shown in Figure 19b. Similar to the borehole logging system, the in situ prompt y-ray analysis on a pIanetary surface requires a neutrcn source in connection with a y-ray detector on board a spaceship. Feasibility and limitations regarding such exploration using the ISPGAA technique have been estimated by Johnson and S e n f t l ~ , 6as ~ .concluded ~~ below. NaI(T1) detectors are preferable for space application due to their resistance to high-temperature conditions. In order to suppress spectral background due to activation on the detector crystal, especially three possible neutron sources were considered: (1) a the generation of 25-min IZBIand 15-h 24Na, neutron generator that can be turned off during space flight; (2) an isotopic (cx,n) source that is "unmixed" until it amves on the planetary surface; and (3) an isotopic source that can be kept physically separated from the detector during space flight.

ta)

i

r --1

I--f I I

I

GAMMA -RAY

CHANNEL NUMBER

Figure 18 (a) Geometrical arrangements of the neutron probe with respect to the lunar surface. (b) Gamma-ray spectrum from inelastic scattering for horizontal and vertical probe orientation. (Reprinted with permission from Science, 152, Caldwell. R. L. st at., Combination neutron experiment for remote analysis. 460461, Copyright 1966. American Association for the Advancement of Science.)

In response to the use of a semiconducting germanium detector for planetary exploration, the CO, solid-cryogen cooling system was designed by Nakano et al.'" The operating temperature of the gemanium detector is 130K, much higher than the liquid nitrogen temperature but somewhat lower than the criticaI temperature of 135K, above which the detector resolution begins to degrade appreciably." Senftle et al. used the Monte Carlo calculation technique to estimate the thermal neutron and induced y-ray fl uences at various distances from a "2Cf neutron source on typical lunar soil with water content up to 10% by weight.69The results inc!icated that the nonhydrogeneous condition encountered on the planetary surface restricts the use of captured and delayed y-ray analysis due to low concentrations of hydrogen or water. In that cme, inelastic prompt y-rays induced by fast neutron will dominate the yray spectra for analysis of the major and a few minor elements on the planetary surfh~e.'~

Figure 19 (a) 252Cfsource Ge(Li) detector experiment configuration. (b) Prompt capture and activation y-pulse-height spectrum. (Reprinted with permission from Nucl. Instr. Meth. 87, Trombra, J. I. et al., Neutron radiativecapture methods for surface elemental analysis, 37-38, Copyright 1970, Elsevier Science Publishers B. V.)

Neutron source

I

0

L

5

10

I L

15

20

I

I

I

25

30

35

1

4 0

45

crn

Figure 20 Sectional contour diagram showing sea bed contributions to ISPGAA probe countrate (from unit volume) a s percentages of the maximum value close to the surface. (Reprinted with permission from Int. J. Appl. Radiat. Isot. 34, Thornas, B. W. et at., Mineral exploration of the sea bed by towed sea bed spectrometers, 439, Copydght 1983, Pergamon Press Ltd.)

The seabed minerals are often formed as a result of a deposition process. Major interest centers on the potential of economic extraction of manganese oxide noduIes and strategic metal-bearing muds. The exploitation of these minerals is subject to knowledge of the locations and extent of seabed mineral deposits. Practical techniques for surveying mineral deposits and assaying value are underinvestigated. A variety of nuclear techniques have been developed and applied to mineral exploration. These include the measurement of natural y-radiation, a range of neutron interaction techniques, and energy dispersive X-ray fluorescence analysis. Among them, the natural y-ray towed seabed spectrometer is potentially feasible for the discovery and delineation of minerals containing radioactive isotopes. The towed spectrometer based on the ISPGAA technique can be used to identify the concentration and spatial distribution of many nonradioactive minerals currently of interest in marine exploration. An ISPGAA-based spectrometer consisting of a 'Cf neutron source and HPGe detector has been designed by Tharnos et al.," as shown schematically in Figure 20. The performance of this probe was demonstrated in a test tank filled with seawater and simulated seabed, consisting of silica sand seeded with known elements of interest. The capture prompt y-rays induced by a 100-pg W f source was collected by an HPGe detector. The performance of the probe, including the MDC for elements of economic interest, effective sampling depth below seabed, factors affecting distribution of neutron fluence in samples, and the influence of interfering elements on detection limits are reported in detail. Some pertinent results are described For a number of elements of economic interest, MDC values were calcuIated using the most intense capture y-rays indrmci by a 100-pg '*Cf neutron source for a counting period of 15 min, as listed in Table 4. The detector response to capture y-rays as a function of seabed depth was inferred using the contours of the percentage contribution to the total count rate, showing an effective sampling depth to 15 cm at the surface of h e seabed, and subject to the variation of seabed environment. Chlorine is the major interfering element in limiting detection sensitivity in neutron-induced y-ray spectrometly of the sealaed. This effect can he improved using a noncylindrical probe.

Table 4 Some Elements of Interest and Their Characteristics

Atomic mass Element Nickel Copper Zinc Molybdenum Silver Tin Gold Mercury Lead Chromium Cobalt Selenium Niobium Antimony Barium Tantalum Tungsten Platinum Vanadium Manganese Iron Zirconium Magnesium Titanium Cadmium

A 58.71 63.54 65.37

Thermal neutron absorption cross-section a.barns

4.6 3.9 1.1

intensity 1 (%), photons1 100 neutrons 42 28 12

Lowest limit of detection in Si02 matrix (counting time 15 min Sensitivity source 100 pg 262Cf), lulA wt% 3.2 1.7 0.2

0.2 0.3 2.0.

Reprinted with permission from Int. J. Appl. Radiat. Isot. 34, Thomas, B. W. et al., Mineral exploralion of the sea bed by towed sea bed spectrometers, 446, Copyright 1983, Pergamon Press, Ltd.

The Ocean Drilling Program (ODP) using nuclear logging techniques for ocean floor in situ s u r v q The ~najjor was implemented to determine the parameters characterizing the ocean floor geo~hemicalIy?~ advantages of nuclear measurements over traditional geophysical logs is the ability to deploy the technique for measuring the formation properties through drill pipe, providing a continuous record for analysis.

ISPGAA techniques have expanded to almost all aspects of bulk analysis for the last three decades. With the advent of innovative nuclear instrumentation and technology dedicated to these fields, sophisticated detection and detail evaluations became possible andlor practical, both from technological and economicai points of view. Large efforts still concentrate on the use of the ISPGAA technique in petroleum industries, keeping pace with the increased need in oil production in an economical way. The ISPGAA method performs elemental analysis for oil rock formation, permitting an understanding of mineral constituents, which is complementary to other techniques for identification of rocks with potential productivity, not just for the purpose of exploration. The economic and environmental importance of in situ borehole analysis of constituent elements in a coal mine before production result in the enhanced development in ISPGAA techniql-2s. Ash content evaIuation is always the prime concern for coal quality determination. Besides, due to stringent environmental protection limitations, sulphur content determination has become a necessity recently. An ISPGAA technique using an isotopic neutron source for surveying water polIutants and salinity has the advantages of large sampling volume and less radiation safety concerns; it is practical in

determining toxic elements in effluents from factories or a heavily polluted water body. Rapid survey and timely response to the accidental release can possibly be taken. Applications of ISPGAA in outerspace exploration and seabed mineral investigation show its unique &aracteristics for remote analysis; in such hostile environments, restrictions in detection instruments and samples to be analyzed should be considered.

REFERENCES 1. Stelson, P. H., Dickens, J. K., Raman, S., and Trammell, R. C., Deterioration of large Ge(Li) diodes caused by fast neutrons. Nucl. Irtstr. Meth. 98. 481. 1972.

2. Kraner, H. W., Pehl, R. H., and Maller, E. E., Fast neutron radiation damage of high-purity germanium detectors, IEEE Nucl. Sci. 22, 149, 1975. 3. Lee, C. J. and Chung, C., Performances of gamma ray measurement using bismuth germanate detector after thermal neutron bombardment, Appl. Radiat. Isor. 42. 729, 1991. 4. Shope, L. A., Berg, R. S., O'Meal, M. L., and Barnaby, R. E., Operation and life of the Zetatron: A small neutron generator for borehole logging, IEEE Nucl. Sci. 28, 1696, 1981. 5. Grau, J. A. and Scllweitzer, J. S., Prompt y-ray spectral analysis of well data obtained with NaI(T1) and 14 MeV neutrons, Nucl. Geophys. 1, 157, 1987. 6. Vartsky, D., Wielopolski, L., Ellis, K. J., and Cohn, S . M., High count rate proble&s in elemental analysis using pulsed neutron inelastic scattering, Nucl. Instx Meth. 206, 575, 1983. 7. Senftle, F. E,Macy, R. J., and Mikesell, J. L., Determination of the optimum-size californium-252 neutron source for borehole capture gammamy analysis, Nucl. Instr: Meth. 158, 293, 1979. 8. Rozsa, C., Dayton, R., and Raby, P., Characteristics of scintillators for well logging to 225 "C, IEEE Nucl. Sci. 37, 966, 1990. 9. Melcher, C. L., Manente, R. A., and Schweitzer, J. S., Applicability of barium fluoride and cadmium tungstate scintillators for well logging, IEEE N~rcl.Sci. 35, 1188, 1989. 10. Melcher, C. L. and Schweitzer, .I. S., Gamma-ray detector properties for hostile environments, IEEE Nucl. Sci. 35. 876. 1988. 1 1. Melcher, C. L, Schwdtzer, J. S., Manente, R. A., and Peterson, C. A., Application of GSO scintillators for well logging, IEEE NucL Sci. 38, 506, 199 1. 12. Traeger, R K. and Lysne, P. C, High temperature electronics application in well logging, IEEE Nucl. .Sci. 35, 852, 1988. 13. Melcher, C. L., Schweitzer, 3. S., Libeman, A., and Sirnonetti, J., Temperature dependence of fluoresence decay time and emission spectrum of bismuth germanate. IEEE Nucl. Sci. 32, 529, 1985. 14. Tanner, A. B., Moxharn, R. M., and Senftle, F. E., A probe for neutron activation analysis in a drill hole using 252Cfand Ge(Li) detector cooled by a melting cryogen, Nml. Instr: Meth. 100, 1 , 1972. 15. Boynton, G. R,Canister cryogenic system for cooling germanium semiconductordetectors in borehole and marine probes, Nucl. Instr. Mefh. 123, 599, 1975. 16. Mellor, D. W., Cryogenics for semiconductor detector neutron activation logging tools, Int. J. Appl. Rndiat. Dot. 36, 295, 1985. 17. Tittle, C. W., A history of nuclear well logging in the oil industry, Nucl. Geophys. 3, 75, 1989. 18. Schultz, W. E. and Smith, IB. I).,Laboratory and field evaluation of a carbonloxygen (C/O)well logging system, J. Pet. Tech. Oct., 1103, 1974. 19. Lock, G. A. and Iloyer, W. A., Carbon-oxygen (C/O) log: Use and interpretation. J. Per. Tech. Sept., 1044, 1974. 20. Culver, R B., Hopkinson, E. C., and Youmans, A. M., Carbontoxygen (C/O) logging instrumentation. Soc. Petrol. Engineers J. Oct., 463, 1974. 21. Kerr, S. A. and Worthington, P. F., Nuclear logging techniques for hydrocarbon, mineral and geological applications, IEEE N~rcl.Sci. 35, 794, 1988. 22. Schweitzer, J. S. and E l k , n.V., Review of nuclear techniques in subsurface geology. lEEE Nucl. Sci. 35. 800, 1988. 23. Sanders, L. G., The application of Monte Carlo comp~rtationsto formation analysis by neutron interactions, Int. J. Appl. Radiat. Isot. 34, 173, 1983. 24. Hertzog, R. C., Neutron-excited gamma-ray spectrometry for well logging, IEEE Nucl. Sci. 26, 1558, 1979. 25. Dewan, J. T., Stone, 0. L., and Morris, R. L., Chlorine logging in cased holes, J. Pet. Tech. June, 53 1. 1961. 26. Mckinlny, P. F. and Tanner, H. I,., The shaie-compensated chlorine log. J. Pet. Tech. Feb., 164, 1975. 27. Herrnn, M. M., Future i~pp!icalioriso f elcmcnti~lconcentr;~tionsI'rom geophysicnl logging, N ~ r c l .Grol~ilys. 3. 197, 1987. 28. HerZzog, R. C., ElemenraI conctmrations from neutron induced gamma ray spectroscopy, IEEE Nttcl. Sci. 35, 827. 1988.

29. Scott, H. D., New developments in remote elemental analysis of rock formations, J. Pet. Tech. July, 7 11, 1986. 30. Schweitzer, J. S., Developments in elemental concentration logging and applications, IEEE Nucl. Sci. 38. 497, 1991. 31. Karus, E. V. and Shimelevich, Yu. S., Nuclear geophysics in prospecting, exploration and development of oil and gas fields, Int. J. Appl. Radiat. Isot. 34, 95, 1983. 32. Senftle, F. E. and Mikesell, J. L., The nuclear ratio technique applied to borehole exploration for industrial metals and coal, Nucl. Geophys. 1, 227, 1987. 33. Lederer, C. M. and Shirley, V. S., in Table of Isotopes, John Wiley & Sons, New York, 1978. 34. Lone, M. A., Leavitt, R. A., and Harrison, D. A., Prompt gamma rays from thermal-neutron capture, Atomic Data and Nuclear Data Tables, 26, 5 I 1-559, I98 1. 35. Mills, W. R., Stmrnswold, D. C., and Allen, L. S., Advances in nuclear oil well logging, Nucl. Geophys. 5, 209, 1991. 36. Schweitzer, J. S., Nuclear techniques in the oil industry, Nucl. Geophys. 5, 65, 1991. 37. Schweitzer, J. S., Ellis, D. V., Grau, J. A., and Hertzog, R. C, Elemental concentrations from gamma-ray spectroscopy logs, Nucl. Geophys. 2, 175, 1988. 38. Grau, J. A. and Schweitzer, J. S., Elemental concentrations from thermal neutron capture gamma-ray spectra in geological formations. Nucl. Geophys. 3, 1. 1989. 39. Schweitzer, J. S., Hertzog, R. C., and Soran, P. D., Nuclear data for geophysical spectroscopic logging, Nucl. Geophys. 1,213, 1987. 40. Evans, L. G., Lapides, J. R., Trombka, J. I., and Jensen, D. H., In situ elemental analysis using neutroncapture gamma-ray spectroscopy, Nucl. Instr. Meth. 193, 353, 1982. 41. Evans, L. G., Trombka, J. I., Jensen, D. H., Stephenson, W. A., Hoover, R. A., Mikesell, J. L., Tanner, A. B., and Senftle, F. E., Inter-pulse high-resolution gamma-ray spectra using a 14 MeV pulsed neutron generator, Nucl. Instr: Meth. A219, 233, 1984. 42. EI-Kady, A. A., Abulfaraj, W. H., and Abulfattah, A. F., Evaluation of a prompt gamma ray sonde consisting of an isotopic neutron source and a high purity germanium detector, Nucl. Instr. Meth. A28 I , 236, 1989. 43. Charbucinski, J., Borsam, M., Eisler, P. L., and Youl, S. F., In situ borehole determination of ash content of coal using gamma-gamma and neutron-gamma techniques, in Proc. Advisory Group Meeting on Gummu, X-Ray and Neutron Techniquesfor the Coal Industry, IAEA Vienna, 1986, 60. 44. James, W. D., Activation analysis of coal and coal effluents, in Activation Analysis, LI, CRC Press, Boca Raton, FL, 1990, 364. 45. Charbucinski, J, Youl, S. F., Eisler, P. L., and Borsaru, M., Prompt neutron-gamma logging for coal ash in water-filled boreholes, Geophysics, 51, 11 10, 1986. 46. Eisler, P., Borsaru, M., Youl S. F, and Charbucinski J., Quantitative spectrometric borehole logging techniques for coal mining, Nucl. Geophyr. 2, 43, 1988. 47. Charbucinski, J., Eiler, P. L., and Borsasu, M., Quantitative nuclear borehole logging based on neutron excited gamma-reactions, Nucl. Geophys. 2, 137, 1988. 48. Borsaru, M., Charbucinski, J., Huppert, P., Youl, S., and Eisler, P., Coal ash determination in dry boreholes by the neutron capture technique, NucL Geoplrys. 2, 201, 1988. 49. Borsaru, M., Charbucinski, J., Eisler, P., and Ceravolo, C., Neutron gamma logging in coal seams of variable iron content, Nucl. Geophys. 5, I 17, 1991. 50. Zhao, J. Q., Liu, J. S., Ni, H. F., Liu, D., Wuru, G. S , Guo, T. C., Wu, S. Y, and Liu, J. L., Neutron capture gamma-ray spectral well logging ustng a %f-HPGe probe, Nucl. Geophys. 5, 123, 1991. 51. Clayton, C. G., Hassan, A. M., and Wormald, M. R, Multi-element analysis of coal during borehole logging by measurement of prompt y-rays from lhcrmaf neutron capturc, Inf. J. App1. Rudiar. Isof. 34.83, 1983. 52. Wormald, M. R. and Clayton, C. G., In situ analysis of coal by measurement of neutron-induced prompt y-rays. Int. J. Appl. Radiat. Isot. 34, 71, 1983. 53. Clayton, C. G. and Wormald, M. R., Coal analysis by nuclear methods, Int. J. Appl. Radiat. Isot. 34.3, 1983. 54. Underwood, M. C. and Petler, J. S., High resolution prompt (n,y) spectroscopy using a down hole logging tool, J. Radioanal. Nucl. Chem. Articles, 114, 379, 1987. 55. Underwood, M. C. and Petler, J. S., Bulk material analysis using energetic neutrons, Nucl. Geophys. 3, 235, 1987. 56. Mikesell, J. L., Senftle, F. E., and Tanner, A. B., Borehole elemental analysis of coal using fast neutrons, in Proc. Advisory Group Meeting on Gamma, X-Ray and Neutron Techniques for the Coal Industry, IAEA Vienna, 1986, 25. 57. Chung, C. and 'Ikeng, T. C., In situ prompt gamma-ray activation analysis of water pollutants using a shallow =Tf-HPGe probe, Nucl. Inst,: Meth. A267, 223, 1988. 58. Chao, J. H. and Chung, C., Optimization of in situ prompt gamma-ray analysis of lake water using a HPGeI5Tf probe, Nucl. Instr. Meth. A299, 651, 1990.

59. Chao, J. H. and Chung, C., In situ prompt gamma-ray measurement of river water salinity in Northern Taiwan using HPGe-U2Cfprobe, Appl. Radiat. Isot. A42, 723, 1991. 60. Chao, J. H. and Chong, C., In sit![ lake pollutant survey using prompt-gamma probe, Appl. Radiat. Isot. A42.735, 1991. 61. Chao, J. H. and Chung, C., In siru elemental measurements in an environmental water body by prompt gamma-ray spectrometry, Nucl. Geophys. 7, 469, 1993. 62. Chao, J. K., Neutron-induced gamma rays in germanium detectors, Appl. Radiar. Isor. 44, 605, 1993. 63. Wooster, W. S., Lee, A. J., and Dietrich, G., Redefinition of salinity, Deep Sea Res. 16, 321, 1969. 64. International Commission on Radiological Protection (ICRP) Report 21, Pergamon Press, Oxford, 1973. 59. 65. Eisenbud, M., Environmental Radioactivity, 3rd ed., Academic Press, Orlando, FL, 1987, 149. 66. CaIdwell, R. L., Mills, FY. R., Allen, L. S., Bell, P. R., and Heath, R. L., Combination neutron experiment for remote analysis, Science, 152, 457, 1966. 67. Trombka, J. I., Senft!e, E, and Schmadebeck, R., Neutron radiative capture methods for surface elemental analysis, Nucl. Instrr Meth. 87,37, 1970. 68. John, R. G., Evans, L. G., and Tromhka, 3. I., Neutron-gamma techniques for planetary exploration. IEEE Nucl. Sci. 26, 1574, 1979. 69. Senftle, F., Philbin, P., Moxham, R., Boynton, G., and Tkombka, J., Problems &countered in the use of neutron methods for elemental analysis on planetary surfaces, Nucl. in st^ Meth. 117, 435, 1974. 70. Nakaho, G. H., Imhof, W. I,., and Johnson, R. G., A satellite-borne high-resolution Ge(Li) gnmma-ray spectrometer system, IEEE Nrrcl. Sci. 2 1, 159 1974. 71. Nakaho, G. H. and Imhof, W. L., Performance of large germanium detectors at elevated temperatures for satellite applications, IEEE NucL Sci. 18, 258, 1971. 72. Englcrt, P., Bruckner, J., and Wanke, H., Planetary gamma-ray spectroscopy, a special form of prompt charged particle and prompt neutron activation analysis, J. Radioanal. Nucl. Chem. Articles. 112, 11, 1987. 73. Thomas, B. W., Clayton, C. G., Ranasinghe, V. V. C., and Blair, I. M., Mineral exploration of the sea bed by towed sea bed spectrometers. Int. J. Appl. Radial. Isot. 34. 437, 1983. 74. Lovell, M. A., Harvey, P. R., and Anderson, R. N., The application of nuclear logs in the ocean drilling program. IEEE Nucl. Sci. 37. 1386, 1990.

Chapter 9

On-Line Applications Jiunn-Using Chao CONTENTS I. Introduction ............................................................................................................................... 159 11. On-line Analysis of Coal .......................................................................................................... 159 A. Importance of On-line Analysis of Coal ......................................................................... 159 B. OLPGAA Coal Analysis .................................................................................................... 160 C. Development of OLPGAA Techniques .............................................................................. 160 D. Commercial OLPGAA Systems ......................................................................................... 164 111. Mineral On-line Process Control ........................................................................................ 164 A. Metal Industry ..................................................................................................................... 166 B. Geothermal Power Utilities ................................................................................................ 170 IV. Explosive On-line Interrogation ............................................................................................... 170 A. Detection of Explosives by OLPGAA .......................................................................... 170 B. Current OLPGAA System for Explosive Detection ..................................................... 172 C. OLPGAA Using Fast Neutrons .......................................................................................... 174 D. Chemical Warfare and Ammunition Identification ......................................................... 175 V. Conclusions ............................................................................................................................. 175 References ...........................................................................................................................................175

Applications of PGAA techniques to various industries for on-line bulk analysis of coal, mineral ore, nitrogen-contained explosives, as well as, geothermal water have proven to be an attractive alternative to conventional methods. It is not surprising that the penetrating neutron, passing through the matter being analyzed and inducing prompt y-rays through either thermal neutron capture or fast neutron composition from deep parts of the inelastic scattering processes, can carry information of eleme~~tal sample to be measured. As a result, hundreds of batch samples can be analyzed directly on the conveyor in a short period of time without tedious preparation prior to measurement. Recent investigations using on-!ine prompt y-ray activation analysis (OLPGAA) methods have concentrated on the coal industry for economic reasons. Potential applications to the fields of process control in the mineral industry, explosive interrogation for airport security, and water analysis in geothermal power plants were also studied and proven to be promising. For the ISPGAA measurement introduced in the previous chapter, the c!etection system moves from place to place to collect information from the geological samples with fixed positions; whereas, the OLPGAA analyzes the samples moving through a fixed detection system, to which the counting conditions, such as temperature, pressure, humidity, and shielding design, can be adjusted or optimized to meet the requirements of specified measurement. More complex and sophisticated instruments can be incorporated into the system to upgrade the detection performance, rather than a small, compact detecting probe in the cases of geological ISPGAA measurement.

!I. ON-LINE AMALYSPS OF COAL A. IMPORTANCE OF ON-LINE ANALYSIS OF COAL Thus far, coal is still one of the most important sources of energy; extensive reserves make it continue to control the economic and industria1 developments in the world. At the same time, growing demand for rapid on-line analysis of coal quality and compositions, which determine the effectiveness of combustion in a coal-fire(! power plant or process contro! in a coal tvashery, results in the development of various on-line analyzers. The most important parameters influencing coal quality are combustion heat, moisture, ash, and sulphur contents. For instance, for the purpose of electricity production in a O.ti493-5 149-9/95/$0.00+$.~0 8 1995 by CRC Press, Jnc.

power plant, the combustion heat of coal is of prime concern. Moisture and ash content of coal can also affect the combustion process and should be known prior to coal combustion. In addition, an understanding of ash concentrations and the ash composite elements can prevent heat exchangers in steam generators from slagging and fouling. Today, the strict regulatory standards for air pollution control call for the determination of sulphur content before coal combustion; more efforts have focused on this measurement as a requisite in coal analysis.

B. OLPGAA COAL ANALYSIS In response to the demand for on-line rapid determination of coal quality in an electricity power station, a variety of nuclear methods including prompt y-techniques were developed. The main function of these techniques is to determine ash content and then to estimate the carbon content, or combustion heat, of coal by subtracting the ash content frcrn total amount; this is usually accomplished by dual-y transmission, Compton backscatter, and y-pair-production techniques. For the measurement of coal moisture, microwave attenuation and fast neutronly techniques are popular. During the past 15 years, OLPGAA techniques with some unequaled advantages over other current on-line techniques for coal analysis were rapidly developed to meet the requirements of the coal industry, and to act as a supplement to other on-line analyzers. The newly developed OLPGAA systems are based on the requirements of economic and environmental aspects; up to 14 elements can be simutaneously detected to infer the parameters related to coal quality. Basic principles and performances of the OLPGAA technique are stated below:

1. The OLPGAA measurement can directly relate combustion heat to the caxllon content in coal, or in a more detailed manner, incorporate the composite elements H, 0, N, and S concentration for evaluation.' Some studies pointed out that the combustion heat can be calculated as the concentrations of C , H, 0 , N, and S are known, or for simplicity, by use of amount of C as an index element for evaluation of combustion heat. The relationship between combustion heat and C content of coal was obtained in an experimental measurement,' yielding the value of 0.443 M J k g %C. But the carbon-dependent value is subject to various types of coals with different ash and moisture contents. Corrections between heat combustion of coal with its ash and moisture contents were reported for coals from various mines. 2. The elemental content of coal ash can be determined by knowing some indicative elements (Al, Ca, Fe, and Si), which are readily measured by OLPGAA methods due to their high cross-sections for capture reactions, reflecting the quantity of corresponding ash constituents of A1203,CaO, Fe203,and SO2. Besides, minor minerals in coal, such as S, C1, Ti, and Na, can be measured simultaneously from the observed characteristic prompt y-ray spectrum. 3. OLPGAA provides more penetrating radiations than that of the traditional X-ray fluorescence method for determining sulphur content. Sulphur content in massive coal samples can be determined by measuring the prompt photon of 2380 keV in the S(n,y) reaction. 4. Similar to all other nuclear techniques, OLPGAA does not tell apart different chemical bonds of the elements, but allows only the determination of elemental concentration. Consequently, the microwave attenuation technique is still a requisite for determination of moisture in coal. However, OLPGAA techniques show some limited success in determining water content in coal produced from a known geologica! formation in which the ratio of carbon to organicbonded hydrogen is a constant.2J For instance, raw lignite in Germany has a mean C/H ratio of 12.73 for five samples with various water content^.^ Basically, the parameters mentioned above are derived from the knowledge of the elemental composition of coal. The characteristic prompt y-rays from the elements of interest in coal analysis are summarized in Table 1. C. DEVELOPMENT OF OLPGAA TECHNIQUES The OLPGAA technique for coal analysis has been continuously modified over the past 15 years. In the early stage, investigators performed 1aboram-y measurements on 50- to 150-kg coal samples to ensure the feasibility of this technique for coal a n a l y s i ~ .This ~ . ~ includes the identification of proper nuclear reactions and prompt y-lines for coal composite element analysis; the optimization of irradiation and detection geometry; the determination of effective coal thickness in the conveyor belt for various

Table 1 Prompt y-Rays from Elements Influencing Coal Quality Prompt y-ray energy, keV

Parameters of .-

Elements

coal quality

Neutron reaction

Combustion heat Ash content

Moisture

Sulphur content Note: Data. taken from References 1 and 2.

intensities of neutron sources; the selection of suitable neutron sources as well as detectors; and the improvement of accuracy in a short counting period. A pioneer OLPGAA detection system for coal analysis for laboratory testing, assembled by Sowerby? is illustrated in Figure 1. The fast neutrons emitted from the nonnmderated 238P~-Re neutron source irradiate the coal sample, resulting in the emission of 4445-keV y-rays for the determination of carbon, an element indicative of specific energy. The moisture content of coal was derived by measuring the

lScm

0x

IOcm

T h ~ c kN a l ( T t ) Det~tor

0 oron Trioxtde Shl el d

Tunqsttn /Lead Sh~cld

a

d m Scolr

(

mnt

Figure 1 Cross-sectional view of thc- backscatter gauge used for the determination of inelastic scatter

and capture y-ray count rates from bulk samples. (Reprinted with permission from Nucl. Instr: Meth. 160, Sowerby, 8. D.,Measurement of specific energy, ash and moisture in bulk coal samples by a combined neutron and gamma-ray method, 175, Copyright 1979, Elsevier Science Publishers B. V.)

2223-keV prompt y-ray, generated through the neutron capture reaction with hydrogen in coal, and also by the measured CIH ratio. The NaI(T1) detector was used to measure these prompt y-rays and a typical spectrum, measured by Sowerby: is shown in Figure 2. Another laboratory expcrimcnt using a W f isotopic neutron source and germanium detector to perform a multielemental analysis was also r e p ~ r t e d This . ~ system was optimized by compromising the sensitivity and background at various coal thicknesses. Up to 12 elements can be detected simultaneously in a 20-min irradiation and counting period. Most of the neutron sources used for OLPGAA are isotopic neutron sources; for example, 252Cfand 238Pu-Be.They are frequently used in OLPGAA applications due to their properties of high neutron yield, long-lasting output, and small size. However, using a D-T neutron generator in OLPGAA for determining carbon and oxygen contents was also in~estigated;~ their cross-sections for the (n,nly) reaction, producing inelastic prompt y-rays, are higher than those using isotopic neutron sources. The D-T neutron generator, unfortunately, has to be replaced after some hundreds of hours service due to the depletion of D/T ion source. Pair and Compton-suppressed y-ray spectrometric techniques were used in OLPGAA to improve spectral quality without loss of detection efficiency. A pair spectrometer is preferably used to detect high-energy y-rays, which tend to induce the pair-production effect in the primary detector; the subsequent emission of two annihilation photons (51 1 keV) in opposite directions are then dctected by the guard NaI(Tl) detectors surrounding the primary detector; this coincidence spectral technique improves the signal-to-noise ratio with reasonable detection efficiency. The Compton-suppressed technique, on the other hand, operates in anticoincidence mode to reject the Compton-scatrered photons escaping from the primary detector, thus reducing the spectral background intensity. Figure 3 shows the spectra collected in pair as well as Compton-suppressed mode when analyzing coal samples using 19 NaI(T1) detector Higher accuracy is achieved using PGAA than existing laboratory methods. The complexity of the hardware required is not expected to create major difficulties in an industrial environment based on present operating experience, which has been largely maintenance-free. During recent years, the German corporation Staatliches Materiaiprufungsamt has carried out, at the request of industrial customers, a number of feasibility studies based on PGAA on-line analyzers using

I

I 2 00

I

I LOO

I

I

600

1

800

1001

CHANNEL NUMBER

Figure 2 Prompt y-ray spectrum with a ccal sample containing 76.4 wt.% carbon. (Reprinted with permission from Nucl. Instr. Meth. 160, Sowerby, B. D., Measurement of specific energy, ash and moisture in bulk coal samples by a combined neutron and gamma-ray method, 177, Copyright 1979, Elsevier Science Publishers 6.V.)

Gamma

energy ( M e V )

Figure 3 (a) Pair spectrum of a limestone sample as used in the manufacture of cement; (b) Compton-

suppressed spectrum for a coal from Easington colliery (Reprinted with permission from Nucl. Geophys. 3, Warmald, M. R., A bulk materials analyzer using pair and Compton-suppressed gamma-ray spectrome.. try, 465, Copyright 1989,Pcrgamon Press Ltd.)

Table 2

Summary of Present Commercial OLPGAA Systems in the Coal Industry

Manufacturer

Instrument

MDH-Motherwell Inc.

ELAN

Science Applications International Corporation (SAIC) Gamma-metrics

Nucoalyzer CONAC Gamma-metrics

Mineral Control Company

Coalscan

WCI)

Performances

Ash, moisture. sulphur, calorific value, elemental analysis Ash, moisture, sulphur, calorific value, elemental analysis Ash, moisture, sulphur, calorific value, elemental analysis Ash, moisture, sulphur, calorific value, elemental analvsis

Note: Data taken from References 8 and 9.

252Cfirradiator~.~ The main purpose of the experiments was the investigation of parameters influencing the accuracy and reproducibility of the analytical results. The work has concentrated on the analysis of hard coal, lignite, and raw glass mixtures.

D. COMMERCIAL OLPGAA SYSTEMS At the present time, there are 26 commercial OLPGAA analyzers installed or on order in the coal industry in the U.S. and Canada. The major suppliers,&'2 as listed in Table 2, are Gamma-metrics, MDH, SAIC, and MCI. Most of the gauges are based on the utilization of a z2Cf source and NaI(T1) detectors in a transmission geometry and analyze the coal as it passes down a vertical chute. The elemental analyzer (ELAN), designed and supplied by MDH-Motherwell Xnc.,'O has been installed in a coal-cleaning plam at Homer City, PA, since 1983, as shown in Figure 4a. Its configuration and system performances are briefly described below. As shown schematically in Figure 4b, two =%f neutron sources, each with 100 pg, are placed on the right-hand side of the coal chute opposite the 15 cm D X 18 cm H NaI(T1) detector.I0 As mentioned in the previous chapter, although the NaI(T1) detector suffers from poor resolution for y-ray detection, it is not so critical for coal analysis due to prominent lines produced from the composite elements of Fe, Si, and Al. Apart from this, the ratio of e s q x peak to full-energy peak of NaI(T1) is small relative to that of a germanium detector, making the y-lines clear for analysis. The NaI(T1) detector is teinperaturestabilized at 54 "C to obtain a faster output than that at room temperature, and the coupled PMT is capable of tolerating continuous counting rates of 400,000 cps. Borated polyethylene and a large-volume coal sample comprise the safety shield, reducing the radiation level at 1 nl from the instrument surface to below 7 p,Sv/h for personnel safety and licensing requirements; in addition, it also prevents interferences from capture y-rays induced by structure materials reaching the detector. In the signal processing stnge, thc nonlinear cffects duc to peak pile-up, energy nonlinearity, and density change should be minimized. Pile-up can be measured by subtracting the sum of two singlesource spectra from the spectrum that results from both sources together, resulting in a residual pileup spectrum. A theory based on measured pulse shapes and the hardware pile-up rejection mechanism was developed to predict the residual pile-up spectrum. The composite spectrum after pile-up stripping can be approximated as a linear superposition of the thermal capture element spectra, two inelastic scattering spectra, four background spectra, and lhree spectra reflecting neutron interactions in the NaI(T1) detector. Calibration of the ELAN includes deriving each of these determined spectra from measurements of various samples of known composition. Figure 5 is an envisioned composite spectrum after pile-up stripping for coal samples at the Homer City pIant. Nearly all major elements, including oxygen, which is inferred from other known elements, can be determined in order to infer the ash and heat contents of coal.

Ill. MINERAL ON-LINE PROCESS CONTROL The application of OLPGAA techniques to mineral processing plants is required to control the variations of constituent elements for economic reasons. Fast neutron OLPGAA is quite suitable for bulk analyses

(a) Installation of ELAN and (b) schematic view of ELAN for the on-line analysis of coal. (Reprinted with permission from Nucl. Geophys. 3, Marshall, J. H., Ill and Zumberge, J. F., On-line measurements of bulk coal using prompt gamma neutron activation analysis, 446-448, Copyright 1989,

Figure 4

Pergaman Press Ltd.)

-

80.478 (SHIELDING)

Figure 5 Typical coal spectrum after pile-up stripping, dual 65+g 252Cf sources. (Reprinted with permission from Nucl. Geophys. 3, Marshall, J. H., Ill and Zumberge, J. F., On-line measurements of bulk coal using prompt gamma neutron activation analysis, 456, Copyright 1989, Pergamon Press Lld.)

of metallic elements owing to their significant neutron inelastic cross-sections; on the other hand, first neutrons aie a more penetrating radiation to metallic minerah than other types of radiation. The detection sensitivities of characteristic prompt y-rays following an inelastic scattering (n,nfy) process are quite dependent upon the neutron bombarding en erg^.'^-'^ The incident neutron energy should be greater than thc lirst cxcilcd cncrgy lcvcl of un ubundunLshblc isotopc of thc clcrncnl. Figurc 6 r;hows lhc tipcclra of an iron sample bombarded with 1.5, 2.5, and 3.5-MeV neutrons, respectively. More high-energy prompt y-ray peaks can be seen in the spectrum with 3.5-MeV bombarding neutrons. However, highenergy neutrons also induced more background and interfering peaks; the optimum neutron energy for analyzing the iron sample was therefore determined to be 2.5 MeV.

A. METAL INDUSTRY In practical cases, isotopic neutron sources of 239Pu-Beor Z'OPo-Be,positioning in an annular geometric arrangement, as illustrated in Figure 7, are preferred to a neutron generator. The tungsten block positioned in between them serves as a y-shield for reducing y-radiation from the neutron source, and the Bz03 embracing the detector is used to minimize thermal neutron bombardment on the detector.'' Both highresolution germanium and NaI(T1) detectors used for detection of prompt y-rays were evaluated. Correction factors for various samples with different bulk densities were also estimated and normalized at y-energies of 0.5, 1.0, and 2.0 MeV, allowing measurement of ore samples with densities of 1.2 to 3.3 g/cm3, as shown in Figure 8. A pulsed height spectrum was obtained with a Pb3O4sample in Figure 9, indicating the possible elements existing in the ore and their relative abundances. The prompt y-rays induced through fast inelastic reactions for essentral elements in mineral industries and their normalized y-responses to HPGe and NaI(T1) detectors were determined to help identification of element^.'^ Five geometries used for the bulk analysis of lead sinter samples were investigated by Cunningham et a1.;I6 their properties are summarized in Table 3. Selecting the most suitable geometry depends on accuracy, counting time, convenience of installation, and radiation safety. The backscatter geometry

Channels

, encrgg

Fe(n,n'y)

2.5MeV neutrons 0

,

Channels energy

-t 10' c

C 0

L

Za

10'

~n

F3

10

0

U

1 200

600

1000

1400

1800 2200 2600 3060

3QMl

Channels ;energy

Figure 6 y-Ray spectra of an iron sample at three bombarding energies. (Reprinted with permission from J. Radioanal. Nucl. Chem. Articles, 46, Yates, S. W. et a!., Elemental analysis by gamma-ray detection following inelastic neutron scattering, 343, Copyright 1978, Elsevier Sequoias. A.)

Figure 7 Cross-sectional view of the annular geometry used to determine neutron inelastic scatter y-ray count rates from bulk samples. (Reprinted with permissionfrom Int. J. Appl. Radiat. Isot. 35, Cunningham. J. 8. et al., Bulk analysis of sulphur, lead, zinc and iron in lead sinter feed using neutron inelastic scatter y-rays, 638,Copyright 1984, Pergamon Press Ltd.)

Figure 8 Calculated density correction factors applied to the measured y-ray yields. (Reprinted with permission from Nucl. Instl: Meth. 166, Sowerby, B. D., Elemental analysis by neutron inelastic scatter gamma rays with a radioisotope neutron source, 572, Copyright 1979, Elssvier Science Publishers 8. V.)

100

200

2 BULK OENSllY I g cm']

1

3 00

LOO

500

600

3 )

700

CHANNEL NUMBER

Figure 9 Pulse height spectrum obtained with a Pb304sample. (Reprinted with permission from Nucl. I n s t ~Meth. 166, Sowerby, B. D., Elemental analysis by neutron inelastic scatter gamma rays with a radioisotope neutron source, 572, Copyright 1379, Elsevier Science Publishers B. V.)

Table 3 Details of the Five Geometries Used for the Bulk Anabsis of Lead Sinter Sam~les

Neutron source Source output (neutronds) NaI(T1) detector dimensions (cm) Sample thickness (cm) Total. count rate in S window (countds) Net count rate in S window (counts/s/wt% S) Counting statistical error (10 min) . (wt% S)

Backscatter (ignition layer)

Backscatter (maln layer)

24'Am-Be

U 8 PBe ~

lo7

7.7 X 106

2x

(GI5 X 20

410 X 10

$15 X 10

10 X 10 X 15

10

-20

-20

5

-40

725

2550

1400

1080

-2350

10.9

6.5

2.6

-8.5

0.27

0.33

0.73

0.33

Annular

Semi-annular

24'Am-Be

"' Am-Be

7.7 X 106

7.7

$10 X I0

5.3 0.29

.

X

lo6

Backscatter (brass box)

238P~-Be 2x

lo7

Reprinted with permission from Int. J. Appl. Radiat. Isot. 35, Cunningham, J. B. et aI.. Bulk analysis of sulphur, Iead, zinc and iron in lead sinter feed using neutron inelastic scatter y-nys,639, Copyright 1984, Pergamon Press Ltd. was chosen for plant testing. The S, Pb, Zn, and Fe in lead sinter feed can be determined to within about 0.3, 0.9, 0.2, and 0.3 wt %, respectively. The development and application of a thermal neutron OLPGAA technique to the aluminum production process was described by Liu et aLI7 The detection system includes a 100-pg z2Cf neutron source placed in the measurement piping and a germanium detector for detecting the capture prompt y-rays for determination of A120,, Fe02, CaO, and S i 0 2 in a pipeline, as illustrated in Figure 10. The system has 'Seen tested and was expected to reduce plant costs by over U.S.D 3,000,000 annually by saving energy consumption, rapid surveying of pulp quality, and reducing labor costs. The measurement error of A1203, Fe02, CaO, and SiOz is 0.3, 0.1, 0.4, and 0.4%, respectively.

Figure 10 Diagram of on-line measurement assembly for aluminum process control. (Reprinted with permission from J. Radioanal. Nwcl. Chem. Articles, 151, Liu, Y et al., Development and applications of an on-line thermal neutron prompt-gamma element analysis system, 86, Copyright 1991, Elsevier Sequoia

Although iron is an essential element for arra!ysis by thermal neutron capture because of its large cross-section and intense high-energy y-rays, the carbon and oxygen in steel have low concentrations and low capture cross-sections, making thermal neutron capture techniques unacceptable for determination of their concentrations in steel. Shirakawa evaluated the feasibility of fast neutron techniques for on-line determination of carbon and oxygen contents in steel by computer ~imulation.'~ The 14-MeV neutrons produced from a D--2 generator are used to excite carbon and oxygen with inelastic scattering crosssections of 0.35 and 0.43 barns, respectively. A high-efficiency BGO detector is preferred for the measurements of the 4.43-MeV y-rays emitted from carbon and 6.13 MeV from which both high energy photopeaks are well separated in the spectrum despite the poorer resolution of the BGO detector. Computed results based on Monte Carlo calculations showed that the accuracy of the measurement of carbon content was t O . l wt % in the range 0.4 to 2.5 wt % for 60-s irradiation with neutron source strengths of 0.5 to 1.0 X loSnls, using both transmission and the backscatter geometric arrangements. The accuracy of measuring oxygen was within f0.01 wt % in the range 0.1 to 0.2 wt % for a 60-s count following 60-s irradiation in both geometries.

B. GEOTHERMAL POWER UTlLlTlES The extraction of the heat from hot rock near the surface of the earth by circulating the coolant may generate electrical power. Instead of sending steam from the geothermal reservoir directly to the turbine, a primary heat exchanger to make secondary steam for the turbine is preferred; this arrangement isolates the mineralized solutions that will be at much higher temperatures and pressures than with current natural steam. Chemical problems with the water circuits (e.g., corrosion and solid precipitation) are foreseen and provide analytical information for planning the geothermal operations and for the plants. Nuclear spectroscopic methods were proposed to analyze the mineral composition in the circulated waters, which are difficult to access by other analytical sampling techniques due to their high temperature and pressure conditions. The OLPGAA technique based on the measurement of the characteristic prompt y-rays provides a solution to this problem. -4 laboratory assembly for this purpose was constructed to demonstrate its performance for on-line anaiysis of geothermal waters.I9 The on-line analyzing system neutron source and a 20-cm3 Ge(Li) detector was described, as illustrated in Figure using a 3.4-mg 252Cf 11, Both neutron source and detector are placed outside the pipes; the induced prompt y-rays can penetrate the wall and are measured for analysis. Spectra measured for distilled water and various spring waters are shown in Figure 12. Chlorine is responsible for corrosion problems and can be determined to very low concentrations. Valuable minerals to be extracted, such as copper, could be determined and the information fed to related process coritrols on chemical additions. The understanding of water minerals is helpful for the engineers to design and operate a geothermal power plant. The information on water content could assist in plant control. Sensing the change in water impurities is mandatory. IV. EXPLOSIVE ON-LINE INTERROGATION The detection of explosive materials in baggage by the OLPGAA technique is a newly proposed alternative to screeti suspected matter for airport security. The basic theory of such a PGAA method is to measure the unique prompt y-rays from nitrogen, it being a major element in explosives. The mean nitrogen density of various explosives is found up to 0.026 5 0.012 mo1/cm3, higher than the common nitrogen-contained materials such as foods, fabrics, and polymers;20therefore, the nitrogen content can be referred to as an indicator for explosives.

A. DETECTION OF EXPLOSWES BY OLPGAA From the detection point of view, the nitrogen caplure y-ray has the highest y-energy of 10.83 MeV; other neutron-induced prompt and decayed photopeaks in the spectrum do not superimpose and interfere with its full-energy md single-escape peaks. For such measurement, the use of a high-efficiency ydetector over a high-resolution detector is preferred. The NaI(T1) detector is still the most popular choice, but the investigation using a BGO detectw for detection of explosives demonstrated its superiority to NaI(Ti) in counting effi~iency.~'In Figure 13, a comparison of NaI(Tl) and BGO detectors for measuring nitrogen prompt y-peaks, with 2 kg urea as an explosive simulator irradiated by thermal neutrons from a "2Cf source, is illustrated. The peak at 10.83 MeV is obvious in the spectrum measured with the BGO detector. In a practical deteciim system, a multidetector array is necessary to improve the detection sensitivity, or shorten the counting duration. Although no other photopeaks can interfere

7 POLYETHYLENE

Ge ILi) DETECTOR

CAY05lAT

N

BLOCKS

I 1 Flgure 11 12Cf storage drum irradiation arrangement; all dimensions in inches. (Reprinted with permission from Nucl. Tech. 27, Duffey, D.,Analysis of geothermal power plant water using gamma rays from capture of californium-252 neutrons, 491, Copyright 1975, American Nuclear Society, Inc.)

CHANNEL

Figure 12 Prompt y-ray spectra obtained from (a) distilled water and (b) hot spring, irradiated with a

252Cf source. (Reprinted with permission from Nucl. Tech. 27,Duffey, D., Analysis of geothermal power plant water using gamma rays from capture of californium-252 neutrons, 495, Copyright 1975,American Nuclear Society, Inc-)

Gamma ray energy ( M e V )

la) 2.5"

1

2

3

4

5

a 2" BGO

6

7

8

9

1011

12

13

14

Qornma ray energy ( M e V )

Flgure 13 Prompt y-ray spectra for explosive-like material measured using (a) 2.5" x 2" BGO and (b) 3" x 3" Nal(T1) scintillator. (Reprinted with permission from Appl. Radiat. /sot 42, Lee, C. J. el al., tlighenergy gamma-ray spectrometer using bismuth germanate detectors, 550, Copyright 1991, Pergarnon Press, Ltd.)

with the nitrogen prompt y-ray of 10.83 MeV, background in this energy region is attributed mainly to random coincidence counting, and Gaussian spreading accounts for 65 and 2596, respectively, as estimated by Wang using an NaI(T1) detector.22It is concluded that improving shielding and using a high-resolution detector can reduce the background counting rate in the nitrogen region.

B. CURRENT OLPGAA SYSTEM FOR EXPLOSIVE DETECTION The capture of slow neutrons by nitrogen results in the highest energy y-ray normally observed in such reactions. SAIC made use of this fact to develop the first prototype thermal nitrogen analysis (TNA) system for airport security.23In this system, the luggage moves through a screen of thermal neutrons produced by a ZS2Cfsource; the detected 10.83-MeV y-rays are tomographically analyzed to give the spatial distribution of nitrogen. After extensive testing, the first SAIC system was put into service at JFK International Airport, New York, in Janua-y of 1990. Up to five units are scheduled to be operational by now at airports in the U.S. and E u r ~ p e . ~For ' radiation safety considerations, the leakage radiation from the system, induced radioactivities in thc irradiated luggage, potential hazard of malfunctions, and shipping and handling of the neutron source were carefully studied. The U.S. Nuclear Regulatory Committee (NRC) znd the relevant agencies found the TNA acceptable for airport lobby installations.

(A)

z N absorber Lead

1

-+!=='

PA BGO 1

TSCA

Figure 14 Layout of the explosive detection assembly designed for airport security inspection; and electronic block diagram of the y-ray counting system. (Reprinted with permission from Appl. Radiat. Isot. 43, Chung, C. eta!., Feasibility study of explosive detection for airport security using neutron source, 1426, Copyright 1993, Pergamon Press Ltd.)

TSCA

- + T s cPA ABGO + 5- ~

For normal operation of the system and anticipated abnormal events, the radiological impact to both the worker and the public is well below the dose guidelines imposed by the authorities. For increasing counting effkiency or reducing scanning time to make the PGAA explosive detection system more effective in practical use, the high-density BGO detector has been considered. An OLPGAA system, consisting of five 2" X 2" BGO detectors and a 20-pg 252Cf neutron source, was assembled for this purpose,24as shown in Figure 14. For each detector, an amplifier/TSCA was connected to select the proper energy range with respect to the 10.83-MeV nitrogen prompt y-ray; this included the 10.83MeV full-energy and single-escape peaks; urea with 42.7% nitrogen content was used as an explosivelike material in this test. Other prompt y-rays, originating mainly from hydrogen (2.2 MeV), chlorine (6.1 MeV), and iron (7.6 MeV), do not interfere with the designated photopeak. The net counting rate collected from the detecting system is around 0.4 countls/kg urea, and the derived detection limit is 0.5 kg for a 5-minscan. It is concluded that using a more intense 252Cf neutron source and bilateral detector arrays are necess~uyto improve the detection eficiency and shorten the counting period.

Elemental Composition, in weight %, of Chemical Warfares and an Explosive Element TNT Sarin VX Mustard Lewisite

Table 4

Reprinted with permission from IEEE Nucl. Sci. 39, Caffrey, A. J. et al., Chemical warfare agent and high explosive identification by spectroscopy of neutron-induced gamma rays, 1422, Copyright 1992, The Institute of Electrical and Electronics Engineers, Inc.

C . OLPGAA USING FAST NEUTRONS The thermal neutron-based OLPGAA will evolve and improve in the coming years. OLPGAA using fast neutron inelastic scattering facilitates the simultaneous determination of oxygen, carbon, and nitrogen by measuring their respective prompt y-rays at 6.13, 4.44, and 2.31 MeV, respecrively, yielding twoand three-dimensional images of the elements for. identifying a bomb and its position; this technique is now being developed by several groups.'O Recently, Science Applications International Corporation (SAIC) developed an Explosive Detection System (EDS) based on the Pulsed Fast Neuiron Activation (PFNA) technique." The induced prompt y-rays of 160,I2C,I4N,and 35Clcan be measured in the defined time intervals immediately after irradiation of a neutron burst; the determined ratios among these elements help specify explosives and even narcotics.

D. CHEMICAL WARFARE AND AMMUNITION IDENTlFlCATlON Inspection of chemical warfare agents and ammunitions may be possible by observing neutron-induced y-rays folkwing detection with a high-purity germanium detector.26The elemental conlposition of' some typical cheniical war';ue and high-explosive munitions are listed in Table 4. For Instance, the lewisite with a high content of arsenic can be identified clearly by the unique prompt y-rays. A 25'Cf neutron source and a high-resolution germanium deteztx are preferred for such measurement design, as shown in Figure 15. This system has proven successful in verifying declarcd chcinical warfare agellts and ammunitions.

V. CONCLUSIBMS The OLPGAA technique has spread into the coal industry for econon~icand environmental consideration. A number of commercial OLPGAA devices dedicated to coal analysis have been installed, for which a high-yield lS2Cfneutron source and scintillation detector array are usually coupied to chance counting efficiency. At the same time, sophisticated techniques, such as high-rate counting. coincidence measurement, and the spectrum stripping method, are incorporated to shorten the counting period without the loss of accuracy. The determination of major and minor constituents by the OLPGAA technique in many industrial process lines opens a new frontier for operational control, although it is still in the developmental stage. Fast neutrons are suihble for analysis of metallic matter due to its high penetrating ability and the reasonable reaction cross-section for these elements. The highest energy capture y-rays of 10.83 MeV from nitrogen plays an important role in on-line measurement of explosive material in baggage; this unique characteriStic y-ray is most discernible in the y-ray spectrum. With the improvement in radiation safety concerns for the public, such an OLPGAA device will soon be popular for the purpose of airport security. A pulsed'fast neutron activation technique allows determination of major elements in explosives, providing detailed information on explosive identification. Declared chemical warfare and ammunitions can be classjfied by their elemental compositions directly specified by the measured capture prompt y-rays as irradiated with a thermal neutron source.

Figure 15 Schematic neutron capture reaction for chemical warfare and ammunition identification. (Reprinted with permission from IEEE Nucl. Sci. 39, Caffrey, A. J. et al., Chemical warfare agent and high explosive identification by spectroscopy of neutron-induced gamma rays, 1422, Copyright 1992, The Institute of Electrical and Electronics Engineers, Inc.)

REFERENCES

1

Cywicka-Jnkiel, T. and Loskiewicz, J., Correlation Methods in Calorific Value Measurements of Coal, in Proc. Advisory Grorrp Meeting on Gamma,X-Ray and Neutron Techniques for the Coal Industry, IAEA, Vienna, 1986, 191. Sowerby, B. D., Measurement of specific energy, ash and moisture in bulk coal samples by a combined neutron and gamma-ray method. Nucl. Insrr. Meth. 160, 173, 1979. Henog, W., Prompt gamma neutron activation analysis of hard con]. raw lignite and a raw glass mixture, Nucl. Geophys. 3, 467, 1989. Wilde, H. R. and Ncrz,og, \K, On-line analysis of coal by neutron induced gamma spectrometry, J. Radioanal. Nucl. Chem. article.^, 7 1 , 253, !982. Henenberg, C. L., Use of small accelerators in coal analysis and coal sluny flow measurements, IEEE Nucl. Sci. 26, 1568, 1979. Wormald, M. R., A bulk materials analyzer using pair and Compton-suppressed gamma-ray spectrometry, Nucl. Geophys. 3, 461, 1989. Wormald, M. R., Pair spectrometer NaI(T1) array for neutron-induced prompt gamma-ray analysis. Nucl. Geophys. 3, 373, 1989. Sowerby, B. D., On-line nucleor techniques in the coal industry, ~Vucl.Geophys. 5, 491. 1991. Surman, P. L., Nuclear Techniques for the On-line Analysis of Coal in Electricity Generating Stations, in Proc. Advisory Group Meeting on Gamma. X-Rny and Neiitron Techniques for the Coal Industry, 1AEA. Vienna, 1986, 181. Marshall, J. H., 111 and Zumherge, J. F., On-line measurements of bulk coal using prompt gamma neutron activation analysis, Nucl. Geophys. 3, 445, 1989. Mcquaid, J. H., Brown, D. R., Gozani, T., and Bozorgmnnesh, H., High rate spectroscopy for on-line nuclear coal analyzer, IEEE Nucl. Sci. 28. 304, 1981. Gozani, T., Advances i n bu!k t:lemental analysis u s q neutroo interactions, Nucl. Geophys. 2, 163, 1988. Yates, S. W., filo, A. J., Chcn~,C. Y., and Coopc, D. E, Elemental analysis by gamma-my detection following inelastic neutron scnttering. J. R~dioarml.Nucl. CIlcin. Arficlcs. 46, 343, 1978.

14. Jiggins, A. H. and Habbani, F. I., Prompt gamma-ray analysis using 3.29 MeV neutron inelastic scattering, Int. J. Appl. Radiat. Isot. 27, 689, 1976. 15. Sowerby, B. D., Elemental analysis by neutron inelastic scatter gamma rays with a radioisotope neutron source, Nucl. Instr: Meth. 166, 571, 1979. 16. Cunningham, J. B., Sowerby, B. D., Rafter, P. T., nnd Greenwood-Smith, K., Bulk analysis of sulphur, lead, zinc and iron in lead sinter feed using neutron inelastic scatter y-rays, Int. J. Appl. Hadiat, h o t . 35, 635, 1984. 17. Liu, Y., Lu, Y., Xie, Y., Wang, Y., Du, Y., Tan, J., Uonian, M., and Seymour, R. S., Development and applications of an on-line thermal neutron prompt-gamma element analysis system, J. Radioanal. A1ucl. Chern. Articles, 151, 83, 1991. 18. Shirakawa, Y., A feasibility study to determine the carbon and oxygen contents of steel using fast neutron techniques, Nucl. Geophys. 5, 5 19, 1991. 19. Duffey, D., Balogna, J. P., and Wiggins, ?. F., Analysis of geothermal power plant water using gammarays from capture of californium-252 neutrons, Nucl. Technol. 27, 488, 1975. 20. Grodzins, L., Nuclear techniques for finding chemical explosives in airport luggage, Nucl. Instl: Merti. B56/ 57, 829, 1991. 21. Lee, C. J., Chao, J. H., and Chung, C., High-energy gamma-ray spectrometer using bismuth germanate detectors, Appl. Radial. Isot. 42, 547, 1991. 22. Wang, H. and Waana, C. M., IVNAA investigation of factors affecting the background in the measurement of nitrogen, J. Radioanal. Nucl. Chem. Articles, 151, 293, 1991. 23. Shea, P., Gozani, T., and Bozorgmanesh, H., A TNA explosives-detection system in airline baggage, Nucl. Instr. Meth. A299, 444, 1990. 24. Chung, C., Liu, S. M., Chao, J. H., and Chan, C. C., Feasibility study of explosive detection for airport security using neutron source, Appl. Radiar. Isot., 44, 1425, 1993. 25. Sawa, Z. P. and Gozani, T., PFNA technique for the detection of explosives, in Proc. 1st Inf. Synip. on Explosive Detection Technology, Virginia, 1992, 82. 26. Caffrey, A. .I., Cole, J. D., Gehrek, R. J., and Greenwood, R. C., Chemical warfare agent and high explosive identification by spectroscopy of neutron-induced gamma rays, IEEE Nucl. Sci. 39, 1422, 1992.

Appendix I

Thermal Neutron Capture Gamma-Rays

The energy and photon intensity of y-rays as seen in thennal-neutron capture are presented in two tables, one in as ending order of y energy and second organized by 2, A. Only those y-rays with E(y) 2 500 ke and intensity of 2 5 % of the strongest transition are included. In cases where there are more-than-one transitions for a nucleus within 0.1 keV of energy difference, the strongest one is kept. In the energy-ordered table, the three strongest transitions, if their intensities are 2 5 % of the strongest, are indicated in each case. Where the nuclide mass number is not indicated, the natural target was used. The nuclide given is the residual nucleus in the capture reaction. The y energies given ate in keV. The y intensities given are relative to 100 for the strongest transition. All data for A > 44 are talken from Evaluated Nuclear Str~rctureData File (8/91), a computer file of evaluated nuclear structure data maintained by the National Nuclear Data Center, Brookhaven National Laboratory, on behalf of' :he Nuclear Stnlcture and Decay Data network, coordinated by the International Atomic Energy Agency, Vienna. These data are published in Nuclear Data Sheets, Academic Press, San Diego, CA. The data for A 5 4 4 is taken from "Prompt Gamma Rays from Thermal-Neutrov Capture," M. A. Lone, R. A. Leavitt, D. A. Hamson, Atomic Data and Nuclear Data Tables 26, 51 1 (1% 1). This research was supported by the Office of Basic Energy Sciences, U.S. Department of Energy.

d

178 Table 1 Capture Gammas-Ordered

by Nucleus

Table 1 Capture Gammas-Ordered

by Nucleus (continued)

7282.2[ 8380.7

"I.

Table 1 Caprure Gammas-Ordered

by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by Nucleus (continued)

"Yo

.'Yo 844.0 722.7 787.4 1023.0

1230.2 1432.3 6624.0

1

.'Yo

0.3

30.6 100.0 7.7

15.8 7.0 16.1

I

Table 1 Capture Gammas-Ordered by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by Nucleus (continued)

185

Table 1 Capture Gammas-Ordered by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by Olucleus (continued)

--

Table 1 Capture Gammas-Ordered

7.0 18.9 31.1 7.0

13.1 7.8 11.3

6.2 ti. 7 7.1 5.2 8. 1 3.7

le.9

0.1 8.1 13.1 7.0

20.3 7.0 5.5 5.7 5.2 0.2

5.5 5.7 1.6

12.7 13.2 8.3 5.6 9.0 8.8 8.4 5.6 5.5 7.4

8.3 7.4

8.3 5.1 8.3 7.4 5.2

8.2 14.9 7.0 1.1 7.3 0.8 7.1 18.1 3.7

7.8 7.1

5. 5 13.1 18.0 6.0 22.1

74.a 32.3

by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by Nucleus (continued)

Table 1 Capture Gammas-Ordered

by ~ucleus'(continoed)

Table 2

Capture Gammas-Energy

$3

M u c l .ua (Strongoat

Sorted

3 y'.)

Huc 1 o u a (Stroagoat 3 y e a )

'

,I

Table 2 Capture Gammas-Energy

Sorted (continued)

"'Yb

181,2.720.0.635.4

" * T s ~ ~ o ~ . 149.7.144.51 s.

Table 2 Capture Gammas-Energy Sorted (continued) # u c 1 *us (Strongest 3 7 . 8 )

Table 2 Capture Gammas-Energy

Sorted (continued)

Table 2 Capture Gammas-Energy

Sorted (continued)

195

Table 2 Capture Gammas-Energy Sorted (continued)

196 Table 2 Capture Gammas-Energy

Sorted (continued)

197 Table 2 Capture Gammas-Energy

Sorted (continued)

"'Yb 6614.7.804.7,776.4 1*1~m[167.8.62.1.1,147.6] 'aVf22.8.12S.1.8517-2)

Table 2 C a ~ t u r eGammas-Enerqv

Sorted (continued)

Table 2 Capture Gammas-Energy Sorted (continued)

sea.

8

970.2

200

Table 2 Capture Gammas-Energy Nuclaua ( S t r o n g a a t 3 y'a)

Sorted (continued)

Table 2 Capture Gammas-Energy

Sorted (a

Table 2 Capture Gammas-Energy

Sorted (continued)

Wuc 1 .us

(Strongeat 3 y'm)

Table 2 Capture Gammas-Energy

Sorted (continued)

204 Table 2 Capture Gammas-Energy N u c l bur ( S t r o n g e a t 3 7's)

Sorted (continued)

Table 2 Capture Gammas-Energy

Sorted (continued)

Table 2 Capture Gammas-Energy Sorted --- (continued)

Table 2 Capture Gammas-Energy

Lorted (continued)

208 Table 2 Capture Gammas-Energy N u c 1* u s ( S t r o n g ~ s t 3 1's)

Sorted (continued) Nua l - ~ u a (Strongeat

kr 3 1'0)

Table 2 Capture Gammas-Energy Sorted (continued) It?) (l-V)

(xl(Y))

Nuc l bus (slroogont 3 y 8 r )

Table 2

Capture Gammas-Energy

Sorted (continued)

Nucl eua ( S l r o n ~ m a l3 y'r)

"'RU

""Nd a''Th

1

174.0.130.1.284.4)

852.8.4838.Q.708.2 3473.0.586.8.472.31

Table 2 Capture Gamm~ts-Energ y Sorted (continued)

212

Table 2 Capture Gammas-Energy Soried (continued)

213

Table 2 Capture Gammas-Energy Nuclmur (Strontast 3 ? ' a )

Sorted (continued)

Table 2 Capture Gammas-Energy

Sorted (continued)

-

--

--- -

-

.

2.:.

Huolaus (ILrorgemL 3 7 ' 8 )

I " ' I "Kr

I

4573.8.l181.6,012.6 1475.9.3382.6.531.9

84Nb 5104.2.5491.0,6832.4) 21.1

18.2 LOO. 0

Table 2 Capture Gammas-Energy Nut l e u 8 ( S t r o n g o ~ t 3 7's)

Sorted (continued)

216 Table 2

Capture Gammas-Energy

Sorted (continued)

Table 2 Capture Gammas-Energy Nuolbua (Strongest 3

7's)

Sorted (continued)

21 8

Table 2 Capture Gammas-Energy

Sorted (continued)

Table 2 Capture Gammas-Energy Nuc 1our (strongom1 3 y'.)

Sorted (continued) Nuoleur (Srroagemt

3

7 . 8 )

220 Table 2

Capture Gammas-Energy

Sorted (continued)

Table 2 Capture Gammas-Energy N u c lbur (Strongest 3 7 ' 8 )

Sorted (continued) Nuc 1 r u t (3tro.go.t

3 7's)

"Po 810.8.883.6.1674.2) *eZn~7883.b,?Jf. 0 , 7 0 6 0 . 0 '7"Hf1125.7.5842.2.670.0

Table 2 Capture Gammas-Energy Nuolous (Strangont

3 7'8)

Sorted (continued)

Table 2

Capture Gammas-Energy

Sorted (continued)

Appendix /I

Other Radiation-Related Properties of Prompt Gamma Activation Facility Chien Chung CONTENTS

I. Introduction ..................................................................................... :.-........................................ 11. Construction Materials for PGAA Facility .............................................................................

III. Residual Radioactivity around PCAA Set-up ......................................................................... IV. Radiation Safety for the Operation of PGAA Facility ............................................................ References ...........................................................................................................................................

Although neutron behavior and prompt y-ray characteristics are certainly of major importance in the design work of a PGAA facility, the choice of satisfactory materials of construction is essential for a safe and practical PGAA set-up. Even if the structure materials with suitable physical and mechanical properties are available, they must have low neutron cross-section to avoid the interference of prompt y-ray measurement emitted from the irradiated sample. In the first section of this Appendix, emphasis will be placed on materials and material specifications that are unique to the construction and assembly of a PGAA facility. One of the major interferences for the PGAA measurements is the residual radioactivity induced by neutrons impinging on construction materials, neutronlgamma shields, prompt y-ray spectrometric unit, and the target sample as well. Activated short-lived radionuclei may obscure the identification of weak prompt y-rays in the spectrum; while longer-lived radionuclei may saturate the y-ray detector and disable it as a spectrometer. In the second section of this Appendix, the nuclear decay properties of frequently encountered radionuclei in PGAA measurement are tabulated. In PGAA measurement, scientists and researchers as well as the patients in in vivo medical scan are subject to radiation exposure. The radiation exposure may arise from neutron and y-sources external to the human body: from radionuc!ides that have entered the human body through inhalation and absorption through skin, or from the internally induced radioactivity by neutron irradiation. Although maximum safety efforts have been applied to the PGAA operation, scientists must be informed of the limit of radiation exposure and allowable intake in order to avoid excess exposure that may lead to radiation injury. In the last section of the Appendix, the allowable concentration of some radionuclei frequently encountered in PGAA operation as we!! as sensitive organs and tissues to radiation, recommended by regulatory authority, are listed. Unlike the conventional INAA, prompt y-ray activation measurements encounter unique radiationrelated problems although the PGAA operation has been widely recognized as an effective analytical method. Such problems include careful selection of construction material, radiation damage on the nuclear instruments, and radiation safety for both workers and irradiated patients. The Appendix provides necessary information and data for material selection, residual radioactivity evaluation, and radiation protection for both external and internal exposures. The values quoted in this Appendix may be used as references for preliminary calculation only and should not be used for design purposes; in the latter case, qualified engineers and licensed professionals should he consulted, in particular for the safety matter concerning the PGAA operation.

11. COMSTRUCYlON MABERIALS FOR PGAA FAClLITY Makrial requirements of PGAA sel-up can vary quite widely, in particular for the field applications such as in situ PGAA survey in a hostile geophysical environment, or in vivo PGAA diagnosis to which 0-8493-5 149-9/'?5/$0.00+5.50

O I995 by CRC Press, Inc.

I

the patient is irradiated by neutrons. In all c s e s , however, construction materials, neutron moderators and absorbers, and radiation shielding materials must be seiecred to meet appropriate requirements. The values and inforxation listed in the following tables, also to be found elsewhere,'-3 should b e ' k e d only for the preliminay design calculation.

Table 1 Properties of Construction Materials Used for PGAA Setup with Low Thermal Neutron Cross Sections --

--

-

-

-

Atomic number Element

(z)

Atomic weight (g/M)

Normal density (dcm3)

Thermal neutron cross section (barn)"

Beryllium Magnesium Zirconium Aluminum Niobium Yittrium Iron Molybdenum Chromium Copper Nickel

4(Bc) 12(Mg) 40(Zr) 13(A1) 4 l(Nb) 3907 26(Fe) 42(Mo) 24(Cr) 29(Cu) 28(Ni)

9.0 122 24.3 12 9 I -22 26.98 15 92.906 88.906 55.847 95.94 5 1.996 6334 58.7 1

1.85 1.74 6.50 2.70 8.57 5.5 1 7.87 10.20 7. I9 8.96 8.90

0.0092 0.063 0.185 0.230 1.15

a

1.28

2.55 2.65 3.10 3.79 4.43

Data taken from Reference 4.

Table 2 Austenitic Stainless Steel Used for PGAA Constructiona AlSi Type

Carbon, % (maximum)

Chromium, %

Nickel, %

Other Elements

" Data taken from Reference 5.

Table 3 Composition by Weight of Various Concretes for PGAA Setupa Concrete (Weight percent) Element (2)

Iron

Barytes

Regular

Aluminum (AI) Barium (Ba) Calcium (Ca) Iron (Fe) Hydrogen (H) Oxygen (0) Sulphur (S) Silicon (Si) -

Densitv( g/crn3)

" Data taken from Reference 6.

3.5

-

4.5

--

2.35

Table 4

Fast Neutron Removal Cross Section for Neutron Moderator Used in PGAA Setupa Atomic or

Material (2 or form)

Cross sections

molecular weight (g/M)

Density

27

9 10.8 2 12

1.70

Aluminum (Al) Beryllium (Be) Boron (B) Deuterium (D) Graphite (C)

,

Macroscopic

2.70

1.3 1

1.84

1.07 0.97 0.92 0.81

0.079 0.132

-

I 55.85 207.19 16 91.2

Hydrogen (H) Iron (Fe) Lead (Pb) Oxygen (0) Zirconium (Zr)

Microscopic (barns)

(gJcm3)

1.OO 1.98 3.53 0.92 2.36

7.87 11.34

-

6.5

--

18 20

Water (HzO) Heavy water (D20) Concrete, Regular Concrete, Barytes Concrete, Iron

Be0 B4C Diphenyl (C6HJC6HJ) Hydrocarbon oil (CH2) ZrH2

-

1.00 1.10 2.35 3.50 4.50

25 55.25 154

3-0 2.2 0.96

93.2

5.61

-

.

2.8

-

-

0.56 5.1 2.9 . 2.8 2.35

-

(1/cm)

-

-

0.065

0.168 0.118

-

0.101 0.103 0.092 0.089 0.105 0.159 0.111 0.12 0.01 1

-

0.085

' Data taken from Reference 1

Table 5 Thermal Neutron Absorbers Used in PGAA Setupa Nuclide

Natural abundance

Li-6 B-10 Rh- 103 Cd-1 13 a

Data taken from Reference 7. Reaction products labeled with

7.5% 19.8% 100% 12.2%

"*"

are radioactive.

Thermal cross section (barn)

+

942 2 3838 -t 2 134 It 4 19800 2 200

Nuclear reactionb "Li(n,a)'T* 10B(n.~)7Li '03Rh(n.r)'MRh* '"Cd(n,r)"'Cd

Table 6 Mass Attenuation Coefficient (cm2/g)for y-Ray Shielding Calculationa y-ray energy, MeV -

Material

W Pb Air

Hz0 Concrete

Human tissue a

-

0.1

0.5

1.o

1.5

2

4

6

4.2 10 5.290 ,151 .I67 .I69 ,163

.I250 .I450 .0868 .0966 .0870 .0936

,0640 ,0684 .0636 .0706 .Of535 .0683

.0492 .05 12 .05 17 .0575 .0517 .0556

.0437 ,0457 ,0445 ,0493 .OM5 ,0478

,0402 ,0420 .0307 ,0339 .0317 .0329

,041 .043 .025 .027 .026 .026

Data taken from Reference 8.

Fast Neutron Removal Cross Section for Neutron Moderator Used in PGAA Setupa

Table 4

Atomic or nlofecular weight (g/M)

Material

(2or form) Aluminum (Al) Beryllium (Be) Boron (B) Deuterium (D) Graphite (C)

Cross sections

Density (Cl1cm3)

Microscopic (barns)

Macroscopic (1Icm)

27 9 10.8 2 12

2.70 1.84

1.31 1.07 0.97 0.92 0.8 1

0.079 0.132

25 55.25 154

3.0 2.2 0.96

1.70

-

0.065

Hydrogen (H). Iron (Fe) Lead (Pb) Oxygen (0) Zirconium (Zr) Water (HzO) Heavy water (D20) Concrete, Regular Concrete, Barytes Concrete, Iron Be0 R4C Diphenyl (C6H,C6H5) Hydrocarbon oil (CH2) ZrH2

-

-

93.2

5.61

0.56 5.1 2.9 2.8 2.35

' Data taken from Reference 1.

Table 5 Thermal ~ e u t r o n Absorbers Used in PGAA Setupa Nuclide

Natural abundance

Li-6 B-10 Rh- 103 Cd-1 13

" Data taken from Reference 7. Reaction products labeled with

7.5% 19.8% 100% 12.2%

"*"

are radioactive.

Thermal cross section (barn)

Nuclear reactionb

942 t 2 3838 2 2 134 rt 4 19800 I+_ 200

bLi(n,a)'T* '"B(n,a)7Li 10IRh(n,r)lMRh* 113Cd(n,r)"4Cd

Table 6 Mass Attenuation Coefficient (cm2/g)for y-Ray Shielding Calculationa .. Material

H Be C N 0 Na Mg A1 Si P

S

K Ca Fe Cu Mo

W Pb Air H20 Concrete Human tissue

y-rs-j energy, MeV

0.5

1.O

I.5

2

4

6

.295 ,132 ,149 ,150 .151

.I730 ,0773 .0870 .0869 .0870

,1260 ,0565 ,0636 ,0636 .0636

,1030 ,0459 .05 18 ,0517 .05 18

,0876 ,0394 .OM .0445 .0445

,0579 ,0266 ,0304 ,0306 .0309

,044 .02 .024 ,024 .025

,151 ,160 .I61 .I72 ,174 ,188

.0833 .0860 ,0840 ,0869 .0846 ,0874

,0608 ,0627 .0614 ,0635 .0617 ,0635

.IN96 .05 12 .0500 .0517 ,0502 .0519

,0427 .0442 .0432 ,047 ,0436 ,0448

.0303 .03 15 .03 10 .0323 ,0316 ,0328

,02 .026 .026 ,027 ,027 ,028

.215 .238 ,344 .427 1.030

,0852 .0876 ,0828 .0820 ,0851

.06 18 ,0634 .0595 .0585 ,0575

.0505 .05 18 ,0485 .0476 .0467

.0438 .0451 ,0424 .0418 .0414

.0327 ,0338 .0330 ,0330 ,0349

.028 .030 .030 .030 .034

4.210 5.290 ,151 .I67 .I69 .I63

.I250 .I450 ,0868 .0966 ,0870 .0936

.0640 .0684 ,0636 ,0706 .0635 .0683

.0492 .05 12 ,0517 ,0575 .05 17 .0556

.0437 ,0457 ,0445 .0493 ,0445 ,0478

,0402

.M .043 .025 ,027 .026 .026

0.1

' Data taken from Reference 8.

,0420 .0307 .0339 .03 17 .0329

Table 9 Decay Properties of Radionuclides Induced by (nth,r)Reactions with Half-Lives 1 h < T,, 5 1 day Cross sectlon Half-life Decay Natural Nuclear reaction

(barn)

(hours)

0.10 5 0.02 0.108 2 0.002 0.64 f- 0.05 1.46 0.03 13.3 0.2

+

15.030 2.62 1.827 12.361 2.578

t 0.03 t 0.2 2 0.2 t 0.06 t 0.02

2.520 12.699 14.12 1.380 11.3

+ 0.4

4.42 1.83 4.48 1.134 2.805

IT, r IT, r

9.104 2.914 1.38 9.0 19.2

P-, r IT, r P-, r EC, P+, r @-, EC, r

abundance

23Na(n.r)24Na 30Si(n,r)3'Si 'OAI-(~,~)~'A~ 41K(n,r)42K ssMn(n,r)S6Mn

100% 3.1% 99.6% 6.73% 100%

MNi(n,r)6-'Ni 63Cu(n,r)"Cu 7'Ga(n.r)72Ga "Ge(n~)~'Ge "jGe(r~,r)~Ge

0.91 % 69.2% 39.9% 36.5% 7.8%

1.49 4.4 4.6 0.36 0.06

79Br(n,r)80"Br s2Kr(n,r)83mKr "Kr(n,r)8SmKr 84Sr(n,r)85mSr 86Sr(n.r)nmSr

50.69% 11.6% 57.0% 0.56% 9.8%

2.4 20 0.09 0.59 0.84

+

2 4

t 0.01 2 0.06

t- 0.05

mode P-, r P-. r P-. r

P-.

P-. P-, P', P-. P-. P-.

T-

r r EC, r r r r

P-. n'. I.

IT, EC, r IT. EC, r

P-.

r EC, r IT, r P-, r EC, r PI,

P-. r EC, P+, r EC, r P-. r 13-. r 3.684 5.5 23.85 16.98 13.10

P-. r IT. r P-. r P-. r IT, r

19.15 18.3 1.57 23.8 3.253

P-.

--

1931r(n,r)'9JIr '~Pt(n,r)"Tt '%Pt(n,r)'gmPt '%Hg(n,r)'"'"Hg 208Pb(n,r)2wPb

62.'7%

25.3% 25.3% 0.15% 52.3%

110 5 30 0.7 t 0.1 0.05 t 0.02 120 t 20 5.0 -1 0.1, x lo-4

r P-, r

IT, P-. r IT, EC, r

P-, r

Note: Decay properties are taken from Refep.xce 7. Decay mode: P-, beta; p+,positron; EC, electron capture; IT, isomeric transition: r, gamma transition.

Table 10 Decay Properties of Radionuciides Induced by 1 day < T,,, 11 month

Nuclear

Natural

reaction

abundance

31P(n,r)32P '6Ca(n,r)47Ca MCr(n,r)5'Cr 70Ge(n,r)7'Ge 7sAs(n,r)76As

100% 0.0035% 4.35% 20.5 %

100%

Cross sectlon (barn) 0.18 0.7 15.9 3.25 4.4

2 0.02 2 0.2

+ 0.4

2 0.13 2 0.2

(n,,,r) Reactions with Half-Lives Half-life (days) 14.282 4.540 1 27.701 11.15 1.097 1.4727 1A60 8.82 2.669 2.75 1 2.88 16.96 2.224 14.0 1.128

Decay mode

PP-, r EC, r EC

P-. P-.

r

r

P-, r P-. EC. r P-. r P-. r

EC.

EC, r

EC, r

P-.

r

IT, r

I3-

P-. P2.

r

P-.

m,

EC. r EC, r

IT, r IT, r

P-.

r

IT, r EC, r IT, EC, r 1.678 1.43 1.38 10.98 1.95

P-.

I.

IT, EC, r P-. r P-. r P-. r

P-. r P-. r

15.4 1.28 3.00 4.33 4.020

EC, r IT, r IT, r

2.697 2.672 5.013

EC, r P-. a,

Note: Decay properties are taken from Reference 7. Decay mode: a, alpha; @-, beta; capture; IT, isomeric transition; r; gamma transition.

P-. r

P+, positron; EC, electro

Table 9 Decay Properties of Radionuclides Induced by (n,,,r) Reactions with Half-Lives 1 h < TI,* 5 1 day Nuclear Natural Cross section Half-life Decay reactlon abundance (barn) (hours) mode

IT, r IT, r

P-, ITI r IT, EC, r IT, EC. r

'"Xe(n,r)'35Xe '33Cs(n,r)'"mCs '38Ba(n.r)'39Ba "Te(n~)'~~Ce 141Pr(n,r)142Pr

10.4% 100% 7 1.7% 0.19% 100%

0.25 2.5 0.4 6 7.6

t 0.03 t 0.4 0.1 2 1 t 0.7

9.104 2.914 1.38 9.0 19.2

'48Nd(n,r)'"9Nd '51Eu(n,r)'sZ"'Eu 's'E~(n,r)'S2m%~ 15BGd(n.r)'S9Gd '56Dy(n,r)'57Dy

5.7% 47.9% 47.9% 24.8% 0.057%

2.5 0.5 3200 200 4 2 1 2.4 Ifi 0.4 33 2 3

1.73 9.30 1.6 18.56 8.06

'64Dy(n,r)'6SDy '62Er(n,r)'63Er '64Er(n,r)'65Er '70Er(n,r)'7'Er 176Yb(n,r)'nYb

28.1% 0.14% 1.56% 14.9% 12.6%

900 19 13 5.7 2.4

+

"+

+ 300

2.334 1.25 10.34 7.52 1.88

Ifi 2 2 2 ? 0.2

t: 0.2

3.684 5.5 23.85 16.98 13.10 'q31r(n,r)'941r '"Pt(n,r)IqPt '"Pt(n,r)'"'"Pt IWHg(n,r)'97mHg 208Pb(n,r)209Pb

62.7% 25.3 % 25.3% 0.15% 52.3%

110 0.7 0.05 120

? 30

t 0.1 +- 0.02 ? 20

5.0 2 0.1, x ~ o - ~

Nore: Decay properties are taken from Reference 7. Decay mode: isomeric transition: r, gamma transition.

19.15 18.3 1.57 23.8 3.253

P-,

r

IT, r

P-. EC,

r

P+,

r

P-.

EC, r

P-,

r

flz, EC, r IT, r

P-:

r

EC, r

P-.

r

EC, P+. r EC, r

v. r B-, P-.

r r

IT, r

PA. r P-. r IT, r

P-. fi-,

r

IT, P-, r I?: EC, r

P-.

I .

P-,bera: P*, positron: EC, electron capture; IT,

Table 10 D e c q Properties of 1 day < TIl2 5 7 month

Radionuclides Induced by (n,,r) Reactions with Half-lives .4

Nuclear reaction

Natural abundance

Cross section (barn)

Half-life (days) 14.282 4.540 1 27.70 1 11.15 1.097

%e(~~,r)'~'*Xe '32Xe(n,r)133Xe

4.1% 26.9%

+

0.4 0.2 0.4 i: 0.1

100% 0.15% 100%

98.8 2 0.3 3000 t 200 3-01? It 0.002

Note: Decay properties are taken from Reference 7 . Decay mode: a, alpha; capture; IT, isomeric transition; r, gamma transition.

mode

P-

P-. r EC, r,.. EC B-. r

2.88 16.96 2.224 14.0 1.128

EC, r EC, r P-. r IT, r

9.625 2.68 1 16.78 1.25

P-. r P*, EC. r

0-

EC, r

P-,IT, r

8.89

IT. r

11.770 5.245 2.19 12.0

IT, r [3-, r IT,r EC.. r

1.43 1.38 10.98 1.95

IT,EC, r P-. r

1.118 9.40 4.19 6.7 1 3.777

P-, P-, P-. P-,

15.4 1.28 3.00 4.33 4.020

1*Au(n.r)'98A~ '%Hg(n~)'~~Hg 209B i(n.r)Z'oBi

Decay

2.697 2.672 5.0 13

.

.

P-, r B-. r r r r r

B-. EC. r

P-. r

P-.

r EC, r IT,r IT. r

P". r EC, r P-,

a. r

P-, beta; P+, positron; EC, electron

Table 13 Risk Factor and Weighing Factor for Human Tissues and Organs Sensitive to Ionization Radiations ICRP 26 Publication (1977) Risk

Tissue or organ

factor

Weighing factor

ICRP 60 Publication (1991)

Risk

factor

Weighing factor

Gonads Red bone marrow Colon Lung Stomach Bladder Breast Liver Esophagus Thyroid Skin Bone surface Remainder Total Note: Although the factors released in the recent publication (ICRP-60) are more conservative, the factors published in 1977 (ICRP-26) are still adopted by many authorities.

' Remaining organs include liver, kidneys, heart, intestine, and stomach. Remaining organs include adrenals, brain, small intestine, upper large intestine, kidneys, muscle, pancreas, spleen, thymus, and uterus.

REFERENCES 1. Lamarsh, J. R., Introducrion to Nmdear Engineering, 2nd ed., Addison-Wesley, Menlo Park., CA. 1983. 2. Cember, H., Inrrod~rctionto Healfh Physics, 2nd ed., Pergamon Press, New York. 1983. 3. Attix, E H., Introduction to Radiological Physics and Radiation Dosimetry, John Wiley & Sons, New York, 1986. 4. Garber, D. I. and Kinsey, R. R, Neutron Cross Section. Vols. I & II.3rd ed., BNL-325, Brookhaven National Laboratory, New York, 1976. 5. A. S. T. M. Special Publication 276, Materials in Nuclear Applications, ASTM, Washington D.C., 1960. 6. American Concrete Institute, Concrete for Radiation Shielding, 2nd ed., ACI, New York, 1962. 7. Lederer, C. M. and Shirley, V., Tclhle of Isotopes, 7th. ed., John Wiley & Sons, New York, 1978. 8. Templin, L. T., Reactor Physics Cotistam, Report ANL-5800, 2nd ed.. ANL, Chicago, 1963. 9. International Commission on Radiological Protection, Limits for Intakes of Radionuclides by Workers, ICRP publication 30, Parts 1, 2, 3, with associated Supplements and Addendum. Annals of ICRP 2-8, 19, Pergamon Press. Oxford, 1979-1988. 10. International Commission on Hndiological Protection, Recornmendation of the ICRe ICRP Publication 26, Annals of ICRP 1(3), Pergamon Press, Oxford, 1977. 1 1. International Commission on Radiological Protection, Age-Dependent Doses to Members of the Public from Intake of Radionuclides, ICRP Publication 56, Part 1. Annals of the ICRP 20(2), Pergamon Press, Oxford, 1989. 12. International Commission on Radiological Protection, 1990 Recommendations of the ICRP, ICRP Publication 60, Annals of the ICRP 21(1-3). Pergamon Press, Oxford, 1991. 13. International Commission on Radiological Protection, Annrral Lbnifson Intake of Radionuclides by Workers Based on the 1990 Recommentlations, ICRP Publication 61, Pergamon Press, Oxford. 1992.

INDEX A Absolute counting efficiency, 47, 49, 50 Absolute thermal flux. 54 ADC, see Analog-to-digital converter Airport security inspection of luggage. 24 OLPGAA facility used for, 34 Aluminum measuring of by PGNAA, 66 process control. 169 Alzheimer's disease, brain tissues from, 70 Analog-to-digital converter (ADC), 5. I I , 14,25, 133 Anticoincidence, 62 Anti-Compton annuli. 13 coincidence system, 35 mode, background reduction in. 24 shield, 85 spectrometer. 19, 39 Attenuation coefficient. 62

Backscattering. 26 gauge, 161 system, 25 Beam catcher, 31 heterogeneity. 96 Beryllium oxide, use of for PGAA facility. 29 BGO, see Bismuth gemanate Bilateral detector arrays. 173 Bilateral irradiation, 78 Biological shield. 3 1 Bismuth collimator, 80, 107 germanate (BGO). 14.15, 106 germanate detector, 17, 19, 24.43.47, 142 energy resolution of, 134 resolution of, 170 use of for PGAA facility, 29 Blood, measurement of trace clement content in, 70 BNCT, see Boron neutron capture therapy Body composition. 101 nitrogen determination of, 117 measurement. 118 Bone disease, 104

surface dose rate equivalents for, 126

sensitivity of to radiation, 119 Borehole measurements, neutron flux perturbation in, 140 probe, 136 Boron -doped filter, 34 determination of by PGNAA. 65 measurement of by NaI(T1) detector, 66 rneasurement of in plant tissues, 66 neutron capture therapy (BNCT), 68 use of for PGAA facility, 29 Borosilicate glasses, measurement of boron in, 69 Rrain mercury concentration. 115, 1 16

Cadaver studies. 89 Cadmium, 28 concentration, detection limit of. 113 measurement of by NaI(T1) detector, 66 ratio (CR),39, 71, 95 shutter. 59 use of for PGAA facility, 29 Calcium. mensuring of by PGNAA, 66 Calibration spectra, 86 Californium neutron source, 32 Carbon, determination of. 80, 118 Carbodoxygen logging, 137 Cation exchange capacity (CEC), 138, 139 CEC, see Cation exchange capacity 252Cf source, disadvantage of, 75 Chemical analysis, nuclear techniques of, 1 Chemical warfare, 175 Chlorine, 25 capture y-rays. 137 measmement of body. 118 measurement of in plant tissues, 66 prompt y-ray spectra, ?7 signal. 8 1 Clay minerals, 138 Clinical diagnosis, 101 Coal mdysis. 160 ash delineation, 140 in sitir analysis of. 85 rnensurement of sulphur content in, 143 neutron capture reaction with hydrogen in, 162 quality, elements influencing, 161 as source of energy. 159 Coaxid detectors, 45 Cold neutron(s), 61, 97 sensitivity with, 86 source, 95

Collimator(s), 26, 105, 110 lithium-dopd polycstcr, 89 measurement of Ca with, 84 at NIST. 63 Comparator. 110 Compton continuum background of, 19 reduction of, 13 Compton scattering, 8-10. 29, 77 Compton suppression, 1 1, 20, 21, 69 factor, 19 mode, 62 spectm, 22.23 spectrometer, 35 Constant fraction discriminator. 12 Construction materials choice of satisfactory, 225 y-ray background from, 39 properties of, 226 Converging guides, 98 Converted dose qoivalents, 121 Copper, use of for PGAA facility, 29 Counting efficiency, 20, 173 Counting statistics. 35 CR, see cadmium ratio Critical kidney burden level, 127 Cyclotron, fast neutrons from. 109

Deionized water, use of for PGAA facility, 29 Delayed analysis. 2 Delayed Gamma Neutron Activation Analysis (DGNAA), 59.79 Detection limits, 83 Detection sensitivity, 170 Detection systems. 10-12 anticoincidence Compton suppression. 10 coincidence double-escape counting, 11-1 2 Detector(s) crystal, 150 deterioration, 30 efficiency, 110. 146 -phnntom nxis, 39 photopeak energy resolution for. 15 radiation protection, 43 sensitivity, inadequate. 87 shield, 13, 19, 78 susceptibility of to damage, 62 DGNAA, see Delayed Gamma Neutron Activation Analysis Dialysis therapy, 118 Dje away measurements, 150 Diphenyl, use of for PGAA facility, 29 Dose rate equivalents, 122, 123 Double-escape peak, 9, 11.30, 68, 77, 85 Drinking water. 150

E ELAN, see Elemental analyzer Electrode polarity, 46 EIectron, binding energy of, 7 Elemental analyzer (ELAN), 164, 165 Elemental concentration, absolute measurement of. 110 Elements, detection limit of essential, I19 Energy nonlinearity, 164 resolution, post-irradiation recovery of, 49 Environ~nenlalprotection codes, 139 Epithermal neutrons, 3 Ethylene glycol, 86 Explosive detecting assembly, 35.42 Explosive Detection System. 174 Explosives. use of BGO detector for detecting, 170 Extrinsic detectors. 7

Fast analog signals, transmission of, 85 Fast neutron@),3, 32 bombarding, 46 energy, 28 fluence, 34.41.49 flux, 53.54 moderator. 29 removal, 54,227 Fat-free body mass, increased hydration of, 80 Fat-free mass (FFM). 8 1 F e d age theory, 76 FFM, see Fat-free mass Field Effect Transistor preamplifier, 6 Fluorocarbon refrigerants. 135 Flux density, 96 intensity. 54 monitor. 112 Foil activation technique, 39.42 Food poisoning. 115 Formation analysis, elements important to, 139 Free electron, 8 Full width at half rnaxianutn (FWHM), 6, 7, 25, 47

Gadolinium orthosilicate detector, 134 Gamma detectors, 5-7 scintillation detectors Na(T1) and BGO, 5-6 solid-state ionization detector, 6-7 Gas turbine engine. 97 Gaussian spreading, 78, 172 Genetic studies, 68 Geological formations, measurements of. 88 Geothermal waters, on-line analysis of, 170

Gemium detector, 1l,44 y-ray spectrum, 48.51 operating temperature of, 151 isotopes. 39, 45 purification technology, 7 recoil energy of. 45, 52 Gold, use of for PGAA facility, 29 Graphite collimator. 62 reflectors. 63 use of fof PGAA facility. 29 y-rayw attenuator. 29 capture of, 93 count rate. 52 detector, 105 spectrometry, 7.26 y-spectrum, shape of, 7-10

Heart disease, 104 dose rate equivalents evaluated in. 125 Heavy water, use of for PGAA facility, 29 High-flux reactor. 61 High-purity germanium (HPGe) detectors, 15 HPGe detectors, see High-pnrity germanium detectors Hydrocarbon contamination, 96 Hydrogen, liquid, 95 measurement of in plant tissues. 66

ICP-MS. 59 ICT. see Insulating core transformer INAA, see Instrumental methods of neutron activation analysis In-beam measurements, 56 Indium foils, 41 use of for PGAA facility. 29 Industrial workers, 127 Inelastic scattering. 79 interactions, 43 reaction. 89 In situ applications, 131-157 . coal mine in situ analysis. 139-145 coal ash delineation using scintillation detectors, 140-142 multielemental analysis using high-resolution detectors, 142-145 environmental water body in situ survey, 145-150 field survey using WPGAA probe, 148-149 performance test, 146-14,8

radiation safety concerns, 149-150 ISPGAA instrumentation, 131-235 detectors, 134-1 345 ISPGAA probe, 131-132 neutron sources, 132-134 oil formation in situ analysis. 135-139 bulk properties of rock formation, 137-138 multielemental analysis. 138-139 planetary in situ exploration, 150-152 seabed mineral in situ survey. 153-154 In situ PGAA (ISPGAA). 23. 25, 131. 135. 145 Instrumental methods of neutron nctivntion analysis (INAA), 102 Instnimentd photon activation analysis (IPAA), 102 Instruments, shielding and, 13-36 advanced instrumentation, 17-25 anti-Compton and pair spectrometers, 19-23 field instruments and spectroscopy. 23-25 prompt y-ray spectroscopy, 14-17 scintillation detectors, 16-17 semiconducting detectors, 15 shielding of PGAA facility, 25-35 biological and detector shields, 29-35 requirements for shielding, 26-29 Insulating core transformer (ICT). 4 Internal radiation. 49 Intestinal rare earth marker, 69 Intestine, dose rate equivalents evaluated in. 125 In vivo PGAAA (IVPGAA), 23 facilities comparison of capability of operational, 109 detector shields for. 33 . radiation shield design for, 31-32 index, 1 10 measurement, 30. 124 neutron sources used for, 103 station, 34 Ionization radiations, risk factors for human tissues sensitive to. 235 IPAA, see lnstrumentd photon activation analysis Iron. use of for PGAA facility. 29 Irradiation port, 83, 107 Isoflux contour curves, 41 Isotope capturing. 96 radioactive, 153 ISPGAA, see In situ PGAA Itd-itai disease. 104 IVPGAA, see In vivo PGAAA

Kidney cadrnirlni, prompt y-my spectrum of. 1 13 concentrntion of cndmiurn in, 89, 112 detection limit of mercury in, 116 dose rate equivalents evaluated in, 125

dysfunction. 104 mercury, detection limit of, 115 lrcutron flux distribution in, 120 KUR, see Kyoto University Reactor Kyoto University Reactor (KUR), 68 .

Lake pollutant surveys. 149 LBM, see Lean body mass Lead collinmtors. 68 -photon interaction. X-rays induced by, 39 sinter samples, bulk analysis of, 166 use of for PGAA facility. 29 Leakage radiation, 172 Lean body mass (LBM). 79 Light Water Reactor (LWR), 3 Light work condition, 233 Limestone, pair spectrum of, 163 Liquid hydrogen, 95 Liquid phantom. 39 Lithium polyethylene shield, 82 use of for PGAA facility, 29 Liver concentration of cadmium in, 89, 112 detection limit of mercury in, 116 dose rate equivalents evaluated in. 125 dysfunction. 114 Living tissues simulator, prompt y-ray emission from, 69 Lucite effective atomic number of, 110 phantom, 123 Lunar rock sample, 67 Lunar surface, geometrical arrangements of neutron probe with mspect to, 151 Lung@) . contaminant. 116 disease, 104 dose rate equivalents for, 125, 126 LWR, see Light Water Reactor

Manganese, measuring of by PGNAA, 66 Marine exploration, 153 Mass attenuation coefficient, 228 Mathematical model, for transport of neutrons, 76 MCA, see Multichannel analyzer MDC, see Minimum detectable concentration Medical community, IVPGAA adopted by, 101 Medical diagnosis elements importance to. 104 IVPGAA, 233 modified THMER facility for IVPGAA, 108

Mercury amalgams, dentists exposed to, 83 measured brain content of. 76 toxicity. clinical symptoms of, 115 use of for PGAA facility, 29 Metabolic disturbance, 104 Mineral processing plants, 164 Minimum detectable concentration (MDC),146, 147, 153 Mobile reactor, 32 Monazite rock samples, 86 Monte Carlo code. 80 simulation, 87 techniques, for tracking neutron, 136 Multichannel analyzer (MCA), 3, 7, 10, 14, 25 Multichannel pulse height analyzer, 5 Multidetector array, 35 Multielement analysis, by neuron activation, 83 MURR, see University of Missouri Research Reactor

National Institute of Standards and Technology (NET), 93,97 Neutron absorption, 78, 95 activation analysis, 97 amplifiers, 5 beam, 32 direction. 38 guided cold, 93 guided, 96 modulation of collimated, 59 profile, 39 profiles, 30 shutter system. 108 bombardment accumulated, 41 induced radioactivity after, 43 capture, 102 geophysical profiles given by, 142 logs, 137 reactions, 32 cold, 97 collimation. 27 counter. 112 cyclotron-induced secondary, 115 damage, reduction of, 25 detector damage caused by fast. 132 dosimeter, 57 emission anisotropic, 53 rate, 50 energy spectrum, 50, 121, 122 field mixed, 52

overexposure in, 57 fluence accumulated, 37 data. 49 flux. 78 contour map, 41 delivery of to irradiated patient, 105 measurements, 39.40, 57 nonthermal. 39 of research nuclear reactors, 3 uncertainty of, 56 unifonnity, 85. 119 focusing, 98 generator, 4, 143. 102 guides. 61. 96. 98 -induced y-rays, 140 inelastic scatterings. 76 moderator, 86 monitoring device, 49 premoderation of, 106 refractive index for, 94 scattering. 43, 96 shield, 26. 34. 37, 42 source, 3-5,33 detector distance. 82 : isotopic, 102 neutrons from accelerators and neutron generators, 4 properties of, 4- radioactive neutron sources, 4-5 specificiation of, 131 types of, 132, 143 thermalization of, 87 transport code. 42 wavelength, 94 Neutron beams, ~ r o m p gamma t activation analysis with guided, 93-100 accuracy, 96-97 advantages of guided neutron beams 93 apparatus. 95 facilities for PGAA with guic: .3 beams, 97-98 production of guided beams. 97 results and applications, 96 trends, 98-99 Neutron damage, induced effects of :clt nr *, ments used and, 37-58 neutron damage on detectors, 43- :g neutron damage on scintillation detec s. 4 6 4 9 neutron damage on semiconducting cle! ors, 44-46 neutron flux distribution around instruments, 37-43 neutrons around PGAA field instnlments, 40-43 neutrons around reactor-based PGAA instn~ments, 38 4 0 neutron-induced effec~son detectors, '19-56 ~

using an HPGe detector as fast neutron monitor. 50-54

using an HPGe detector as thermal neutron monitor. 54-56 NIST, see National Institute of Standards and Technology Nitrogen amount of inside body. 89 liquid, 135 measurement, 84 Nonthermal neutrons, slowing down of, 28 Nuclear interaction, rate of, 1 Nuclear reactor, y-photons with, 59 Nuclide mass number. 177 production of activated. 149 Nutrition, elements related to human balance of. 102 Nutritional disturbance. 104

Ocean-bottom deposits, analysis of, 87 Ocean Drilling Program, 154 Oil reservoir, 137 OLPGAA, see On-line PGAA On-line applications, 159-176 explosive on-line interrogation, 170-175 chemical warfare and ammunition identification, 175 current OLPGAA system for explosive detection, 172-1 73 detection of explosives by OLPGAA. 170-172 OLPGAA using fast neutrons, 174 mineral on-line process control, 164-170 geothermal power utilities. 170 metal industry, 166-170 on-line analysis of coal, 159-164 commercial OLPGAA systems, 164 development of OLPGAA techniques, 160-164 importance of on-line analysis of coal, 159-160 OLPGAA coal analysis, 160 On-line measurement assembly. 169 On-line PGAA (OLPGAA), 23.25, 159 facility, layout of. 34 technique, development of thermal neutron, 169 Organic materials, contamination with. 67 Osteoporosis, 1 14

PA, see Preamplifier Pair production, 9 spectrometer, 21, 162 spectroscopy, detection system for. 1 1 Palladium, 4 Peak-to-Compton ratio, 20 Performance degradation, 47

PET scan, 125 PGAA, sce Prompt gamma activation analysis PGNAA, sce Prompt gaxnrna neutron activation analysis Phmtom distribution of thennal neutron flux in, 120 female-like lucite, 110. 111 liquid. 39 Iucite, 123 man-like liquid, 110. 1 11 Phosphor, 5 Photo sources, monoenergetic, 48 Photomultiplier, 86 Photomultiplier tube (PMT), 14, 15, 43, 48, 107 Photopeak. 9. 11 area, quantitative determination of, 49 channel, drift of, 46 counts, 115 energy. 19.46 identification of, 13 Pile-up stripping. 166 Plastic scintillators, 13 Platinum shield. 86 PMT, see Photomultiplier tube Pole-zero cancellation filter, 106 Polyethylene borated, 164 detector shielded by baiated, 86 matrix blocks. 31 Pork meat, concentration of nitrogen in. 117 Potassium, measuring of bli PGNAA. 66 Preamplifier (PA), 14. 16 Prompt gamma activation analysis (PGAA), 93 disadvantages in using, 57 experiment, effect of Compton suppression in, 20 facilities neutrons emitted from source of. 28 reactor based, 13, 29 suitability of. 35 field applications, detectors used for, 43 measurement y-ray spectrometer in. 57 major interferences for, 225 systems. detection sensitivity of. 25 Prompt gamma activation andysis, in vivo, 101-130 clinical applications. 112-1 19 partial-body scan, 112-1 16 whole-body scan, 116-1 19 IVPGAA facility, 102-1 12 gmeraI layout, 103-1 10 phantom calibration, 110-1 12 radiation doses to patients. 119-127 doses for partial-body scan. 122-124 doses for whole-body scan, 125-127 neutron flux distribuson in body, 119-122 Prompt gamma activation fxility, other radiation-related properties of. 225-235

construction materials for PGAA facility, 225-228 radiation safety for operation of PGAA facility. 233-235 residual radioactivity around PGAA set-up. 229-233 Prompt gamma neutron activation analysis (PGNAA), 2, 3, 9, 59 facility, detection system of, 64 disadvantage of, 59 y-ray detection systems used for, 10 Prompt photopeak, 45, 52 Propane, liquid, 135 Protein depletion, 80 Pulse height spectrum, 45 analysis, 106 Pulses. categories of. 43 Pyrolytic graphite crystals, 98

Radiation damage, principal consequence of, 45 detectors. 37 exposure prime concerns of, 233 scientists subject to, 225 safety, 27. 103, 107. 172 skin doses, 124 tissues sensitive to, 119 Radiative capture, 2, 88, 112 Xadioactive decay, measurement of, 6 Kadioactivity, induced, 30 Radioisotope sources, PGNAA with neutron generators, charged particle accelerators. and, 75-92 accelerators. 89 14-MeV neutron generators, 87-89 radionuclide sources, 75-87 UIArn-Be sources, 75-77 252Cfsources, 82-87 Pu-Be sources, 77-82 Radionuclide(s). 60 decay properties c ; 229-233 derived air c: -.'ation of submerged, 234 disintegration : . , 60 encou%e,r:l i i i PGAA operation, 225 pr!'jtexxies of, 44 qtia:?a:ive identification of induced, 51 sourc:.. 'f.5 Random coinc ,.!ence counting, 78. 172 Rare earth eleilients, 29, 66 Raw glass mixtures, 84 Reaction core, 30, 32, 39 y-rays, suppression of, 62 high-flux, 6 1 power level, 108

probability. 1 thermal neutrons from. 60 Reactor neutrons, prompt gamma neutron activation analysis with, 59-73 practical applications. 65-71 techniques with PGNAA with nuclear reactors, 59-65 detection. 62-65 filters and collimators, 62 Red bone marrow dose rate equivalents for, 126 sensitivity of to radiation, 119 Reflection, increased neutron flux by, 83 Relative neutron flux. 54 Renal failure, disorder of phosphoms in patients with, 1 1 8 Renal tabular damage, 114 Research reactor, 102 Rhodium, use of for PGAA facility. 29

Salinity determination. 147, 148 Samarium. 69 Sample-to-detector distance, 39, 57 SAMPO, 20 Scattering inelastic, 88 neutron absorber, 31 peaks of inelastic, 87 Science Applications International Corporation, 174 Scintillation detector. 6, 49 Seawater, measurement of manganese nodules in, 86 Selective pulse integration, 78 Semiconducting detectors, 15, 134 Semiconductor materials. 7 Sensitivity to background, 147 Shadow shield, 81 Shielding materials. 32 Shutter device. 26 Signal-to-noise ratio (SM ratio), 80, 82, 132 Silica-bearing dust, occupational exposure to, 116 Silicon, measuring of by PGNAA, 66 Silver, use of for PGAA facility, 29 Single-channel analyzer, 12 Single-crystal quartz, 62 Single-escape peak, 9.30. 61. 85 Skin doses. 124 Slagging, 160 Slowpoke reactor, 3 SM ratio, see Signal-to-noise ratio Sodium iodide detectors, 13 measuring of by PGNAA, 66 Source -to-detector distance. 52 -detector tube, 82 storage safe, 83

Spectrometry timing scheme, inelastic mode. 133 Spontaneous fission source. 75 Stomach, dose rate equivalents evaluated in, 125 Sulfur content determination. 85. 154. 159. 160 measurement of in complex materials, 67 Surgical illness, 80

Target-detector assembly. 95 TBCI, see Total body chlorine TRF, see Total body fat TBH, see Total body hydrogen TBN measurement, 84 TBN, see Total body nitrogen TBW, see Total body water TFW, see Total fat weight Therapy evaluation, 101 Thermal capture reactions, 48 Thermal neutron absorber, 27, 29. 43. 84. 149 bombardment, semiconductor response to, 54 capture, 77 energy of y-rays in, 177 gamma-rays, 177 reactions of. 43 chopper. high-speed. 59-60 fluences, 41 flux. 31. 38, 39 distributian, 3 1, 40, 42 monitored, 54 of THOR facility, 55 moderator, primary, 42 Thermal nitrogen analysis (TNA), 172 Thermoluminescent dosimeters, 54 Thermoluminescent detector (TLD), I22 THMER, see Tsing Hua Mobile Educational Reactor THOR, see Tsing Mua Open-pool Reactor Thyroid, dose rate equivalents for. 125. 126 Time-to-Amplitude Converter, 12 Timing single-channel analyzer (TSCA), 24, 173 Titanium, 4 compressor blade, 97 measuring of by PGNAA, 66 oxide, 67 TLD, see Thermoluminescent detector TNA, see Thennal nitrogen analysis Total body carbon, measurement of, 88 chlorine (TBCI), 8 1 fat (THF), 79, 81 hydrogen (TRH), 79, 88 nitrogen (TRN), 77

watet (TWW). 80

Total fat weight (TFW), SO Total neutron emission rate, 75 Total weight (TW), 79 Transmission counting, 26 Transmission detection system. 25 Transport disturbance. 104 Triple coincidence. 12, 62 Tritium dilution, 80 TSCA. see Timing single-channel analyzer Tsing Hua Mobile Educational Reactor (THMER), 17, 108 critical assembly, 33 facility. 34, 116 Tsing Hua Open-pool Reactor (THOR), 15.38 neutrons from. 3 1 PGAA measurement using. 48 thermal column of, 47 Tungsten shielding. 76 TW,see Total weight Two-collimator facility, 83

U U.S. Nuclear Regulatory Committee (NRC), 172 University of Missouri Research Reactor (MURK), 63

W Waste discharge, 145 Water attenuation, 147 phantom, measurement of, 87 pollutant survey, 25, 40 shielding, 75 Wet chemical analysis, 84 Whole-body carbon measurements, I27

z Zetatron, 132, 133 Zirconium, 4 Zirconium hydride. 27