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Half-lives of 90Sr and 137Cs
J. laorg. Natl. Chem., 196~. Vol. 27. pp. 21 to 23. Pergamon Press Ltd. Printed in Northern Ireland
HALF-LIVES OF °°Sr AND 157Cs* K. F. FLYNN, L. E. GLENDENIN, A. L. H A R K ~ and E. P. S I F r . ~ o Argonne National Laboratory, Argonne, Illinois (Received 18 May 1964)
Abaract--The half-fives of ,.sr and , - c s have been measured both by specific activity and by direct decay over periods of 6-10 years. The results of the decay measurements are 28.0 ± 0.4 years for "Sr and 29.9 -4-0.5 years for z'Cs; the values obtained by spec/fic activity measurements are 28.5 -4- 0.9 years for 'eSr and 30.9 4- 0.7 years for :"Cs. Recommended "best" values for these half.fives b~___,,~_on current literature values are 28.1 -4- 0.3 years for 'eSr and 30.0 -4-0.2 years for I|TCs .
WxJ.Es and TOMUNSON(x) have reported a value of 27.7 -4- 0.4 years for the half-life of 'eSr. The/r determination was based on a specific activity measurement using a /~-proport/onal counter to measure the activity and a mass spectrometric, isotope dilution technique to determine the number 0 f ' ° S r atoms. Several investigators have measured the half-life o f ~TCs(S--l°) using this method as well as others which will be described later. The importance o f an accurate determination of the half-life of ~ C s has been pointed out by LYON in his report on the Availability and Use o f Radioactivity Standards. (xx) ~ r is not used for a standard as extensively as a~Cs, but an accurate determination of its half-life is o f s0me~interest. This paper describes determinations of the half-lives o f 'eSr and ~a~Cs both by specific activity measurements and by decay measurements over a period of several years. A summary o f the current literature values and recommended " b e s t " average values for the half-lives are given. EXPERIMENTAL Aged fission product material was used as' the source of seSr and xs'Cs. Decay measurements wece made with radiochemicatly purified samples evaporated on platinum disks. The samples were counted st about three month intervals for a period.of ten years using a 2~r #-proportional counter. A "T¢ standard (half-life 2.1 × "I0' yrs) was used to ensure reliable counter behaviour. The '*St was free of contaminating radioactivity; however, a trivial correction for luCs was applied to the I"Cs data. The amount of ~uCs present initially was determined by ~-analysis using pure a4~s standards of known p.activity and found to ~ 0.4 per oent of the Cs #-activity. The decay data were fed to an IBM 704 computer programmed for least squares analysis. • Based on work performed under the auspices of the U.S. Atomic Energy Commission. ci~D. M. W,~i and R. H. T O ~ N , Canad. J. Phys. 33, 133 (1955). ('~ F. Bl[OWN,G. R. HAIL and A. J. W A L ~ , J. Inarg. Nucl. Chem. I, 241 (1955). c,~ H. Fx_.mu~,A. K. DASOuI'rAand R. H. T o ~ N , Canad. J. Chem. 39, 681 (1961). c,) M. P. ~ v , A. I. Gaxv~ovx, B. A. ZAui~, and V. A. ~ , At. Ency. I0, 622 (1961). (' ) H. D . Cool[, Cmtlinburg Conference on Analytical Chemistry In Nuclear Reactor Technolooe7 (1962). (') D. G. 1 ~ , I. V. Bu-aov~x and V. P. N ~ r n o v , At. ~ r g . 13, 592 (1962). c7~S. G. Goaazcs, W. E. Ku-Nz and A. E. NAStI,Nucleonics 21, 63 (1963). c,) L. A. Dnn'z, C. F. PAat~Jcgx and G, A. LAin), Analyt. Chem. 35, 797 (1963). c,) B. F. RIDn, J. P. ~ N , Jlt. and C. P. Rmz, Nuci. $¢L/~g~. 1S, 284 (1963). cle) S. A. R]rrt~i2~, Oak Ridge Quarterly Report ORNL-3537, p. 77 (1963). c:1~W. S. LYON, Analyt. Chem. 36, 31A (1964). 21
22
K . F . FLYNN, L. E. GLENDENn~,A. L. I-IAm~ESSand E. P. STeINB~O
The samples used for the specific activity measurements were radiochemically purified by estabfished carrier free procedures.~l~,TM Aliquots were counted in a 4~rE-proportional counter to determine the absolute disintegration rate, The ~Sr was counted in equilibrium with its 64 hr daughter ~Y. The observed counting rate of xa~Cswas corrected for the presence of la~"Ba(in equilibrium with the x*'Cs) and for the small amount of xuCs as determined by 7-analysis. The number of atoms present in the samples was determined by standard isotope dilution techniques using a 60° sector field mass spectrometer. RESULTS AND DISCUSSION The results o f the direct decay measurements for b o t h tOSr and lS~Cs are given in Table 1. The stated errors are the least squares errors as given by the 704 computer. TABLEI.--HAI~-LIV~ OF 'eSr and x*TCs~Y DECAYMEASUREMENTS Decay time (years)
Number of points
Half-life (years)
NSr
10.3
$0
28.0 -4- 0.4
xsTCs-I xITCs-2
10"2 6-6
50 29 Weighted average
29.3 4- 0"6 31.4 4- 0.9 .29.9 4- 0.5
Sample
The results o f the specific activity measurements are given in Table 2. Each value listed in the table represents an independent fission p r o d u c t source and specific activity determination. The errors are the standard deviations o f the measurements. TABLe 2.--HALr-L~V~ OF '°Sr AND l|TCs "BY SPECIFIc MEASUREMENTS
Sample
Half-life (years)
'eSr
28.5 4- 0.9
ls~cs-I 1*7Cs-2 tS~Cs-3 xs~Cs-4 tsTCs-5 xSTCs-6 1'~Cs-7 l*'Cs-8 lt~Cs-9 xs~cs-10 Average
31 "0 30"9 32"0 30.2 30-7 32"0 31"8 29-7 30"3 30"7 30.9 -4- 0.7
A s u m m a r y o f the current literature values for the half-lives o f 9eSr and ~ C s is given in Table 3. Older values with large uncertainties cx~le~ and values clearly in cxl~ L. E. GLSNDENIN, Radio Chemical Series: Fission Product N.N.E.S. IV, Vol. 9, p. 1466, McGraw-
Hill, New York (1951). clij E. K. HYDE, J. Amer. Chem. So¢. 74, 4181 (195. ~x,~L. E. GLENDENmand g. P. METCALF,Radio Chemical Series: Fission Product N.N.E.S. IV,
Vol. 9, p. 1067, McGraw-Hill, New York (1951). ~xsJD. M. WILES,B. W. SMITH, R. HORSLI~Yand H. G. THODI~,Canad..r. Phys. 31,419 (1953). c1,~L. E. GL~NDE~ and C. D. CORYELL,Radio Chemical Series: Fission Product N.N.E.S. IV, Vol. 9, p. 687, McGraw-Hill, New York (1951).
Half-lives of *eSrand I'TCs
23
error (1~,1s) have not been included in the table. The recommended "best" values of the half-lives were calculated by simple averaging with no attempt to weigh the various determinations. The errors o n t h e averages are the standard error of the mean of the values listed. The eleven determinations for the half-life of 18~Cs have a maximum deviation of 3 per cent from the mean with a standard deviation of 2 per cent. For ~°Sr the three
~r
laTCs
T~LE 3.--SUMMARYOF SeSrand talCs HALFoLIV~ Reference Method Half-fife (years) 1 Specific activity (47rfl and mass 27-7 4- 0-4 spectrometer) This work Specificactivity (4¢rfl and mass 28.5 4- 0"9 spectrometer) This work E-decay(10 years, 2~r fl proportional 28.0 4- 0"4 counter) Recommended "best" value 28.1 4- 0.3 Specific activity (4,r ~ and mass 30.0 ± 0"4 2 spectrometer) lSTBagrowth (mass spectrometer) 30.4 -4- 0"4 3 Specific activity (4~ ~ and mass 29 4- 1 4 spectrometer) Specific activity (,br fl and mass 29.40 4- 0"18 5 spectrometer) Specificactivity (liquid scintillator 30-1 4- 0.7 6 and mass spectrometer) /3-decay(3 years, ionization 29.68 4- 0.05 7 chamber-electrometer) Decay (1.5 years, mass spectrometer) 30.35 4- 0.38 8 l'TBa growth (mass spectrometer) 29.2 4- 0.3 9 ~,-decay(4 years) 30.7 4- 0.6 10 Specificactivity (4~r~ and mass 30.9 4- 0.7 This work spectrometer) decay (10 years, 2~ fl pro29.9 4- 0-5 This work portioned counter) Recommended "best" value 30.0 4- 0.2 /
determinations have a standard deviation of 1.5 per cent. These deviations reflect the uncertainties involved in measurements of this type. Thespecific activity determinations are limited by uncertainties in both absolute beta counting and mass spectrometric isotope dilution techniques, whereas the laTBa growth measurements are limited only by the errors in the latter. The direct decay measurements are limited by the accuracy with which one can measure small changes in activity. It is interesting to note that the average values for lsTCs determined by the three different techniques are 30.2 4 - 0 . 4 y r s (direct decay), 29.9 4-0.6 years (specific activity), and 29"8 40.6 years (lSTBa growth). The good agreement among these values lends confidence to the recommended "best" value given in Table' 3. Similar good agreement is observed among the three 9°St half-life determinations.
Acknowledgements--The authors wish to thank Dr. R. J. HAYDENfor some of the early mass spectrometric measu~ments. ~1,~D. M. WILESand R. H. TOMIANSON,Phys. Rev. 99, 188 (19/55). ~1,~R. I. PowEns and A. F. VOIOT,Phys. Rev. 79, 175 (1950).
HALF-LIVES OF °°Sr AND 157Cs* K. F. FLYNN, L. E. GLENDENIN, A. L. H A R K ~ and E. P. S I F r . ~ o Argonne National Laboratory, Argonne, Illinois (Received 18 May 1964)
Abaract--The half-fives of ,.sr and , - c s have been measured both by specific activity and by direct decay over periods of 6-10 years. The results of the decay measurements are 28.0 ± 0.4 years for "Sr and 29.9 -4-0.5 years for z'Cs; the values obtained by spec/fic activity measurements are 28.5 -4- 0.9 years for 'eSr and 30.9 4- 0.7 years for :"Cs. Recommended "best" values for these half.fives b~___,,~_on current literature values are 28.1 -4- 0.3 years for 'eSr and 30.0 -4-0.2 years for I|TCs .
WxJ.Es and TOMUNSON(x) have reported a value of 27.7 -4- 0.4 years for the half-life of 'eSr. The/r determination was based on a specific activity measurement using a /~-proport/onal counter to measure the activity and a mass spectrometric, isotope dilution technique to determine the number 0 f ' ° S r atoms. Several investigators have measured the half-life o f ~TCs(S--l°) using this method as well as others which will be described later. The importance o f an accurate determination of the half-life of ~ C s has been pointed out by LYON in his report on the Availability and Use o f Radioactivity Standards. (xx) ~ r is not used for a standard as extensively as a~Cs, but an accurate determination of its half-life is o f s0me~interest. This paper describes determinations of the half-lives o f 'eSr and ~a~Cs both by specific activity measurements and by decay measurements over a period of several years. A summary o f the current literature values and recommended " b e s t " average values for the half-lives are given. EXPERIMENTAL Aged fission product material was used as' the source of seSr and xs'Cs. Decay measurements wece made with radiochemicatly purified samples evaporated on platinum disks. The samples were counted st about three month intervals for a period.of ten years using a 2~r #-proportional counter. A "T¢ standard (half-life 2.1 × "I0' yrs) was used to ensure reliable counter behaviour. The '*St was free of contaminating radioactivity; however, a trivial correction for luCs was applied to the I"Cs data. The amount of ~uCs present initially was determined by ~-analysis using pure a4~s standards of known p.activity and found to ~ 0.4 per oent of the Cs #-activity. The decay data were fed to an IBM 704 computer programmed for least squares analysis. • Based on work performed under the auspices of the U.S. Atomic Energy Commission. ci~D. M. W,~i and R. H. T O ~ N , Canad. J. Phys. 33, 133 (1955). ('~ F. Bl[OWN,G. R. HAIL and A. J. W A L ~ , J. Inarg. Nucl. Chem. I, 241 (1955). c,~ H. Fx_.mu~,A. K. DASOuI'rAand R. H. T o ~ N , Canad. J. Chem. 39, 681 (1961). c,) M. P. ~ v , A. I. Gaxv~ovx, B. A. ZAui~, and V. A. ~ , At. Ency. I0, 622 (1961). (' ) H. D . Cool[, Cmtlinburg Conference on Analytical Chemistry In Nuclear Reactor Technolooe7 (1962). (') D. G. 1 ~ , I. V. Bu-aov~x and V. P. N ~ r n o v , At. ~ r g . 13, 592 (1962). c7~S. G. Goaazcs, W. E. Ku-Nz and A. E. NAStI,Nucleonics 21, 63 (1963). c,) L. A. Dnn'z, C. F. PAat~Jcgx and G, A. LAin), Analyt. Chem. 35, 797 (1963). c,) B. F. RIDn, J. P. ~ N , Jlt. and C. P. Rmz, Nuci. $¢L/~g~. 1S, 284 (1963). cle) S. A. R]rrt~i2~, Oak Ridge Quarterly Report ORNL-3537, p. 77 (1963). c:1~W. S. LYON, Analyt. Chem. 36, 31A (1964). 21
22
K . F . FLYNN, L. E. GLENDENn~,A. L. I-IAm~ESSand E. P. STeINB~O
The samples used for the specific activity measurements were radiochemically purified by estabfished carrier free procedures.~l~,TM Aliquots were counted in a 4~rE-proportional counter to determine the absolute disintegration rate, The ~Sr was counted in equilibrium with its 64 hr daughter ~Y. The observed counting rate of xa~Cswas corrected for the presence of la~"Ba(in equilibrium with the x*'Cs) and for the small amount of xuCs as determined by 7-analysis. The number of atoms present in the samples was determined by standard isotope dilution techniques using a 60° sector field mass spectrometer. RESULTS AND DISCUSSION The results o f the direct decay measurements for b o t h tOSr and lS~Cs are given in Table 1. The stated errors are the least squares errors as given by the 704 computer. TABLEI.--HAI~-LIV~ OF 'eSr and x*TCs~Y DECAYMEASUREMENTS Decay time (years)
Number of points
Half-life (years)
NSr
10.3
$0
28.0 -4- 0.4
xsTCs-I xITCs-2
10"2 6-6
50 29 Weighted average
29.3 4- 0"6 31.4 4- 0.9 .29.9 4- 0.5
Sample
The results o f the specific activity measurements are given in Table 2. Each value listed in the table represents an independent fission p r o d u c t source and specific activity determination. The errors are the standard deviations o f the measurements. TABLe 2.--HALr-L~V~ OF '°Sr AND l|TCs "BY SPECIFIc MEASUREMENTS
Sample
Half-life (years)
'eSr
28.5 4- 0.9
ls~cs-I 1*7Cs-2 tS~Cs-3 xs~Cs-4 tsTCs-5 xSTCs-6 1'~Cs-7 l*'Cs-8 lt~Cs-9 xs~cs-10 Average
31 "0 30"9 32"0 30.2 30-7 32"0 31"8 29-7 30"3 30"7 30.9 -4- 0.7
A s u m m a r y o f the current literature values for the half-lives o f 9eSr and ~ C s is given in Table 3. Older values with large uncertainties cx~le~ and values clearly in cxl~ L. E. GLSNDENIN, Radio Chemical Series: Fission Product N.N.E.S. IV, Vol. 9, p. 1466, McGraw-
Hill, New York (1951). clij E. K. HYDE, J. Amer. Chem. So¢. 74, 4181 (195. ~x,~L. E. GLENDENmand g. P. METCALF,Radio Chemical Series: Fission Product N.N.E.S. IV,
Vol. 9, p. 1067, McGraw-Hill, New York (1951). ~xsJD. M. WILES,B. W. SMITH, R. HORSLI~Yand H. G. THODI~,Canad..r. Phys. 31,419 (1953). c1,~L. E. GL~NDE~ and C. D. CORYELL,Radio Chemical Series: Fission Product N.N.E.S. IV, Vol. 9, p. 687, McGraw-Hill, New York (1951).
Half-lives of *eSrand I'TCs
23
error (1~,1s) have not been included in the table. The recommended "best" values of the half-lives were calculated by simple averaging with no attempt to weigh the various determinations. The errors o n t h e averages are the standard error of the mean of the values listed. The eleven determinations for the half-life of 18~Cs have a maximum deviation of 3 per cent from the mean with a standard deviation of 2 per cent. For ~°Sr the three
~r
laTCs
T~LE 3.--SUMMARYOF SeSrand talCs HALFoLIV~ Reference Method Half-fife (years) 1 Specific activity (47rfl and mass 27-7 4- 0-4 spectrometer) This work Specificactivity (4¢rfl and mass 28.5 4- 0"9 spectrometer) This work E-decay(10 years, 2~r fl proportional 28.0 4- 0"4 counter) Recommended "best" value 28.1 4- 0.3 Specific activity (4,r ~ and mass 30.0 ± 0"4 2 spectrometer) lSTBagrowth (mass spectrometer) 30.4 -4- 0"4 3 Specific activity (4~ ~ and mass 29 4- 1 4 spectrometer) Specific activity (,br fl and mass 29.40 4- 0"18 5 spectrometer) Specificactivity (liquid scintillator 30-1 4- 0.7 6 and mass spectrometer) /3-decay(3 years, ionization 29.68 4- 0.05 7 chamber-electrometer) Decay (1.5 years, mass spectrometer) 30.35 4- 0.38 8 l'TBa growth (mass spectrometer) 29.2 4- 0.3 9 ~,-decay(4 years) 30.7 4- 0.6 10 Specificactivity (4~r~ and mass 30.9 4- 0.7 This work spectrometer) decay (10 years, 2~ fl pro29.9 4- 0-5 This work portioned counter) Recommended "best" value 30.0 4- 0.2 /
determinations have a standard deviation of 1.5 per cent. These deviations reflect the uncertainties involved in measurements of this type. Thespecific activity determinations are limited by uncertainties in both absolute beta counting and mass spectrometric isotope dilution techniques, whereas the laTBa growth measurements are limited only by the errors in the latter. The direct decay measurements are limited by the accuracy with which one can measure small changes in activity. It is interesting to note that the average values for lsTCs determined by the three different techniques are 30.2 4 - 0 . 4 y r s (direct decay), 29.9 4-0.6 years (specific activity), and 29"8 40.6 years (lSTBa growth). The good agreement among these values lends confidence to the recommended "best" value given in Table' 3. Similar good agreement is observed among the three 9°St half-life determinations.
Acknowledgements--The authors wish to thank Dr. R. J. HAYDENfor some of the early mass spectrometric measu~ments. ~1,~D. M. WILESand R. H. TOMIANSON,Phys. Rev. 99, 188 (19/55). ~1,~R. I. PowEns and A. F. VOIOT,Phys. Rev. 79, 175 (1950).