Resolving Power of Gamma Ray Coincidence Spectrometry Using Lithium Drifted Germanium Detectors and Its Application to Multiple Radioisotope Analysis I. M. H. Pagden and J. C. Sutherland’ Atlantic Oceanographic Laboratory, Bedford Institute, Dartmouth, NoGa Scotia, Canada
Computer studies have been made of the cascade gamma rays of (n,r) activation products to determine which pairs of gamma ray coincidences could be unambiguously identified in a comprehensive radioisotope mixture. The resolution of the coincidence photopeaks has been investigated as a function of detection resolution and tables condensed from thecomputeroutput are presented. The currently achievable resolving powers of Ge(Li) detectors are such that most coincidence photopeaks can be unambiguously identified and if used for radiochemical analysis, would be, in the chemical sense, interference free. This i s contrasted with simple detector spectra of radiochemical mixtures where the majority of photopeaks exhibit varying degrees of interference. A brief discussion of possible experimental effects resulting from this approach and their bearing on some of the conclusions drawn i s included.
termine only the most intense gamma ray of a radioisotope to measure its activity in a sample, but it is believed that the previous comment still holds good, that a single Ge(Li) detector is not adequate to determine completely the radiochemical components of any mixture. F o r coincidence measurements using a pair of Ge(Li) detectors, the resolving power may be taken as the square of the resolving power of a single detector, and it is of interest to see how this figure, say 16 X lo4, compares with the total number of coincident gamma rays which might be expected to occur in a generalized radiochemical mixture such as that mentioned previously. This has been done in some detail and the results are presented in the following sections.
LITHIUM DRIFTED GERMANIUM gamma ray detectors have been available for some time and find a variety of applications in nuclear physics and radiochemical analyses. I n the latter field their resolving power has enabled radiochemists t o reduce the number of chemical separations required for identification of radioisotopes or, alternatively, to change the chemical methods to take advantage of the increased resolving power. A typical figure for the resolution of a commercially available Ge(Li) detector defined as full width at half maximum (FWHM) of the photo peak is 5 keV. Figures have been reported which are much better than this, but generally apply t o small detectors having a volume of 1 o r 2 c m 3 and with appropriate electronics. The number of gamma rays of various energies which might be emitted by radioisotopes produced by neutron irradiation of a sample containing all the stable elements, is of the order 4000. These gamma rays have different intensities and the parent radionuclides have different half-lives. Considering those radioisotopes having half-lives greater than one day, the total number of gamma rays is still of the order 2000. The gamma rays typically lie in the energy range 0-2 MeV and the distribution is such that the density of the gamma rays is much higher at lower energies, that is there are as many known gamma rays of different energy in the interval 0-500 keV as there are from 500 keV to 2 MeV. If one defines the resolving power of a Ge(Li) detector as the energy interval to be scanned, divided by the resolution, then the resolving power of the average Ge(Li) detector is about 400. This number is somewhat less than the number of gamma rays mentioned previously and indicates that the detector would not resolve all the gamma rays in such a mixture--i.e., other techniques would be required to determine the radiochemical composition of such a mixture. In practical terms it may be necessary to de-
The radioisotope cobalt-60 emits 2 gamma rays of energy 1333 keV and 1173 keV sequentially, and for the purposes of the following discussion these would be referred to as a coincident pair. The radioisotope sodium-24 emits a number of gamma rays, but in its decay a gamma ray of 1368 keV may be emitted in coincidence with a gamma ray of 3850 keV, or it may emit a 1368 keV gamma ray in coincidence with a 2750 keV gamma ray, and it would be described as having two coincident pairs. A listing has been made of all the possible coincident pairs which might be emitted from radioisotopes produced by (n, y ) reactions on stable elements. The coincident pairs have been restricted to those in which the mean life of the intermediate level, or levels, is less than 100 nsec. This figure was taken as a convenient limit for coincidence spectrometry. In the case of terbium-160 there are possibly 290 coincident pairs if one takes the information on the Nuclear Data sheets as a starting point. The total number of coincident pairs of the radioisotopes considered is approximately 5000. Library information on the coincident pairs was assembled on magnetic tapefrom punched cards. A Control Data 3100 computer with 8K memory used this in a program to determine which pairs were interference free. Any coincident pair was defined
l Present address, Control Data Canada Limited, 800 Dorchester Blvd., West, Montreal 2, Quebec.
UNIQUENESS OF COINCIDENT GAMMA RAYS
Table I. Uniqueness Test Half-lives > 3 hours Total interferences
Resolution, AE
-j x 100 3.12 1.56
3880 1083 288
0.78
92
6.25
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383
Table 11. Uniqueness Test Resolution = 5 keV Half-lives > 3 hr Total interferences = 586 = I Isotope Number of interferences = Ni I 2K 5 1Cr 2Ga ??Ge 77As 76Se somBr S2Br ?9Kr 93m~o 9gMo lo3Ru losRu lo3Pd logPd 1 lOmAg '6Cd
Ni
I
Ni
I
Ni
I
Ni
3 1 4 6 4 27 1 1 31 1
l19mSn l25Sn lz2Sb la4Sb 1 2 1mTe 123mTe 1Z7Te 127mTe lzsXe lZ7Xe 129mXe 3Xe 135Xe 134cs 131Ba 133Ba 14oLa
3
3Ce la7Nd 1S3Sm lS2Eu 152mEu lS4Eu lS3Gd l59Gd lo0Tb l57Dy 159Dy 166H0 166mH0 171Er IagYb l15Yb 177Lu
4 37 51 11 4 7 23 26 131 9 14
175Hf 180mHf ls1Hf lS2Ta
14 12 7 88 1 2 3 13 18 35 20 145 1 2 3 3
5
4 1 3 3 5 2
1
2 5
5 4 3 1
6 2 1 25 1 3 39 52 14
1
18 1w
l8lW 1 ~ 5 0 ~
1910s 30s 1921r 19 41r 191Pt 193mpt 1OSmpt
3
79 63 66 1 7
107pt
lg7Hg
Table 111. Pair Production Interferences Isotope = I
I 72Ga 8sRb 7 2Ga 88Rb lz4Sb 4°C
lalBa IS2Eu a lBa ls4Eu 133Ba l@Ho 47Nd 162E~ lS3Sm 2Ta ls2Eu 125Sn leoTb 1S2Eu 165Dy 176rnL~
166mH0 lS2Ta 166mH0 lE2Ta l7lEr l62Eu l7'Er lazEu
E
E
899
1835
899
1835
887
1119
122
1114
123
1276
81
1380
122
1114
100
1122
811
1068
122
1114
88
1060
100
1122
100
1122
122
1114
122
1114
1st gamma ray cascade energies = E E 2nd gamma ray cascade energies = E'E' Resolution = 5 keV E' E' I E 894 1320 l7lEr 899 1324 162E~ 122 812 894 l7lEr 813 899 la2Eu 122 603 890 l7lEr 608 887 l76mLu 88 92 124 I6gYb 92 122 7OGa 174 124 250 l6gYb 123 254 ls2Eu 122 81 355 lS2Ta 81 358 1 16mIn 1085 91 120 183rnW 92 122 la2Ta 100 97 609 1 ~ 5 0 ~ 100 611 13gBa 166 560 811 1 9 3 0 ~ 557 811 sgFe 191 87 119 lg21r 92 122 60Co 1173 88 546 lg4Tr 88 549 125mSn 326 96 96 191Pt 100 100 5gFe 191 96 610 191Pt 100 61 1 I6aHo 81 117 606 19 l p t 122 603 l86H0 81 124 606 197Pt 122 603 5gFe 191 lg7Hg 5gFe 191
as unique if there was no other coincident pair both of whose energies differed from the energies of the first by less than some arbitrarily set limit. If E& = 1,2) are coincident pair energies and E,,@ = 1,2) are any other pair energies then EI,EI,is unique if, for at least one of every pair of expressions with j = n, and j # n, 1 Ei,- Emnl 2 E,imit for all m. Various tests were made, the limits being 6.25, 3.1, 1.56, and 0.78z energy resolutions, respectively, and also a test in which the 384
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
E
1114 1114 1060 1040 1114 1270 1122 1090 1100 1332
1390 1100 1380 1380 1100 1loo
E' 87 92 87 92 87 88 21 18 94 92 66 63 99 100
72 68 73 78 156 151 329 326 82 78 82 81 85 81 77 78 77 78
E' 117 122 124 122 544 549 177 174 118 122 1273 1270 103 100 163 166 196 191 308 3 10 365 368 188 191 360 358 360 358 191 191 191 191
limit was arbitrarily 5 keV. The results of the first series of tests are shown in Table I. The results of the second test are shown in Table 11, which is an abbreviated form of the output from the computer. It is evident from the tables that as the resolution improves the number of interferences between gamma ray cascades from different radioisotopes is reduced, and that near the region of experimentally realized resolutions, a number of radioisotopes
have coincident gamma rays whose photopeaks are interference free. This should be compared with gamma ray energy lists--e.g., A.E.C.L. 1225-where the reasonably interference free region, applying a 5 keV limit, begins at about 1.5 MeV, and many of the lines below that energy have of the order ten other gamma rays from different radioisotopes lying within 5 keV of the particular energy. The foregoing is qualified since the detection process may occur in a number of different ways each with its own probability of occurrence which is dependent on the gamma ray energy. The three dominant processes, photo-electric effect, Compton effect, and pair production may have comparable likelihood of occurrence within the energy range considered. The extent of this governs the freedom from interference which may apply for any spectral line. In addition, the uniqueness test was applied with all gamma ray cascades given equal weight, whereas the intensity varies from cascade to cascade. In a given circumstance the degree of interference will depend on the relative source strengths of the two radioisotopes and the relative intensity of the interfering gamma ray cascades. PAIR PRODUCTION INTERFERENCE
The input data has been subjected to an additional test for interference due to the existence of double escape and single escape peaks showing in the spectra of gamma rays energetic enough to cause pair production. For example, Iz4Sbdecays with the emission of two gamma rays of 1.69 MeV and 0.60 MeV in cascade, among others. Since the 1.69 MeV gamma ray is energetic enough, pair production may occur within the detector with possible escape of either or both of the ensuing annihilation photons whose probability is dependent on the size of the detector used and which will be reflected in the intensity of the single and double escape peaks of the 1.69 MeV gamma ray at 1.18 MeV and 0.67 MeV, respectively. Compared with the time resolution of the Ge(Li) detectors the single and double escape events are prompt and Iz4Sbwill therefore show additional peaks apparently from two gamma ray pairs with energies of 1.18 MeV-0.60 MeV and 0.67 MeV-0.60 MeV, respectively. Thus these two possibilities were compared against the listed gamma ray cascades. In practice one is most likely to use the most intense cascade for identification of any given radioisotope. Therefore these cascades, from isotopes with half-lives greater than 3 hours, were compared with the listed cascades in which either or both of the gamma rays had an energy greater than 1.02 MeV. No lower half-life limit was set for the listed cascades due to pair production and the results are shown in Table 111. It is apparent that in many cases the gamma ray energy which is responsible for pair production is quite close to threshold and one would not expect to see prominent pair production peaks for such a gamma ray. Interference from pair production peaks might be expected in cases where the gamma ray energy is high or, alternatively, where the isotope responsible for the pair production has strong coincident lines or is produced in large quantities. With the additional restraint that for many purposes one needs to consider isotopes which have long enough half-lives for their examination away from the point of production, it may be concluded that pair production would not be expected to produce interference in the coincident spectra which would require to be accounted.
might be produced by neutron irradiation of a sample material. Certain gamma rays emitted by sources decaying to levels in a common radioisotope-e.g., I5%m and ls3Gd decaying to 153E~-are not suitable for instrumental activation analysis by coincidence spectrometry only, although the coincident pairs for such isotopes appear in the listing. It will be seen from the list that the gamma ray energies concerned vary from about 75 keV to 2 MeV and that coincidence gamma ray intensity is also quite variable from 100% down to a few per cent of the disintegrations of the parent radioisotope. This affects the scale of the experimental problem. Published work ( I ) using sodium iodide crystals for coincidence gamma ray spectrometry in association with large anticoincidence shields has enabled a number of radioisotopes to be determined simultaneously and with high sensitivity because of the efficiency of the sodium iodide detectors and the low background experienced using them in a coincidence mode and with additional suppression of background from the anticoincidence shielding. A typical resolving power for a sodium iodide detector is of the order ten and it is immediately apparent that to attempt the identification of the isotopes shown in Table IV using sodium iodide, will be limited in that the system does not have sufficient resolving power since it and the number of coincident gamma rays to be observed are approximately the same. In experiments with’ Ge(Li) detectors the present disadvantage is namely the relatively small size of’the detectors and therefore the coincidence efficiency is low. Experiments so far (unpublished) indicate that useful counting rates can be obtained using Ge(Li) detectors although only a few radioisotopes have been determined in any coincidence spectrum mainly because of the fact that large anticoincidence shields have not been used and therefore the coincidence spectrum is composed mainly of events due to Compton interaction within the detectors. In the event that the experiments are repeated using anticoincidence shields (which have the effect of suppressing the Compton continuum), then additional comments may be made concerning the results of the present computer search. In the first case one would expect any interference from pair production to be reduced owing to the detection of the escaping annihilation quanta in the anticoincidence shield. Secondly, the results to be expected from coincidence spectra would be modified in those cases where the two coincident gamma rays are emitted in a cascade of three gamma rays or more. In this case there is a probability that the third gamma ray would be detected in the anticoincidence shield and consequently the coincidence intensity observed would be reduced by some factor which would have to be determined for each particular radioisotope concerned. A few other comments may be made about the expected shape of the coincidence spectra to be observed assuming that Compton suppressing anticoincidence shields are used. With sodium iodide spectra, the shapes of peaks and Compton edges are well rounded. To that extent one expects to find a small contribution in the coincidence spectrum from photons which have back-scattered from the primary detectors and allowance has to be made for the effect. It is not thought that discrimination between photo peak and Compton edge could be made on the basis of shape alone. In the case of experiments with Ge(Li) detectors, peak shapes tend to be quite different from Compton edges and there is a possibility that in
EXPERIMENTAL CONSIDERATIONS
The computer search indicates that most coincident gamma rays are interference-free and Table IV shows a list of the most intense coincident gamma rays for various radioisotopes which
(1) R. W. Perkins and D. E. Robertson in “Proceedings of the 1965 International Conference: Modern Trends in Activation Analysis,” College Station, Texas, April 19-22, 1965. ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
385
Table IV. Selected Intense Coincident Gamma Rays
E
E
1368 320 500 887 191 1173 370 490 174 835 199 265 215 210 559 136 81 554 403 899 700 264 181 109 318 620 433 656 260 485 280 556 1085 159 811 326 564 603 70 82
2750 1520 810 1119 1100 1332 1114 610 1040 2201 427 368 416 265 657 265 199 777 2570 1835 874 685 740 216 475 810 617 885 263 935 1270 722 1270 161 1068 1390 686 1690 506 214
89
159
10
59 203 59 21 1 455 55 173 40 79
360 215 665 475 540 188 203 196 81
120 0.5 0.1 30 20
NC 103
