- Email: [email protected]
The half-life of 152Eu
Int. J. Appl. Radiat. Isot. Vol. 3-/..No. 6. pp. 891-892. 1983 Printed in Great Britain.All rights reserved
0020-708X;83:060891-02503.00/0 Pergamon Press Ltd
The Half-Life of 152Eu S. BABA, S. I C H I K A W A , K. G U N J I , T. S E K I N E , H. BABA* a n d T. K O M O R I Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki, Japan IReceived 17 May 1982: in revised form 27 August 1982)
The half-lifeof tS-'Eu was determined by measuring the disintegration rate of a tSZEu sample and the number of t~ZEu atoms contained in the sample. The number of t52Eu atoms was determined by isotope-dilution mass spectrometry, while the tS-'Eu activity was measured by the 4nil-7 spectroscopic coincidence method. The half-lifeobtained was 13.12 + 0. I0 yr.
I. Introduction The nuclide tSZEu is widely used as a calibration source for energy and efficiency in high-resolution 7-ray spectrometry, because of the relatively long half-life and emission of multiple 7-rays with appropriate energy intervals and emission probabilities per decay. Many precise measurements of the 7-ray emission rates per decay have been carried out ~t-'~ for t52Eu with the aim of minimizing the uncertainty in the efficiency calibration. If a calibrated ~-'Eu source is to be used as an efficiency-calibration standard over a long period, the accuracy of its half-life is also important. As listed in Table 2, the half-life tends to shift toward longer values in recent experiments. Furthermore, the average of the longer half-life values deviates more than an estimated one standard deviation from that of the shorter half-life values. All t h e reported half-life values of ~52Eu were obtained by tracing the decay for a period much shorter than one half-life:~7-t3~ Karraker et al. ts~ followed its decay with a mass spectrometer while others used radioactivity counters. It is usually very difficult to maintain the stability of a counter over years, although long-lived reference sources can be used to compensate for this. The present authors have attempted to deduce the half-life of tS-'Eu by measuring the disintegration rate of a t52Eu source containing a known amount of t52Eu atoms. The disintegration rate was measured precisely by a method using the 7-ray spectrum in coincidence with signals from a 4rr/~ counter."'L~ The number of ~5-'Eu atoms was obtained using isotopedilution mass spectrometry. Hence, the accuracy of the deduced half-life value depends on both the accuracy of the activity determination and of the massspectrometric measurement;but is free from the errors * Present address: Osaka University, Toyonaka. Osaka. Japan.
arising in the repetitive radioactivity measurements and the analysis of a decay curve.
2. Experimental (a) Preparation of the l ~ 'Eu sample solution Europium oxide with t s IEu enriched to about 90% was irradiated in a reactor. After the irradiation, • europium was separated by the ion-exchange method from the impurities and the daughter nuclides of l~2Eu; namely, t52Sm and t52Gd. The :52Eu sample solution was prepared by dissolving the purified europium in 0.1N HCt. It contained tStEu, 152Eu, t53Eu and t54Eu isotopes, t54Eu activity was 0.7%, that of the t52Eu. A part of the sample solution was used to prepare sources for determination of the activity and the rest was used for mass-spectrometric measurements• (b) Mass spectrometry The fractional abundances of the four europium isotopes in the sample solution were measured with a surface-ionizationmass spectrometer" 5~(the results are shown in the second column of Table l). Then, the total europium concentration of the solution was obtained by applying the isotope-dilution method. A known amount of natural europium (1s t Eu: 48.03%, t53Eu:51.97%) was added as a spike. The europium mass x in a given mass of the sample solution, which contains 15tEu, '~2Eu, tS3Eu and tS~Eu, is related by equation (I) to the mass y of europium in the added spike: .. B-C b Mx x=Y'c-A a My' (1) where A, B and C are the measured t53Eu/t51Eu ratios in the sample, the spike and the mixture solution, respectively; a and b denote the fractional abundances of IStEu in the sample and in the spike solution, respectively; M~ and My are the atomic weights of the europium in the sample and in the spike solution. The numbers of atoms of europium
891
892
S. Baba et aL
Table 1. The fractional isotopic abundances and the concentrations of europium isotopes in the sample solution Fractional isotopic abundance (atom ° o)
Isotope lStEu '~-"Eu tS~Eu tS'~Eu
89.345 2.344 8.300 0.0105
Concentration (atom, rag- soln)
4- 0.022 + 0.008 ___0.021 4- 0.0001
(1.075 + 0.003) x 10 t" (2.819 + 0.014) x l01-" (9.98 + 0.04 ) X 1012 (1.32 4- 0.02 ) x 101°
isotopes in the sample were calculated from ~ and the isotopic abundances, as listed in the third column of Table 1. (c) Radioacticity measurement The activity of tSZEu in a given aliquot of the sample solution was determined by the 4nil-,. spectroscopic coincidence measurement/TM The weighed sample solution was deposited and dried on a thin film of VYNS resin, coated with gold. The thickness of the film with the gold coating was 15--20/.tg/cm 2 as a whole, determined by measuring the absorption of :c particles from an 2'*tAm source. The coincidence counting was carried out with a methane-flow 4zrfl proportional counter and a coaxial type Ge(Li)detector connected to a 4096-channel pulse-height analyzer. The effective counting efficiencies of the 4zrfl counter for fl-rays, conversion electrons, and Auger electrons from l S-'Eu were obtained by taking the intensity ratio for certain 7-rays between the single spectrum and the spectrum coincident with the pulses from the 4zrfl counter. Details of this method is given elsewhere. "'~ The disintegration rate of the 15ZEu was deduced with the standard deviation of 0.6% from the measured effective counting efficiencies and the t S-'Eu decay scheme. 3. R e s u l t s
and
Discussion
The concentrations of the europium isotopes per I mg of the sample solution are listed in Table 1. Each value is the average of three experimental results of mass-spectrometric measurements with different amount of the spike (y in equation (1)). The quoted uncertainty is intended to correspond to one Table 2. Half-life of t~-'Eu Half-life (yr) 13 4- 2 12.2 _ 0.2 13.2 _+ 0.3 13.10 _+ 0.05 13.50 _ 0.05 13.39 + 0.02 13.57 _+ 0.11 13.12 _ 0.10
Period of decay-curve measurement (yr) 3.4 4.9 1.3 3.5 3.0 3.9
Comments Karraker et al. (1952)~s~ Geiger ( 1957¢*~ Emery et al. (1972)"°) Lagoutine etal.(1978) ~l z~ Sch6tzig et al. (1980)(~ Rutledge et al. 11980)~tz~ Hoppes et al. (1982)(13~ Present work
standard deviation of the mean. with the uncertainty in the weighing considered. The 15-'Eu activity in I m g of the sample solution was 4491 + 27s-~ 341 days after the mass-spectrometric measurements. The uncertainty in the decay rate was estimated by summing in quadrature all random and systematic uncertainties. The half-life T of tS"Eu was calculated iteratively using equations (2) and (3): / I n 2-At'~
Oo = o.~xp~ ----~-1
(2~
In2 Do = ~ No,
(31
where No is the measured number of lS'Eu atoms. At is the elapsed time from the mass-spectrometric measurement to activity determination, and D O and D are the disintegration rates of ~SZEu at the times of the mass-spectrometric and radioactivity measurements, respectively. Initially, the experimentally obtained disintegration rate and 13 y are substituted to D and T in equation (2) to calculate Do, which is used in turn together with No to calculate a new T from equation (3). The procedure was repeated until a constant value was found for T. The resultant T is listed in the last row of Table 2. The relation between the uncertainties, AT. ANo and AD, for 7". No and D, respectively, is expressed by
r
=
V\
.Vo /
+
.
14)
The quoted uncertainty corresponds to the standard deviation of the mean. The present result deviates from the recently reported longer half-lifes, but agrees very well with the value of Lagoutine et al. (I t~ References
I. Aum,~ O.. BAaRE'rTEJ., BAaRE'r'rE M. and MONAaO S. Nucl. Instrum. Methods 76. 93 (1969). 2. MOWAT'rR. S. Can. d. Phys. 48, 2606 (1970). 3. NOTEA A. and ELIAS E. Nucl. lnstrum. Methods ~ , 269 (19701. 4. McNELLES L. A. and CAMPBELL J. L. Nucl. Instrum. Methods 109, 241 (1973). 5. GEHRKE R. J., HELMER R. O. and Ga~WOOD R. C. Nucl. Instrum. Methods. 147, 405 (19771. 6. DEBERTI~"K. Nucl. lnstrum. Methods 158, 479 (1979). 7. SCH(STZlG U., DEBEaTL~ K. and WALZ K. F. Nucl. Instrum. Methods 169. 43 (I 980). 8. KARR^I<~a D. G, HAVt)E.~ R. J. and IXGt-IRAMM. G. Phys. Rev. 87, 901 (1952). 9. GEtC;Ea K. W. Phys. Rer. 105, 1539 (1957). 10. E~IERV J. F.. REVXOLDS S. A., Wvarr E. I. and GLEASO~"G. I. Nucl. Sci. Eng. 48, 319 (1972). 11. LAGOUTINE F., LEGRAND J. and BAC C. Int. J. Appl. Radiat. Isot. 29, 269 (1978). 12. RUTLEDGE A. R.. MERRtTT J. S. and S,~nrn L. V. Report AECL-6788, p. 45 (1980). 13. HOPPES D. D., SCHL~IA F. J, Ht;TCHt.~SO,~ J. M. R. and UNTERWEOER M. P., NBS Special Publication 626. 85 (1982). 14. BABA S., ICHIKAWA S.. SEKINE T., ISHIKAWA I. and BABA H. Nucl. lnstrum. Methods 203. 273 (1982). 15. KOMORI T.. TAMURAS.. GUNJI K. and Ooa Z. Mass Spectroscopy (Japan) 21.27 (1973).
0020-708X;83:060891-02503.00/0 Pergamon Press Ltd
The Half-Life of 152Eu S. BABA, S. I C H I K A W A , K. G U N J I , T. S E K I N E , H. BABA* a n d T. K O M O R I Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki, Japan IReceived 17 May 1982: in revised form 27 August 1982)
The half-lifeof tS-'Eu was determined by measuring the disintegration rate of a tSZEu sample and the number of t~ZEu atoms contained in the sample. The number of t52Eu atoms was determined by isotope-dilution mass spectrometry, while the tS-'Eu activity was measured by the 4nil-7 spectroscopic coincidence method. The half-lifeobtained was 13.12 + 0. I0 yr.
I. Introduction The nuclide tSZEu is widely used as a calibration source for energy and efficiency in high-resolution 7-ray spectrometry, because of the relatively long half-life and emission of multiple 7-rays with appropriate energy intervals and emission probabilities per decay. Many precise measurements of the 7-ray emission rates per decay have been carried out ~t-'~ for t52Eu with the aim of minimizing the uncertainty in the efficiency calibration. If a calibrated ~-'Eu source is to be used as an efficiency-calibration standard over a long period, the accuracy of its half-life is also important. As listed in Table 2, the half-life tends to shift toward longer values in recent experiments. Furthermore, the average of the longer half-life values deviates more than an estimated one standard deviation from that of the shorter half-life values. All t h e reported half-life values of ~52Eu were obtained by tracing the decay for a period much shorter than one half-life:~7-t3~ Karraker et al. ts~ followed its decay with a mass spectrometer while others used radioactivity counters. It is usually very difficult to maintain the stability of a counter over years, although long-lived reference sources can be used to compensate for this. The present authors have attempted to deduce the half-life of tS-'Eu by measuring the disintegration rate of a t52Eu source containing a known amount of t52Eu atoms. The disintegration rate was measured precisely by a method using the 7-ray spectrum in coincidence with signals from a 4rr/~ counter."'L~ The number of ~5-'Eu atoms was obtained using isotopedilution mass spectrometry. Hence, the accuracy of the deduced half-life value depends on both the accuracy of the activity determination and of the massspectrometric measurement;but is free from the errors * Present address: Osaka University, Toyonaka. Osaka. Japan.
arising in the repetitive radioactivity measurements and the analysis of a decay curve.
2. Experimental (a) Preparation of the l ~ 'Eu sample solution Europium oxide with t s IEu enriched to about 90% was irradiated in a reactor. After the irradiation, • europium was separated by the ion-exchange method from the impurities and the daughter nuclides of l~2Eu; namely, t52Sm and t52Gd. The :52Eu sample solution was prepared by dissolving the purified europium in 0.1N HCt. It contained tStEu, 152Eu, t53Eu and t54Eu isotopes, t54Eu activity was 0.7%, that of the t52Eu. A part of the sample solution was used to prepare sources for determination of the activity and the rest was used for mass-spectrometric measurements• (b) Mass spectrometry The fractional abundances of the four europium isotopes in the sample solution were measured with a surface-ionizationmass spectrometer" 5~(the results are shown in the second column of Table l). Then, the total europium concentration of the solution was obtained by applying the isotope-dilution method. A known amount of natural europium (1s t Eu: 48.03%, t53Eu:51.97%) was added as a spike. The europium mass x in a given mass of the sample solution, which contains 15tEu, '~2Eu, tS3Eu and tS~Eu, is related by equation (I) to the mass y of europium in the added spike: .. B-C b Mx x=Y'c-A a My' (1) where A, B and C are the measured t53Eu/t51Eu ratios in the sample, the spike and the mixture solution, respectively; a and b denote the fractional abundances of IStEu in the sample and in the spike solution, respectively; M~ and My are the atomic weights of the europium in the sample and in the spike solution. The numbers of atoms of europium
891
892
S. Baba et aL
Table 1. The fractional isotopic abundances and the concentrations of europium isotopes in the sample solution Fractional isotopic abundance (atom ° o)
Isotope lStEu '~-"Eu tS~Eu tS'~Eu
89.345 2.344 8.300 0.0105
Concentration (atom, rag- soln)
4- 0.022 + 0.008 ___0.021 4- 0.0001
(1.075 + 0.003) x 10 t" (2.819 + 0.014) x l01-" (9.98 + 0.04 ) X 1012 (1.32 4- 0.02 ) x 101°
isotopes in the sample were calculated from ~ and the isotopic abundances, as listed in the third column of Table 1. (c) Radioacticity measurement The activity of tSZEu in a given aliquot of the sample solution was determined by the 4nil-,. spectroscopic coincidence measurement/TM The weighed sample solution was deposited and dried on a thin film of VYNS resin, coated with gold. The thickness of the film with the gold coating was 15--20/.tg/cm 2 as a whole, determined by measuring the absorption of :c particles from an 2'*tAm source. The coincidence counting was carried out with a methane-flow 4zrfl proportional counter and a coaxial type Ge(Li)detector connected to a 4096-channel pulse-height analyzer. The effective counting efficiencies of the 4zrfl counter for fl-rays, conversion electrons, and Auger electrons from l S-'Eu were obtained by taking the intensity ratio for certain 7-rays between the single spectrum and the spectrum coincident with the pulses from the 4zrfl counter. Details of this method is given elsewhere. "'~ The disintegration rate of the 15ZEu was deduced with the standard deviation of 0.6% from the measured effective counting efficiencies and the t S-'Eu decay scheme. 3. R e s u l t s
and
Discussion
The concentrations of the europium isotopes per I mg of the sample solution are listed in Table 1. Each value is the average of three experimental results of mass-spectrometric measurements with different amount of the spike (y in equation (1)). The quoted uncertainty is intended to correspond to one Table 2. Half-life of t~-'Eu Half-life (yr) 13 4- 2 12.2 _ 0.2 13.2 _+ 0.3 13.10 _+ 0.05 13.50 _ 0.05 13.39 + 0.02 13.57 _+ 0.11 13.12 _ 0.10
Period of decay-curve measurement (yr) 3.4 4.9 1.3 3.5 3.0 3.9
Comments Karraker et al. (1952)~s~ Geiger ( 1957¢*~ Emery et al. (1972)"°) Lagoutine etal.(1978) ~l z~ Sch6tzig et al. (1980)(~ Rutledge et al. 11980)~tz~ Hoppes et al. (1982)(13~ Present work
standard deviation of the mean. with the uncertainty in the weighing considered. The 15-'Eu activity in I m g of the sample solution was 4491 + 27s-~ 341 days after the mass-spectrometric measurements. The uncertainty in the decay rate was estimated by summing in quadrature all random and systematic uncertainties. The half-life T of tS"Eu was calculated iteratively using equations (2) and (3): / I n 2-At'~
Oo = o.~xp~ ----~-1
(2~
In2 Do = ~ No,
(31
where No is the measured number of lS'Eu atoms. At is the elapsed time from the mass-spectrometric measurement to activity determination, and D O and D are the disintegration rates of ~SZEu at the times of the mass-spectrometric and radioactivity measurements, respectively. Initially, the experimentally obtained disintegration rate and 13 y are substituted to D and T in equation (2) to calculate Do, which is used in turn together with No to calculate a new T from equation (3). The procedure was repeated until a constant value was found for T. The resultant T is listed in the last row of Table 2. The relation between the uncertainties, AT. ANo and AD, for 7". No and D, respectively, is expressed by
r
=
V\
.Vo /
+
.
14)
The quoted uncertainty corresponds to the standard deviation of the mean. The present result deviates from the recently reported longer half-lifes, but agrees very well with the value of Lagoutine et al. (I t~ References
I. Aum,~ O.. BAaRE'rTEJ., BAaRE'r'rE M. and MONAaO S. Nucl. Instrum. Methods 76. 93 (1969). 2. MOWAT'rR. S. Can. d. Phys. 48, 2606 (1970). 3. NOTEA A. and ELIAS E. Nucl. lnstrum. Methods ~ , 269 (19701. 4. McNELLES L. A. and CAMPBELL J. L. Nucl. Instrum. Methods 109, 241 (1973). 5. GEHRKE R. J., HELMER R. O. and Ga~WOOD R. C. Nucl. Instrum. Methods. 147, 405 (19771. 6. DEBERTI~"K. Nucl. lnstrum. Methods 158, 479 (1979). 7. SCH(STZlG U., DEBEaTL~ K. and WALZ K. F. Nucl. Instrum. Methods 169. 43 (I 980). 8. KARR^I<~a D. G, HAVt)E.~ R. J. and IXGt-IRAMM. G. Phys. Rev. 87, 901 (1952). 9. GEtC;Ea K. W. Phys. Rer. 105, 1539 (1957). 10. E~IERV J. F.. REVXOLDS S. A., Wvarr E. I. and GLEASO~"G. I. Nucl. Sci. Eng. 48, 319 (1972). 11. LAGOUTINE F., LEGRAND J. and BAC C. Int. J. Appl. Radiat. Isot. 29, 269 (1978). 12. RUTLEDGE A. R.. MERRtTT J. S. and S,~nrn L. V. Report AECL-6788, p. 45 (1980). 13. HOPPES D. D., SCHL~IA F. J, Ht;TCHt.~SO,~ J. M. R. and UNTERWEOER M. P., NBS Special Publication 626. 85 (1982). 14. BABA S., ICHIKAWA S.. SEKINE T., ISHIKAWA I. and BABA H. Nucl. lnstrum. Methods 203. 273 (1982). 15. KOMORI T.. TAMURAS.. GUNJI K. and Ooa Z. Mass Spectroscopy (Japan) 21.27 (1973).