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Ge(Li) gamma-ray spectrometry as a pilot for NaI(Tl) gamma-ray spectrometry
Talanta,1968, Vol. 15, pp. 1159 to 1163. PergamonPress. Printedin NorthernIreland
Ge(Li) GAMMA-MY SPE~ROMETRY AS A PILOT FOR NaI(TI) GAMMA-RAY SPECTROMETRY V. P. GUINN, F. M. GRMJER, and D. M. FLEISHMAN Gulf General Atomic Incorporated, San Diego, California, U.S.A. (Received4 May 1968. Accepted 9 May 1968) Summary-Lithium-drifted germanium semiconductor detectors give much better resolution than do thallium-activated sodium iodide detectors, but much lower sensitivity. They can often advantageously be used in conjunction with NaIfll) detectors, to show whether corrections must be applied for activities other than the one to be measured and to provide the necessary ~omlation for calculation of the corrections. IN THE early days of activation analysis, post-irradiation radiochemical separations were a necessity, since no means of doing gamma-ray spectrometry were available. Separated activities were usually beta-counted with a Geiger-Miiller or gas proportional counter. Such radiochemical separations, greatly developed and applied by A. A. Smales and his colleagues at Harwell, are still quite necessary in many instances (e.g., Ref. 1). With the advent of the NaI(T1) scintillation detector and the single-channel pulse-height analyser, it was found that certain elemental analyses could be performed by means of purely instrumental activation analysis, based on gamma-ray spectrometry. The subsequent availability of multichannel pulse-height analysers greatly advanced the speed and scope of the instrumental form of the activation analysis method. Step by step, these m~tichannel analysers were improved, from the early 20-channel vacuum-tube discriminator type to the transistorized versions, with 100-512 analysis and storage channels. By this time, the resolution of the pulse-height analyser was so good that the limiting factor was the rather poor resolution of the NaI(T1) detector itself. This limited the practical application of the instr~ental activation analysis technique, since neutron activation of many kinds of samples of interest-but of complex elemental composition- resulted in pulse-height spectra consisting of large numbers of photopeaks, of various sizes and of various gamma-ray energies, many of which overlapped with one another. This problem has been attacked rather effectively by the development of computer programs, for resolving such complex spectra into their various radionuclide components, such as those pioneered by the Harwell group, especially by Salmon.a The most recent advance in the field of gamma-ray spectrometry is, of course, the development of the lithium-drifted germanium semiconductor detector, Ge(Li). Depending on the size, shape, sensitive depth, type, and the gamma-ray energy, these detectors typically exhibit a resolution (FWHM) 10-20 times better than that of even a very good standard 75 x 75 mm NaI(T1) detector. Once the resolution of the detector became better than that of the available pulse-height analysers, analysers with a much larger number of analysis and storage channels were soon developed: first with 1024 channels, then 2048, and now 4096 or 8192. The combination of a 1159 7
1160
V. P. GUINN, F. M. GRABER,and D. M. FLEISHMAN
15 cm3 (10 mm depth) Ge(Li) detector and a 4096-channel analyser, such as is used in the authors’ laboratory, completely resolves almost every photopeak found in the pulse-height spectra of even very complicated matrices that have been activated with neutrons in the reactor. Unfortunately,, although the presently-available, rather small Ge(Li) detectors are vastly superior to NaI(T1) detectors in energy resolution, they are also far less efficient in their detection of gamma-rays-at least at energies greater thana few hundred keV. It is the purpose of this paper to emphasize how a high-resolution, but low-efficiency, Ge(Li) spectrometer can be frequently used very advantageously as a “pilot” for the low-resolution, high-efficiency NaI(T1) spectrometer. A 75 x 75 mm NaI(TI) detector is compared with a 15 cm3 Ge(Li) one unless otherwise stated. EXAMPLES
The idea of using a Ge(Li) spectrometer as a pilot for an NaI(T1) spectrometer can be described best by first considering a general case, and then by giving some actual experimental examples. The general case: a sample is activated and then counted on an NaI(Tl) spectrometer, possibly producing a complicated pulse-height spectrum (many peaks) or perhaps a rather simple one. One peak appears to be due to the induced activity of interest. If the irradiation, decay, and counting times have been optimized for detection of an induced activity of that particular half-life, the probability that that peak is due to that particular radionuclide is increased. Its identity can be checked further by counting at several decay times-to ascertain whether the peak is decaying according to the expected half-life-and possibly by looking also for other gamma rays (if any) emitted by that particular radionuclide. Unfortunately, the additional counting and calculating required to check the half-life is rather timeconsuming. If it is decided that the observed photopeak is due entirely to the radionuclide of interest, the amount of the element present can be calculated by comparison with the corresponding photopeak of a standard sample of that element, activated and counted in an identical fashion. Considering (1) the large number of radionuclide species that can be formed to a significant degree, from the various chemical elements, by thermalneutron and fast-neutron reactions, (2) the very large number of different X-ray and gamma-ray energies possibly being emitted by an activated sample, and (3) the rather poor energy resolution of the NaI(T1) detector, it is apparent that errors can be made. A rather extreme, but instructive, example is shown in Fig. 1. This shows the spectrum of a small sample of the rare-earth mineral, xenotime, that had been activated with neutrons in the reactor, resolved by the NaI(T1) detector and a O-9-cm3 (3 mm deep) Ge(Li) detector. Starting at the low-energy end of the NaI(T1) spectrum, it is seen that there appear to be five broad major photopeaks-of decreasing size with increasing gamma-ray energy. However, the Ge(Li) spectrum reveals that only one of these NaI(T1) photopeaks is really a single-energy peak-the one at the highest energy. Each of the other four NaI(T1) peaks is seen to contain contributions from gamma rays of 2,3, or 4 different energies -in each group too close to one another in energy to be resolved by the NaI(T1) detector. If the only interest lay in analysing the sample for an element of which the induced activity emitted gamma rays of one of the energies falling in one of the broad multicomponent NaI(T1) peaks, an erroneous answer would, of course, be obtained if the
FIG. 1.-Ge(Li)
40
pulse-height
spectrum of a neutron-activated
sample of xenotime.
oo.A
d
z-
h L
8
0.
Id24
Chanii13 I number FIG. 2.-Ge(Li)
pulse-height
spectrum of a neutron-activated
sample of flour containing
8 ppm Br. 1160
2648
Channel number FIG. 3.-Ge(Li)
pulse-height
spectrum of a neutron-activated
aqueous Br standard.
000
Channel number
FIG. 4.-Ge(Li)
pulse-height
speztrum of a neutron-activated
sample of flour containing 5 ppm Hg.
0
b FIG. 5.-Ge(Li)
Channel number
pulse-height
spectrum
of a neutron-activated
aqueous
H,@I)
standard.
GetLi) gamma-ray spectrometry as a pilot for NaI(Tl) gamma-ray spectrometry
1161
NaI(TI) peak were considered to be due only to that particular radionuclide. In such a situation, (1) the use of the NaI(T1) detector could be abandoned, and the calculations based on the Ge(Li) spectrum, (2) radiochemical separations could be used to isolate the activity of interest, (3) the data could be treated by a computer program that includes all possible contributory radionuclides, or (4) the Ge(Li) spectrum could be used as a pilot to indicate exactly which specific gamma-ray energies must be included in a weighted least-squares fitting of the entire NaI(T1) spectrum or of that region of the one broad NaI(TI) peak that includes the gamma ray of interest (in this example, it is assumed that the half-lives of the various interfering species are sufficiently close to that of the radionuclide of interest for simple decay not to help appreciably in solving the problem). The difficulty with resorting entirely to the Ge(Li) detector is that the counting efficiencies of such small detectors are very low at the higher gammaray energies, thus often requiring counting periods of an hour or more to obtain as good photopeak counting statistics as could be obtained in minutes with the NaI(T1) detector. An example of more specific practical interest is the instrumental determination of bromine pesticide residues in foodstuffs, by means of activation analysis using thermal neutrons. Experience at this laboratory has shown that, with a 30;min activation of a 05-g sample, at a thermal-neutron flux of 2 x 10” ncm-* se&, followed by a 2-3 day cooling period before counting on a 75 x 75 mm NaI(T1) gamma-ray spectrometer, bromine can be detected instrumentally in most foodstuffs via the gamma rays of 353&r bromine-82. In untreated samples, the observed bromine levels are usually in the O-1-10 ppm range. B In crop materials grown in soil treated with a nematocide, e.g., dibromochloropropane,4 and in grains and flour fumigated in storage with methyl bromide,5 the bromine levels are frequently in the lO-50ppm range, and sometimes higher. The gamma-ray spectrum of bromine-82 is complex, since this radionuclide emits gamma rays of eight different energies, in the range from O-5 to l-5 MeV: O-554 (66 %), O-619 (41x), O-698 (27 %), 0*777(83 %), @828 (25 %), l-044 (29 %), l-317 (26x), l-475 (17%).6 Because of their greater abundances and better detection efficiencies, the O-554 and/or the O-777 MeV gamma rays are usually used in the calculations. However, as can be seen in Heath’s catalogue,’ the O-554 and O-619 MeV peaks largely overlap, and the O-698,0-777, and O-828 MeV peaks overlap. Furthermore, the two broad composite peaks (with maxima at O-554 and O-777 MeV respectively) are not completely resolved from one another. These five peaks essentially occupy the 05-0~9 MeV region of the spectrum. Therefore, there is always the possibility that the region also includes significant contributions from gamma rays from other radionuclides. As shown in Figs. 2 and 3, this possibility can be readily checked by counting on a Ge(Li) spectrometer. In this example, a fumigated flour sample, found to contain about 8 ppm bromine, was counted on the NaI(T1) and Ge(Li) detectors. As seen in, Fig. 2, the five bromine-82 peaks (0.554-O-828 MeV) show up, completely resolved from one another, and no extraneous gamma-ray peaks are observed in the O-50.9 MeV energy range. Furthermore, comparison with the Ge(Li) spectrum of an activated aqueous bromine standard (Fig. 3), reveals that the five peaks have the same ratios to one another in both the flour sample and the standard. The bromine-82 purity of this region of the spectrum having been ascertained, the broad 0*777-MeV peak in the NaI(T1) spectra was used to calculate the bromine content of the sample.
1162
V. P. GIJINN,F. M. GRABER,and D. M. FLEISHMAN
The mean value obtained on three aliquots, in 30-min counts after l-3 days decay, was 8.05 f 0.93 ppm bromine (mean of 6 measurements ranging from 7.09 to 9.15 ppm). The spread of values was only partially due to counting statistics, since each count had a standard deviation equivalent to only about A@15 ppm bromine. In order to obtain this good a counting precision with the Ge(Li) detector it is necessary to count for 3 times as long if the counts under the three peaks at @698, 0.777 and 0,828 MeV are combined and for almost 5 times as long if only the O-777 MeV peak is employed. The mean value obtained from 14 measurements using separately the 0.554 and O-777 MeV peaks was 8.02 & 0.54 ppm, in good agreement with the NaI(T1) determination. A third example is furnished by a recent analysis of a sample of flour containing about 5 ppm mercury (from seed grain, treated with a mercurial fungicide). The mercury (n, 7) product measured was 65-hr mercury-197. This radionuclide decays by electron capture, emitting predominantly 77.6 keV gamma rays and gold K X-rays (mostly at 68.8 kev). 6 With an NaI(T1) detector, the 68.8 and 77.6 keV photons produce a single slightly broadened photopeak. 7 As can be seen in Figs. 4 and 5 these two peaks can be resolved from one another with a Ge(Li) detector. Since the spectral region included in the broad NaI(T1) peak (cu. 63-82 keV) might also include some contributions from gamma rays of other radionuclides possibly present in the activated flour sample, back-scattered radiation from slightly higher-energy gamma rays (84-121 keV), and K X-rays from the elements from osmium (Z = 76) to bismuth (2 = 83) it was important to check this region with the Ge(Li) detector. It is evident from Figs. 4 and 5 that other elements in the flour do not give rise to interference and that it is safe to compute the mercury content of the sample by means of a simple photopeak area calculation, using the NaI(T1) spectrum. Three aliquots of the flour sample, and an aqueous mercury(I1) standard, were activated for 30 min in the reactor, at a thermal-neutron flux of l-8 x 1012n. cm-2. se&. Then each was counted for 30 min, twice, on a 75 x 75 mm solid NaI(T1) detector after a l-day decay. From the single broad peak attributed to mercury-197, the mercury content was calculated to be 5.13 f 0.21 ppm (with a single-count standard deviation, due only to counting statistics, of about &O-O6ppm). The samples were also counted once for 30 min under similar conditions on the Ge(Li) detector. The mean value from these measurements was 5.13 & 0.15 ppm. The precision is actually slightly better, probably fortuitously, than that obtained with the NaI(T1) detector. The single-count standard deviation, based only on the counting statistics, was equivalent to fO-16 ppm. In the Ge(Li) calculations, only the 77*6-keV peak was employed as the basis. If both the 77.6 and 68.8 keV peaks had been added together in the calculations, the single-count standard deviation, from counting statistics only, would have been lowered to about &to*11ppm. At such low energies, the Ge(Li) is just as efficient as NaI(Tl), but its small area decreases the geometric factor. CONCLUSIONS
The Ge(Li) detector can be used effectively, not only as the primary counter in instrumental activation analysis gamma-ray spectrometry but also as a pilot detector, to assist in the processing of NaI(T1) activation analysis pulse-height data. If the Ge(Li) pilot spectrum shows that the observed broad NaI(T1) peak of interest is due entirely to the radionuclide of interest, the NaI(T1) photopeak data can be used in a
Ge(Li) gamma-ray spectrometry
as a pilot for NaI(T1) gamma-ray spectrometry
1163
simple calculation. Advantage can thus be taken of the generally better NaI(T1) counting statistics (or of comparable statistics from shorter counting periods). If the Ge(Li) pilot spectrum shows that the broad NaI(T1) peak of interest also contains contributions from gamma rays of other radionuclides, these particular interfering gamma rays-and no others-can be included in a least-squares fitting calculation, to resolve the NaI(T1) peak into its components. By reduction of the number of gammaray energies to be considered in the least-squares fitting calculation, greater precision and accuracy are attainable. Although the discussion has centred about the determination of one particular element of interest in a complex sample, the same approach applies also to multi-element determinations. Zusammenfassung-Mit Lithium diffundierte Germanium-Halbleiterde tektoren geben vie1 bessere AufIasung als Thallium-aktivierte Natriumjodiddetektoren, sind aber vie1 unempflndlicher. In Verbindung mit NaJ(Tl)-Detektoren k&men sie oft vorteilhaft verwendet werden, urn zu zeigen, ob fiir andere als die zu messenden Aktivitgiten korrigiert werden muO, und urn die zur Berecbnung der Korrekturen notwendigen Daten zu erhalten. Rhm&Les d&ecteurs B semi-conducteur au germanium-lithium “drift” donnent une bien meilleure r&solution que les d&ecteurs ii iodure de sodium active au thallium, mais une beaucoup plus faible sensibilitk. 11speuvent Btre souvent avantageusement utilisds en liaison avec les dbtecteurs NaI(Tl), pour montrer si des corrections doivent &tre appliqu&s li des activitb autres que celle & mesurer et apporter l’information nt%ssaire pour le calcul des corrections.
1. 2. 3. 4. 5. 6. 7.
REFERENCES A. A. Smales and B. D. Pate, Anulyst, 1952,77,188. L. Salmon, Nucl. Znstr. Methook, 1961, 14,193. V. P. Guinn, World Rev. Pest Control, 1964,3,138. V. P. Guinn and J. C. Potter, Agr. Food Chem. 1962,10,232. D. L. Lindgren, F. A. Gunther, and L. E. Vincent, J. Econ. Entomol. 1962,55, 773. C. M. Lederer, J. M. Hollander, and I. Perlman, Table of Isotopes, 6th Ed., Wiley, New York, 1967. R. L. Heath, Scintillation Spectrometry Gamma-Ray Spectrum Catalqeue, ZOO-16880-l and -2, 2nd Ed. 1964.
Ge(Li) GAMMA-MY SPE~ROMETRY AS A PILOT FOR NaI(TI) GAMMA-RAY SPECTROMETRY V. P. GUINN, F. M. GRMJER, and D. M. FLEISHMAN Gulf General Atomic Incorporated, San Diego, California, U.S.A. (Received4 May 1968. Accepted 9 May 1968) Summary-Lithium-drifted germanium semiconductor detectors give much better resolution than do thallium-activated sodium iodide detectors, but much lower sensitivity. They can often advantageously be used in conjunction with NaIfll) detectors, to show whether corrections must be applied for activities other than the one to be measured and to provide the necessary ~omlation for calculation of the corrections. IN THE early days of activation analysis, post-irradiation radiochemical separations were a necessity, since no means of doing gamma-ray spectrometry were available. Separated activities were usually beta-counted with a Geiger-Miiller or gas proportional counter. Such radiochemical separations, greatly developed and applied by A. A. Smales and his colleagues at Harwell, are still quite necessary in many instances (e.g., Ref. 1). With the advent of the NaI(T1) scintillation detector and the single-channel pulse-height analyser, it was found that certain elemental analyses could be performed by means of purely instrumental activation analysis, based on gamma-ray spectrometry. The subsequent availability of multichannel pulse-height analysers greatly advanced the speed and scope of the instrumental form of the activation analysis method. Step by step, these m~tichannel analysers were improved, from the early 20-channel vacuum-tube discriminator type to the transistorized versions, with 100-512 analysis and storage channels. By this time, the resolution of the pulse-height analyser was so good that the limiting factor was the rather poor resolution of the NaI(T1) detector itself. This limited the practical application of the instr~ental activation analysis technique, since neutron activation of many kinds of samples of interest-but of complex elemental composition- resulted in pulse-height spectra consisting of large numbers of photopeaks, of various sizes and of various gamma-ray energies, many of which overlapped with one another. This problem has been attacked rather effectively by the development of computer programs, for resolving such complex spectra into their various radionuclide components, such as those pioneered by the Harwell group, especially by Salmon.a The most recent advance in the field of gamma-ray spectrometry is, of course, the development of the lithium-drifted germanium semiconductor detector, Ge(Li). Depending on the size, shape, sensitive depth, type, and the gamma-ray energy, these detectors typically exhibit a resolution (FWHM) 10-20 times better than that of even a very good standard 75 x 75 mm NaI(T1) detector. Once the resolution of the detector became better than that of the available pulse-height analysers, analysers with a much larger number of analysis and storage channels were soon developed: first with 1024 channels, then 2048, and now 4096 or 8192. The combination of a 1159 7
1160
V. P. GUINN, F. M. GRABER,and D. M. FLEISHMAN
15 cm3 (10 mm depth) Ge(Li) detector and a 4096-channel analyser, such as is used in the authors’ laboratory, completely resolves almost every photopeak found in the pulse-height spectra of even very complicated matrices that have been activated with neutrons in the reactor. Unfortunately,, although the presently-available, rather small Ge(Li) detectors are vastly superior to NaI(T1) detectors in energy resolution, they are also far less efficient in their detection of gamma-rays-at least at energies greater thana few hundred keV. It is the purpose of this paper to emphasize how a high-resolution, but low-efficiency, Ge(Li) spectrometer can be frequently used very advantageously as a “pilot” for the low-resolution, high-efficiency NaI(T1) spectrometer. A 75 x 75 mm NaI(TI) detector is compared with a 15 cm3 Ge(Li) one unless otherwise stated. EXAMPLES
The idea of using a Ge(Li) spectrometer as a pilot for an NaI(T1) spectrometer can be described best by first considering a general case, and then by giving some actual experimental examples. The general case: a sample is activated and then counted on an NaI(Tl) spectrometer, possibly producing a complicated pulse-height spectrum (many peaks) or perhaps a rather simple one. One peak appears to be due to the induced activity of interest. If the irradiation, decay, and counting times have been optimized for detection of an induced activity of that particular half-life, the probability that that peak is due to that particular radionuclide is increased. Its identity can be checked further by counting at several decay times-to ascertain whether the peak is decaying according to the expected half-life-and possibly by looking also for other gamma rays (if any) emitted by that particular radionuclide. Unfortunately, the additional counting and calculating required to check the half-life is rather timeconsuming. If it is decided that the observed photopeak is due entirely to the radionuclide of interest, the amount of the element present can be calculated by comparison with the corresponding photopeak of a standard sample of that element, activated and counted in an identical fashion. Considering (1) the large number of radionuclide species that can be formed to a significant degree, from the various chemical elements, by thermalneutron and fast-neutron reactions, (2) the very large number of different X-ray and gamma-ray energies possibly being emitted by an activated sample, and (3) the rather poor energy resolution of the NaI(T1) detector, it is apparent that errors can be made. A rather extreme, but instructive, example is shown in Fig. 1. This shows the spectrum of a small sample of the rare-earth mineral, xenotime, that had been activated with neutrons in the reactor, resolved by the NaI(T1) detector and a O-9-cm3 (3 mm deep) Ge(Li) detector. Starting at the low-energy end of the NaI(T1) spectrum, it is seen that there appear to be five broad major photopeaks-of decreasing size with increasing gamma-ray energy. However, the Ge(Li) spectrum reveals that only one of these NaI(T1) photopeaks is really a single-energy peak-the one at the highest energy. Each of the other four NaI(T1) peaks is seen to contain contributions from gamma rays of 2,3, or 4 different energies -in each group too close to one another in energy to be resolved by the NaI(T1) detector. If the only interest lay in analysing the sample for an element of which the induced activity emitted gamma rays of one of the energies falling in one of the broad multicomponent NaI(T1) peaks, an erroneous answer would, of course, be obtained if the
FIG. 1.-Ge(Li)
40
pulse-height
spectrum of a neutron-activated
sample of xenotime.
oo.A
d
z-
h L
8
0.
Id24
Chanii13 I number FIG. 2.-Ge(Li)
pulse-height
spectrum of a neutron-activated
sample of flour containing
8 ppm Br. 1160
2648
Channel number FIG. 3.-Ge(Li)
pulse-height
spectrum of a neutron-activated
aqueous Br standard.
000
Channel number
FIG. 4.-Ge(Li)
pulse-height
speztrum of a neutron-activated
sample of flour containing 5 ppm Hg.
0
b FIG. 5.-Ge(Li)
Channel number
pulse-height
spectrum
of a neutron-activated
aqueous
H,@I)
standard.
GetLi) gamma-ray spectrometry as a pilot for NaI(Tl) gamma-ray spectrometry
1161
NaI(TI) peak were considered to be due only to that particular radionuclide. In such a situation, (1) the use of the NaI(T1) detector could be abandoned, and the calculations based on the Ge(Li) spectrum, (2) radiochemical separations could be used to isolate the activity of interest, (3) the data could be treated by a computer program that includes all possible contributory radionuclides, or (4) the Ge(Li) spectrum could be used as a pilot to indicate exactly which specific gamma-ray energies must be included in a weighted least-squares fitting of the entire NaI(T1) spectrum or of that region of the one broad NaI(TI) peak that includes the gamma ray of interest (in this example, it is assumed that the half-lives of the various interfering species are sufficiently close to that of the radionuclide of interest for simple decay not to help appreciably in solving the problem). The difficulty with resorting entirely to the Ge(Li) detector is that the counting efficiencies of such small detectors are very low at the higher gammaray energies, thus often requiring counting periods of an hour or more to obtain as good photopeak counting statistics as could be obtained in minutes with the NaI(T1) detector. An example of more specific practical interest is the instrumental determination of bromine pesticide residues in foodstuffs, by means of activation analysis using thermal neutrons. Experience at this laboratory has shown that, with a 30;min activation of a 05-g sample, at a thermal-neutron flux of 2 x 10” ncm-* se&, followed by a 2-3 day cooling period before counting on a 75 x 75 mm NaI(T1) gamma-ray spectrometer, bromine can be detected instrumentally in most foodstuffs via the gamma rays of 353&r bromine-82. In untreated samples, the observed bromine levels are usually in the O-1-10 ppm range. B In crop materials grown in soil treated with a nematocide, e.g., dibromochloropropane,4 and in grains and flour fumigated in storage with methyl bromide,5 the bromine levels are frequently in the lO-50ppm range, and sometimes higher. The gamma-ray spectrum of bromine-82 is complex, since this radionuclide emits gamma rays of eight different energies, in the range from O-5 to l-5 MeV: O-554 (66 %), O-619 (41x), O-698 (27 %), 0*777(83 %), @828 (25 %), l-044 (29 %), l-317 (26x), l-475 (17%).6 Because of their greater abundances and better detection efficiencies, the O-554 and/or the O-777 MeV gamma rays are usually used in the calculations. However, as can be seen in Heath’s catalogue,’ the O-554 and O-619 MeV peaks largely overlap, and the O-698,0-777, and O-828 MeV peaks overlap. Furthermore, the two broad composite peaks (with maxima at O-554 and O-777 MeV respectively) are not completely resolved from one another. These five peaks essentially occupy the 05-0~9 MeV region of the spectrum. Therefore, there is always the possibility that the region also includes significant contributions from gamma rays from other radionuclides. As shown in Figs. 2 and 3, this possibility can be readily checked by counting on a Ge(Li) spectrometer. In this example, a fumigated flour sample, found to contain about 8 ppm bromine, was counted on the NaI(T1) and Ge(Li) detectors. As seen in, Fig. 2, the five bromine-82 peaks (0.554-O-828 MeV) show up, completely resolved from one another, and no extraneous gamma-ray peaks are observed in the O-50.9 MeV energy range. Furthermore, comparison with the Ge(Li) spectrum of an activated aqueous bromine standard (Fig. 3), reveals that the five peaks have the same ratios to one another in both the flour sample and the standard. The bromine-82 purity of this region of the spectrum having been ascertained, the broad 0*777-MeV peak in the NaI(T1) spectra was used to calculate the bromine content of the sample.
1162
V. P. GIJINN,F. M. GRABER,and D. M. FLEISHMAN
The mean value obtained on three aliquots, in 30-min counts after l-3 days decay, was 8.05 f 0.93 ppm bromine (mean of 6 measurements ranging from 7.09 to 9.15 ppm). The spread of values was only partially due to counting statistics, since each count had a standard deviation equivalent to only about A@15 ppm bromine. In order to obtain this good a counting precision with the Ge(Li) detector it is necessary to count for 3 times as long if the counts under the three peaks at @698, 0.777 and 0,828 MeV are combined and for almost 5 times as long if only the O-777 MeV peak is employed. The mean value obtained from 14 measurements using separately the 0.554 and O-777 MeV peaks was 8.02 & 0.54 ppm, in good agreement with the NaI(T1) determination. A third example is furnished by a recent analysis of a sample of flour containing about 5 ppm mercury (from seed grain, treated with a mercurial fungicide). The mercury (n, 7) product measured was 65-hr mercury-197. This radionuclide decays by electron capture, emitting predominantly 77.6 keV gamma rays and gold K X-rays (mostly at 68.8 kev). 6 With an NaI(T1) detector, the 68.8 and 77.6 keV photons produce a single slightly broadened photopeak. 7 As can be seen in Figs. 4 and 5 these two peaks can be resolved from one another with a Ge(Li) detector. Since the spectral region included in the broad NaI(T1) peak (cu. 63-82 keV) might also include some contributions from gamma rays of other radionuclides possibly present in the activated flour sample, back-scattered radiation from slightly higher-energy gamma rays (84-121 keV), and K X-rays from the elements from osmium (Z = 76) to bismuth (2 = 83) it was important to check this region with the Ge(Li) detector. It is evident from Figs. 4 and 5 that other elements in the flour do not give rise to interference and that it is safe to compute the mercury content of the sample by means of a simple photopeak area calculation, using the NaI(T1) spectrum. Three aliquots of the flour sample, and an aqueous mercury(I1) standard, were activated for 30 min in the reactor, at a thermal-neutron flux of l-8 x 1012n. cm-2. se&. Then each was counted for 30 min, twice, on a 75 x 75 mm solid NaI(T1) detector after a l-day decay. From the single broad peak attributed to mercury-197, the mercury content was calculated to be 5.13 f 0.21 ppm (with a single-count standard deviation, due only to counting statistics, of about &O-O6ppm). The samples were also counted once for 30 min under similar conditions on the Ge(Li) detector. The mean value from these measurements was 5.13 & 0.15 ppm. The precision is actually slightly better, probably fortuitously, than that obtained with the NaI(T1) detector. The single-count standard deviation, based only on the counting statistics, was equivalent to fO-16 ppm. In the Ge(Li) calculations, only the 77*6-keV peak was employed as the basis. If both the 77.6 and 68.8 keV peaks had been added together in the calculations, the single-count standard deviation, from counting statistics only, would have been lowered to about &to*11ppm. At such low energies, the Ge(Li) is just as efficient as NaI(Tl), but its small area decreases the geometric factor. CONCLUSIONS
The Ge(Li) detector can be used effectively, not only as the primary counter in instrumental activation analysis gamma-ray spectrometry but also as a pilot detector, to assist in the processing of NaI(T1) activation analysis pulse-height data. If the Ge(Li) pilot spectrum shows that the observed broad NaI(T1) peak of interest is due entirely to the radionuclide of interest, the NaI(T1) photopeak data can be used in a
Ge(Li) gamma-ray spectrometry
as a pilot for NaI(T1) gamma-ray spectrometry
1163
simple calculation. Advantage can thus be taken of the generally better NaI(T1) counting statistics (or of comparable statistics from shorter counting periods). If the Ge(Li) pilot spectrum shows that the broad NaI(T1) peak of interest also contains contributions from gamma rays of other radionuclides, these particular interfering gamma rays-and no others-can be included in a least-squares fitting calculation, to resolve the NaI(T1) peak into its components. By reduction of the number of gammaray energies to be considered in the least-squares fitting calculation, greater precision and accuracy are attainable. Although the discussion has centred about the determination of one particular element of interest in a complex sample, the same approach applies also to multi-element determinations. Zusammenfassung-Mit Lithium diffundierte Germanium-Halbleiterde tektoren geben vie1 bessere AufIasung als Thallium-aktivierte Natriumjodiddetektoren, sind aber vie1 unempflndlicher. In Verbindung mit NaJ(Tl)-Detektoren k&men sie oft vorteilhaft verwendet werden, urn zu zeigen, ob fiir andere als die zu messenden Aktivitgiten korrigiert werden muO, und urn die zur Berecbnung der Korrekturen notwendigen Daten zu erhalten. Rhm&Les d&ecteurs B semi-conducteur au germanium-lithium “drift” donnent une bien meilleure r&solution que les d&ecteurs ii iodure de sodium active au thallium, mais une beaucoup plus faible sensibilitk. 11speuvent Btre souvent avantageusement utilisds en liaison avec les dbtecteurs NaI(Tl), pour montrer si des corrections doivent &tre appliqu&s li des activitb autres que celle & mesurer et apporter l’information nt%ssaire pour le calcul des corrections.
1. 2. 3. 4. 5. 6. 7.
REFERENCES A. A. Smales and B. D. Pate, Anulyst, 1952,77,188. L. Salmon, Nucl. Znstr. Methook, 1961, 14,193. V. P. Guinn, World Rev. Pest Control, 1964,3,138. V. P. Guinn and J. C. Potter, Agr. Food Chem. 1962,10,232. D. L. Lindgren, F. A. Gunther, and L. E. Vincent, J. Econ. Entomol. 1962,55, 773. C. M. Lederer, J. M. Hollander, and I. Perlman, Table of Isotopes, 6th Ed., Wiley, New York, 1967. R. L. Heath, Scintillation Spectrometry Gamma-Ray Spectrum Catalqeue, ZOO-16880-l and -2, 2nd Ed. 1964.