ARTICLE IN PRESS
Applied Radiation and Isotopes 60 (2004) 307–310
Measurements of nuclear data parameters of gamma-ray spectrometry
201
Tl by
# Karla C. de Souzaa,*, Monica L. da Silvaa, Jose! U. Delgadob, Roberto Polednab, Ricardo T. Lopesa, Carlos J. da Silvab a
Programa de Engenharia Nuclear - COPPE, Universidade Federal do Rio de Janeiro, Caixa Postal 68509, Rio de Janeiro/RJ CEP:21945-970, Brazil b ! Laboratorio Nacional de Metrologia das Radia@oes * Ionizantes/IRD, Comissao * Nacional de Energia Nuclear, Av Salvador Allende, s/n. Recreio, Rio de Janeiro/RJ CEP:22780 160, Brazil
Abstract 201
Tl is frequently used in radiopharmaceutical applications, and therefore the gamma-ray emission probabilities and half-life have been re-determined by means of gamma-ray spectroscopy. While the activity was calibrated using the sum-peak coincidence method, the half-life was obtained by the reference source method based on simultaneous counting of a reference source and the sample. Both the measurement techniques and assignment of uncertainties are presented and discussed, and the resulting data are shown to be in good agreement with previously published studies. r 2003 Elsevier Ltd. All rights reserved. Keywords: Gamma-ray spectrometry; Gamma-ray emission probability; Half-life determination; Sum-peak method
1. Introduction 201
Tl Has found wide application in nuclear medicine (Coursey et al., 1990). This radionuclide decays by electron capture only, and emits mainly gamma rays with energies of 165.88 and 167.43 keV. 201Tl has convenient nuclear decay characteristics for spectrometeric calibration, emitting photons in the energy range from 20 to 200 keV. The gamma-ray emission probabilities and half-life have been re-determined by means of gamma-ray spectroscopy to improve the known accuracy of these important nuclear parameters.
2. Activity measurements 201
Tl parent solution was taken from an ampoule provided by IPEN/CNEN. Two radioactive point sources were prepared from this parent solution for the measurements. Aliquots of the sample were with*Corresponding author. Fax: +55-21-2442-1605. E-mail address:
[email protected] (K.C. de Souza).
drawn from each parent solution and deposited on 0.05mm thick polystyrene film. After drying, these sources were covered with the same thickness of polystyrene film, and an area 17.5-mm diameter around the centre of each was punched out and placed in the middle of a polyvinyl chloride holder (Bernardes et al., 2002). 201 Tl was calibrated by means of the sum-peak coincidence counting method, using the same procedure for the calibration of 123I (Brinkman et al., 1977) and a coaxial 20% HPGe detector with a resolution of 1.9 keV for the 1332 keV line of 60Co. The sum-peak method is an absolute measurement technique (Brinkman et al., 1963) that combines gamma-ray spectrometry and coincidence counting, requiring only one-photon detector with associated electronics. The activity is determined from the count rates of the full absorption peaks of a single X- and gamma-ray, the sum-peak count rate of photons in true coincidence, and the total count rate of the spectrum (Kim et al., 2003). 201 Tl decays by electronic capture to 201Hg. The exact value of the activity was determined to be 1290.9070.25% kBq/g at the reference date for the 201 Tl point source, which was calculated by the
0969-8043/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2003.11.033
ARTICLE IN PRESS K.C. de Souza et al. / Applied Radiation and Isotopes 60 (2004) 307–310
308
following equation: No ¼ NT þ
Nx Ng Nxg
Nxx Ng2 ; 2 Nxg
ð1Þ
where No is the activity; NT is the total count; Nx is the number of counts in the X-photoelectric peak; Ng is the number of counts in the g-photoelectric peak; Nxg is the number of counts in the X–g coincidence peak; and Nxx is the number of counts in the X–X coincidence peak. The same source was studied independently by means of the full-energy peak efficiency response of another germanium detector. Results were consistent with those determined by the sum-peak method, despite a gamma-ray emission probability of only 0.106 for the 167.5 keV transition. The estimated uncertainty is the quadratic sum of the A- and B-type uncertainties at the 68% confidence level ðk ¼ 1Þ: Radioactive impurities were identified (202Tl), and quantified as a fraction of the activity of 201Tl at the reference time. This study by the sum-peak method will be published separately.
A is the activity of the measured source, traceable to the absolute measurement system. The spectrometers were carefully calibrated for efficiency using the following point sources as references: 54 Mn, 60Co, 109Cd, 113Sn, 133Ba, 134Cs, 137Cs, 139Ce, 152 Eu, 166mHo and 241Am. The total uncertainties in the activities of these references varied from 0.1% to 1.5% ðk ¼ 1), and the efficiency curve for the detector system is shown in Fig. 1. The efficiency calibrations in the energy region below 100 keV is much more problematic than for higher energies; therefore, the gamma lines of 201Tl below 100 keV will be determined in future work through a deconvolution process for the analysis of complex regions of the spectrum. The emission probabilities of the 135.3and 167.4-keV gamma rays of 201Tl are given in Table 1. Previously published evaluations are included for comparison, and all uncertainties were evaluated at the one sigma confidence level.
3. Gamma-ray emission probabilities
4. Half-life determination
The emission probabilities of the 135.3 and 167.4 keV gamma rays were determined using calibrated HPGe spectrometers and the expression
The half-life was determined using the reference source method, which is based on the ratio of the net full-energy peak counts of the radionuclide to be measured and the long half-life reference measured simultaneously (Morel et al., 1992). Although the spectrum suffers the same losses to dead time and pileup, the ratios of net full-energy peak counts are independent of these effects (Parker, 1990).
Pg ¼
C ; eg A
ð2Þ
where C is the count rate in the photo-peak, eg is the fullenergy peak efficiency for the X- or g-ray emission, and
Fig. 1. Photopeak efficiency curve of coaxial HPGe spectrometer.
ARTICLE IN PRESS K.C. de Souza et al. / Applied Radiation and Isotopes 60 (2004) 307–310 Table 1 Gamma-ray emission probabilities of
309
201
Tl
E (keV)
135.3
165.9
167.4
165.9+167.4
Martin (1976) Nass (1977)a Debertin et al. (1979)a Plch et al. (1989) PTB (1990)b NPL (1990)b NIST (1990)b This work
0.03770.004 0.026570.0010 0.028070.0003 — — — — 0.027370.0002
0.001970.0003 0.0018070.00020 0.001670.0002 — — — — —
0.11970.009 0.100070.0017 0.106070.0012 — 0.101870.0010 0.100570.0017 0.098870.0008 —
— — — 0.10670.001 — — — 0.105070.0005
a b
From Plch et al. (1989). From Coursey et al. (1990).
Table 2 Half-life of
201
Tl
Lead author
Number of half-lives followed
T1/2 (days)
Uncertainty (days)
Method
Debertin et al. (1979)
5.5 5.2
3.0420 3.0460
0.0050 0.0046
Ge(Li)-1 Ge(Li)-2
Lagoutine and Legrand (1982)
4.0
3.0408
0.0040
NaI(Tl)
This work
1.5
3.0380
0.0017
HPGe
A point source of 201Tl was prepared with a mixture containing 60Co and 137Cs as reference standards. The mixed point source was measured at a distance of about 20 cm from the end cap of 20% efficient HPGe coaxial detector. A total of 70 consecutive measurements were made for a period of 1.5 half-lives. The 135.3- and 167.4keV gamma rays were chosen for the assay of 201Tl, along with the 1173.2- and 1332.5-keV gamma rays of 60 Co and 661.7-keV gamma ray of 137Cs. The only impurity detected was 202Tl (T1/2=12.3570.17 days) at a level of nearly 0.9% by activity at the beginning of the measurement series. A linear fit was applied to the logarithm of the ratio between the net count of 201Tl and the reference radionuclide as a function of time. A half-life of 3.038070.0018 days was determined, with the uncertainty quoted at the 68% confidence level. This value is in good agreement with other published data measured by means of gamma-ray spectrometers, as can be seen in Table 2.
5. Conclusion Calibrated HPGe spectrometers have been used in conjunction with the sum-peak method to determine the
absolute emission probabilities of the main gamma rays of 201Tl, which are independent of spectral impurity interference. These results are in good agreement with previously published values within the uncertainty ranges, and can be used as a complementary standard for photon spectrometry below 200 keV. The X-ray emission probabilities were not determined because of the presence and interference of 202Tl impurity. Although the uncertainty in the 201Tl half-life has been reduced in comparison with previous studies, our half-life measurement was not performed under ideal conditions. This half-life requires a higher duration for the measurement (about 4 weeks of continuous data acquisition), which will be undertaken in future work.
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Coursey, B.M., Hoppes, D.D., Hirschfeld, A.T., Judge, S.M., Woods, D.H., Funck, E., Schrader, H., Tuck, A.G., 1990. Standardization and decay scheme of 201Tl. Appl. Radiat. Isot. 41, 289–291. . Debertin, K., Pessara, W., Schotzig, U., Walz, K.F., 1979. Decay data of 67Ga and 201Tl. Int. J. Appl. Radiat. Isot. 30, 551–555. Kim, I.J., Park, C.S., Choi, H.D., 2003. Absolute calibration of 60 Co using sum-peak method and HPGe detector. Appl. Radiat. Isot. 58, 227–233. Lagoutine, F., Legrand, J., 1982. P!eriodes de neuf radionucl!eides. Int. J. Appl. Radiat. Isot. 33, 711–713.
Martin, M.J., 1976. Nuclear decay data for selected radionuclides. ORNL-5114. Morel, J., Etcheverry, M., Blanchis, P., 1992. Possibilit!es offertes par la spectrom!etrie x et gamma pour la mesure des p!eriodes radioactives en se fondant sur l’emploi d’un r!ef!erentiel radioactif. Bull. BNM 88, 19–25. Parker, J.L., 1990. Near-optimum procedure for half-life measurement by high-resolution gamma-ray spectroscopy. Nucl. Instrum. Methods Phys. Res. A 286, 502–506. Plch, J., Kov!ar, P., Dry!ak, P., 1989. Absolute measurement of activity and total photon yield in the decay of 201Tl. Appl. Radiat. Isot. 40, 513–518.
