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Decay of 131Ba131Cs
J. Inorg. Nucl. Chem., 1963, Vol. 25, pp. 1079 to 1083. Pergamon Press Ltd. Printed in Northern Ireland
DECAY OF ~ZlBa-lzlCs W. S. LYON Analytical Chemistry Division, Oak Ridge National Laboratory,* Oak Ridge, Tennessee (Received 18 February 1963; in revisedform 15 March 1963)
Abstract--The absolute intensities of eight ~,-rays emitted following electron capture in 131Ba have been determined: 0.130 MeV (0-28), 0.220 MeV (0.28), 0-380 MeV (0.16), 0.496 MeV (0.60), 0.600 MeV (0.032), 0.820 MeV (0.0034), 0.910 MeV (0.0009), and 1.04 MeV (0'017). The method used was to prepare 131Ba free from its 13xCs daughter, allow the ~alCs activity to grow in, separate the 13~Cs activity from the parent and measure its activity, measure the 7-activity of the ~a~Baby ~,-ray spectrometry, and calculate the absolute amount of tuba from parent daughter growth relationships. The half-life of xa~Cs has been determined to be 9.83 + 0.28 days. The ratio of the pile neutron activation cross-sections for the production of la~Ba and 13SBa has been found to be t~ls0/crx32= 1"36 ± 0'08. GAMMA-RAYS following electron c a p t u r e in X31Ba have been studied b y several investigators. BEGGS, et al. m identified at least nine g a m m a rays a n d m a d e m e a s u r e m e n t s o f their relative intensities. AUGUST et al. (2) m e a s u r e d relative intensities o f five o f these ~,-rays. C o n v e r s i o n coefficients for some o f these ~,-rays a n d the lifetime o f a m e t a s t a b l e state at 0.081 M e V have also been r e p o r t e d a n d are t a b u l a t e d in nuclear d a t a collections. (3) Very recently KELLY a n d HOREN t4) r e p o r t the conversion electron spectra o f 131Ba. N o m e a s u r e m e n t has b~en m a d e previously, however, o n the a b s o l u t e ),-ray intensities due p r i m a r i l y to the difficulty in m e a s u r i n g the d i s i n t e g r a t i o n rate o f lalBa. In the present work, the relative intensities o f eight y-rays associated with the d e c a y o f lSlBa have been measured. The absolute intensities have been d e t e r m i n e d b y m e a s u r i n g the active 131Cs d a u g h t e r o f lalBa a n d b a c k calculating to d e t e r m i n e the a m o u n t o f lalBa p a r e n t present. The half-lives o f b o t h 131Cs a n d 131Ba have been r e m e a s u r e d ; a n d in a d d i t i o n , the ratio o f the pile n e u t r o n activation cross-section for p r o d u c t i o n o f lSlBa a n d 13SBa (g130/tr18~) has been obtained. EXPERIMENTAL Barium-131 produced by neutron capture on natural barium in the Oak Ridge Research Reactor was used as a starting material in the barium-cesium separation work. To several millicuries of lSlBa solution, a few milligrams of barium carrier was added and BaC12 precipitated by addition of H a - e t h e r solution, tS~ The precipitate was separated by centrifugation, redissolved in a few drops of H20 and the separation repeated twice more. The time of the final BaC1, precipitation was noted. The BaCI2 was dissolved in H~O and made to a known volume. Cesium-131 was allowed to grow in for twelve days and then a barium-cesium separation was made using the following procedure. ~6~ * Operated by Union Carbide Nuclear Company for the U.S. Atomic Energy Commission. ~1) W. C. BEGGS, B. L. ROBINSONand R. W. FINK, Phys. Rev. 101, 149 (1956). t2~ L. S. AUGUST, R. W. CAMPBELLand M. GOODRICH, Bull. Amer. Phys. Soc. 2, #4, 231 (1957). t3~ D. STROMINGER,J. M. HOLLANDERand G. T. SEAaOR~, Revs. Mod. Phys. 30, 716 (1958). t~ W. H. K~LL~Yand D. J. HOR~N, Bull. Amer. Phys. Soc. 8,#1, 85 (1963). t~) C. L. BURROSand R. L. BROOKSaANK,Method No. 2 21081 (7-1-52), ORNL Master Analytical Manual, TID 7015, Section 2. ~ H. L. FINSTON. Private communication (1957). 1079
1080
W . S . LYON
A 500-/~1 aliquot of the Ba--Cs solution was added to a separatory funnel, 2 ml of a 1 M sodium citrate-0.5 M nitric acid buffer solution added, and the volume made to 3 ml with water. Three ml of a 0.05 M tetraphenyl boron in amyl acetate solution was added as an extractant, and the cesium was extracted into the organic phase by shaking for 30 see. After the phases had separated, the aqueous phase was drawn off and re-extracted with another 3 ml portion of the extractant. The two organic phases were then combined, washed with 3 ml of buffer solution and then the cesium stripped with two, 2 ml portions of 3 N HC1. The cesium fraction was made to volume in a 5 ml volumetric flask; the original aqueous phase containing the barium was likewise made to volume in a 5 ml flask. (The buffer wash and the organic phase after acid stripping were examined in every experiment for residual activity. The amount retained was in every case less than 1 per cent of the total.) One ml aliquots of the cesium and the barium dilutions were dried on 1 in. watch glasses and used as sources for the 3 in. × 3 in. NaI(T1) ),-ray spectrometer equipped with a twenty-channel analyser. In the 181Cssource mount, the K X-ray peak at 29.6 KeV was measured. In addition, the total integral count above 50 KeV was obtained and in all cases found to be background or less. This confirms the observation that 131Csdecays only by electron capture and indicates the selectivity of the extraction method and the purity of the cesium product. The tuBa source mount was used to measure the X-ray region as well as the y-rays from decay of tuBa. The ratio of the K X-ray peak to the ~31Ba496 KeV y-ray was used as a test of the purity of the barium fraction. This ratio was the same in all the separations made and was identical with that obtained from a spectrum obtained from a freshly separated exhaustively purified 13~Basource. This also confirmed the selectivity of the extraction and the purity of the aalBa product. MEASUREMENTS The integrated peak area ( P r ) o b t a i n e d from the Cs 131 K X-ray spectrum was used to calculate the disintegration rate of 131Cs (A2) using the expression:
e~
(1)
A2 - - ~ f K E ~ , A
where
f~ f K E~ A
= solid angle at 9.3 cm (0.0373) = fluorescence yield for Xe = 0.84 ---- fraction of decay t h r o u g h K cap = 0.8T 7) ---- peak efficiency = 1.00 = a t t e n u a t i o n in crystal shielding ----0.97
The activity of 131Ba p a r e n t at the initial separation time (twelve or more days p r i o r to the extraction) was then calculated in the usual m a n n e r : (22 - - ;tl)A2 ,,t 1 = 22(e_alt - - e-a2t) where
(2)
A1 = calculated 131Ba p a r e n t disintegration rate at initial separation time A 2 = observed 131Cs daughter at extraction time 21 ---- decay c o n s t a n t 131Ba 22 = decay c o n s t a n t ~3XCs t = growth time of lalCs (i.e., time between initial separation time a n d extraction).
The absolute n u m b e r (Ne) of 490 KeV y-rays in the 131Ba source was o b t a i n e d from the integrated p e a k area i n the y-ray spectrum, using the published data o f ~?~B. R. Josm and G. M. LEwis, Proc. Phys. Soc. 76, 349 (1960).
Decay of aaXBaJalCs
1081
LAZAR(8). This number (N~) is related to the disintegration rate of 131Ba(ABa) through the factor f A B ~ - Nv f
(3)
wherefis the )'-branching in the decay of 131Ba through the 496 KeV ),-ray. N / f was corrected back to the initial separation time: ABa /i I
-
-
e_a~ t
N~ -
-
(4)
fe_~x t
or
N~, f-
where
(5)
Ale_,h t
A1 = calculated lZlBa present at initial separation time t = decay time of 131Ba ( = growth time of 131Cs) 21 = decay constant of 131Ba
9"2200.496 400
~0 0.630
o
4.0
1.04 ' 0.9t0
0.1
0.0~
I 0
200 400 600 PULSE" HEIGHT
FIG. 1.--y-Ray spectrum of 13tBa. 20 KeV/channel.
f was obtained by substituting from Equation (2) the value for A 1 into Equation (5). From six determinations, each on a different day, f = 0.60 with a standard deviation of 0.03. Figure 1 shows the )'-ray spectrum of 131Ba obtained by use of the 3 in. × 3 in. NaI(Tl) crystal and the twenty-channel analyser. A considerably more active source 18~N. H. LAZAR,I.R.E. Transactions of the Professional Group on Nuclear Science, Vol. NS-5, #3, 138, December, 1958.
1082
W.S. LYON
was used to obtain these data than was used to measure the 0.496 KeV branching, since it was of interest here to determine intensities of the low abundant high energy v-rays. Successive graphical subtraction revealed v-rays as indicated in Fig, 1. Two different sources of 131Ba of different age were used to obtain these spectra; in each of the solutions a slight amount of 13aBa was present, as evidenced by the faint indication of a y-photopeak at 370 KeV. After decay of the a3tBa had proceeded for a number of months, the XaaBa contribution was corrected for by rerunning the y-ray TABLE1.--INTENSITIESOF ~.'-RAYSPRESENTIN 18XBaDECAY Relative intensities y-energy, (MeV) 0.130 0-220 0'380 0.496 0.600 0'630 0"820 0.910 1.04
y's/dis. LYON
BEOGS, AUGUST~
et al. tt~
0"73 0.55 0"36 1'00
0"56 0'42 0"28 1'00
0"03
0'09 0'005 0 019 0030
LYON 0'47 0"46 0"26 1'00 0-053
0'28 0"28 0'16 0'60 0'032
0"0057 0"0016 0.028
0"0034 0"0009 0-017
spectrum, matching the 370 KeV peak to a known t~Ba y-ray spectrum, drawing in this contribution to the lZXBa spectrum, and subtracting. Agreement between the y-ray intensities in the two sources was within a few per cent. The peak at 630 KeV is a composite of a true 600 KeV y-ray photopeak and a coincident-sum peak of 630 KeV, which arises from coincidence summing between the 130 KeV and the 496 KeV y-rays. This was established:. (1) by measurement and comparison of the y-ray spectrum obtained at 9.3 cm with that obtained at 20 cm; (2) by calculation of the contribution to the 630 KeV peak of summing in a manner similar to that described by LAZAR.~s) Both the previous techniques indicated the 630 KeV y-ray photopeak to be due to coincidence summing. Consequently, the observed photopeak intensities for both the 130 KeV and the 496 KeV y-ray have been corrected for the slight loss at 9.3 cm of photopeak pulses due to summing. These y-ray intensities are listed in Table 1, together with previously reported values; the intensities measured in this work were obtained using the method of LAZAR,~s) as previously described in the discussion o f the lZlBa determination. Absolute intensities are also shown in Table 1 and were obtained by multiplying the relative intensities in Table 1 by the experimentally determined absolute intensity value of 0.60 for the 496 KeV y-ray. Inasmuch as the irradiation time of the material and the absolute activities of the 131Ba la3Ba could be calculated, the ratio of the pile neutron activation cross-sections (~r13o/~13~) could be calculated. The activity of the l~Ba was calculated using the branching ratio of LANOEVIN/9) which has been verified in our laboratory and which assigns 75 per cent of the capture processes through the 370 KeV y-ray. The value obtained for this ratio ~130fir13~.is 1"36 -4- 0.08. BEGaS et aL ~1~ have reported a value of ~9)M. LANGEV1N,Ann. Phys. 1, 57 (1956).
Decay of ~31Ba-~a~Cs
1083
1.2 i 0.3 which was obtained using somewhat different assumptions regarding the decay schemes of 131Ba and tuBa. The y branchings for the lalBa y-rays used by BEGGSin the calculation of 131Ba activity lead to a value about 10 per cent higher than that obtained by use of data in this report. A larger difference arises in the lUBa activity calculations which was made by BEGGSet al. from a measurement of the 0.082 y-ray and use of an earlier 0cT value from HAYWARDet alJ 1°~ o f 3-5. More recent measurementstg, 11) indicate the 0~r to be ~-A-6. Measurements made in this laboratory confirm this latter figure. In addition, these authors used 7.5 years as the half-life of ~ B a . Published data from ORNL indicates the half-life of l~Ba to be 10.7 ± 0.2 yr. t~) When one calculates the ratio ~qa0/a132 by using the decay schemes and half-lives suggested in this paper, the value given by BEGGSet al. for the ratio ~1~0/~1z,,is found to be 1-50 4- 0.39; this agrees well with our value of 1.36 ± 0.08. The half-lives of both ia~Ba and 131Cs have been redetermined, 13~Ba by measurements on a high pressure gamma ionization chamber and l~XCs by graphical integration of the K X-ray photopeak obtained by use of the 3 in. × 3 in. NaI(T1) y-ray spectrometer. The half-life of XaXBais in agreement with previously published halflife of 12.0 days obtained at this laboratory, t~3~ The half-life for 131Cs was found to be 9.83 ~ 0.28 days. DISCUSSION
AND CONCLUSION
Nothing reported in this paper affects the level assignments made on the basis of coincidence and other nuclear measurements obtained in the past. Consequently, no discussion of this subject is relevant. The absolute y-branchings as listed in Table 1 should now make it possible to assay accurately ~31Ba by y-spectrometric methods. The reactor neutron cross-section for the reaction Xa°Ba(n,y)131Ba has been previously reported tm as 8.8 barns. Use of the ratio ~130/~ta2 - 1"36 yields a value for la2Ba(n, y)133Ba of 6-5 barns. ~10) R. W. HAYWARD,D. D. HOPPES and H. ERNST, Phys. Rev. 93, 916 (1954). ~m R. K. GUI'TA, S. JHA, M. C. JOSHr and B. K. MADAN, Nuovo Cimento 8, 48 (1958). (12) E. I. WYATT, S. A. R~YNOLDS, T. H. HANDLEY, W. S. LYON and H. A. PARKER,Nucl. Sci. Engng. 11, 74 (1961). tlS~ H. W. WRIGHT, E. I. WYATT, S. A. REYNOLDS,W. S. LYON and T. H. HANDLEY,Nucl. Sci. Enffng. 2, 247 (1957). ~a4~W. S. LYON, Nucl. Sci. Engng. 8, 378 (1960).
DECAY OF ~ZlBa-lzlCs W. S. LYON Analytical Chemistry Division, Oak Ridge National Laboratory,* Oak Ridge, Tennessee (Received 18 February 1963; in revisedform 15 March 1963)
Abstract--The absolute intensities of eight ~,-rays emitted following electron capture in 131Ba have been determined: 0.130 MeV (0-28), 0.220 MeV (0.28), 0-380 MeV (0.16), 0.496 MeV (0.60), 0.600 MeV (0.032), 0.820 MeV (0.0034), 0.910 MeV (0.0009), and 1.04 MeV (0'017). The method used was to prepare 131Ba free from its 13xCs daughter, allow the ~alCs activity to grow in, separate the 13~Cs activity from the parent and measure its activity, measure the 7-activity of the ~a~Baby ~,-ray spectrometry, and calculate the absolute amount of tuba from parent daughter growth relationships. The half-life of xa~Cs has been determined to be 9.83 + 0.28 days. The ratio of the pile neutron activation cross-sections for the production of la~Ba and 13SBa has been found to be t~ls0/crx32= 1"36 ± 0'08. GAMMA-RAYS following electron c a p t u r e in X31Ba have been studied b y several investigators. BEGGS, et al. m identified at least nine g a m m a rays a n d m a d e m e a s u r e m e n t s o f their relative intensities. AUGUST et al. (2) m e a s u r e d relative intensities o f five o f these ~,-rays. C o n v e r s i o n coefficients for some o f these ~,-rays a n d the lifetime o f a m e t a s t a b l e state at 0.081 M e V have also been r e p o r t e d a n d are t a b u l a t e d in nuclear d a t a collections. (3) Very recently KELLY a n d HOREN t4) r e p o r t the conversion electron spectra o f 131Ba. N o m e a s u r e m e n t has b~en m a d e previously, however, o n the a b s o l u t e ),-ray intensities due p r i m a r i l y to the difficulty in m e a s u r i n g the d i s i n t e g r a t i o n rate o f lalBa. In the present work, the relative intensities o f eight y-rays associated with the d e c a y o f lSlBa have been measured. The absolute intensities have been d e t e r m i n e d b y m e a s u r i n g the active 131Cs d a u g h t e r o f lalBa a n d b a c k calculating to d e t e r m i n e the a m o u n t o f lalBa p a r e n t present. The half-lives o f b o t h 131Cs a n d 131Ba have been r e m e a s u r e d ; a n d in a d d i t i o n , the ratio o f the pile n e u t r o n activation cross-section for p r o d u c t i o n o f lSlBa a n d 13SBa (g130/tr18~) has been obtained. EXPERIMENTAL Barium-131 produced by neutron capture on natural barium in the Oak Ridge Research Reactor was used as a starting material in the barium-cesium separation work. To several millicuries of lSlBa solution, a few milligrams of barium carrier was added and BaC12 precipitated by addition of H a - e t h e r solution, tS~ The precipitate was separated by centrifugation, redissolved in a few drops of H20 and the separation repeated twice more. The time of the final BaC1, precipitation was noted. The BaCI2 was dissolved in H~O and made to a known volume. Cesium-131 was allowed to grow in for twelve days and then a barium-cesium separation was made using the following procedure. ~6~ * Operated by Union Carbide Nuclear Company for the U.S. Atomic Energy Commission. ~1) W. C. BEGGS, B. L. ROBINSONand R. W. FINK, Phys. Rev. 101, 149 (1956). t2~ L. S. AUGUST, R. W. CAMPBELLand M. GOODRICH, Bull. Amer. Phys. Soc. 2, #4, 231 (1957). t3~ D. STROMINGER,J. M. HOLLANDERand G. T. SEAaOR~, Revs. Mod. Phys. 30, 716 (1958). t~ W. H. K~LL~Yand D. J. HOR~N, Bull. Amer. Phys. Soc. 8,#1, 85 (1963). t~) C. L. BURROSand R. L. BROOKSaANK,Method No. 2 21081 (7-1-52), ORNL Master Analytical Manual, TID 7015, Section 2. ~ H. L. FINSTON. Private communication (1957). 1079
1080
W . S . LYON
A 500-/~1 aliquot of the Ba--Cs solution was added to a separatory funnel, 2 ml of a 1 M sodium citrate-0.5 M nitric acid buffer solution added, and the volume made to 3 ml with water. Three ml of a 0.05 M tetraphenyl boron in amyl acetate solution was added as an extractant, and the cesium was extracted into the organic phase by shaking for 30 see. After the phases had separated, the aqueous phase was drawn off and re-extracted with another 3 ml portion of the extractant. The two organic phases were then combined, washed with 3 ml of buffer solution and then the cesium stripped with two, 2 ml portions of 3 N HC1. The cesium fraction was made to volume in a 5 ml volumetric flask; the original aqueous phase containing the barium was likewise made to volume in a 5 ml flask. (The buffer wash and the organic phase after acid stripping were examined in every experiment for residual activity. The amount retained was in every case less than 1 per cent of the total.) One ml aliquots of the cesium and the barium dilutions were dried on 1 in. watch glasses and used as sources for the 3 in. × 3 in. NaI(T1) ),-ray spectrometer equipped with a twenty-channel analyser. In the 181Cssource mount, the K X-ray peak at 29.6 KeV was measured. In addition, the total integral count above 50 KeV was obtained and in all cases found to be background or less. This confirms the observation that 131Csdecays only by electron capture and indicates the selectivity of the extraction method and the purity of the cesium product. The tuBa source mount was used to measure the X-ray region as well as the y-rays from decay of tuBa. The ratio of the K X-ray peak to the ~31Ba496 KeV y-ray was used as a test of the purity of the barium fraction. This ratio was the same in all the separations made and was identical with that obtained from a spectrum obtained from a freshly separated exhaustively purified 13~Basource. This also confirmed the selectivity of the extraction and the purity of the aalBa product. MEASUREMENTS The integrated peak area ( P r ) o b t a i n e d from the Cs 131 K X-ray spectrum was used to calculate the disintegration rate of 131Cs (A2) using the expression:
e~
(1)
A2 - - ~ f K E ~ , A
where
f~ f K E~ A
= solid angle at 9.3 cm (0.0373) = fluorescence yield for Xe = 0.84 ---- fraction of decay t h r o u g h K cap = 0.8T 7) ---- peak efficiency = 1.00 = a t t e n u a t i o n in crystal shielding ----0.97
The activity of 131Ba p a r e n t at the initial separation time (twelve or more days p r i o r to the extraction) was then calculated in the usual m a n n e r : (22 - - ;tl)A2 ,,t 1 = 22(e_alt - - e-a2t) where
(2)
A1 = calculated 131Ba p a r e n t disintegration rate at initial separation time A 2 = observed 131Cs daughter at extraction time 21 ---- decay c o n s t a n t 131Ba 22 = decay c o n s t a n t ~3XCs t = growth time of lalCs (i.e., time between initial separation time a n d extraction).
The absolute n u m b e r (Ne) of 490 KeV y-rays in the 131Ba source was o b t a i n e d from the integrated p e a k area i n the y-ray spectrum, using the published data o f ~?~B. R. Josm and G. M. LEwis, Proc. Phys. Soc. 76, 349 (1960).
Decay of aaXBaJalCs
1081
LAZAR(8). This number (N~) is related to the disintegration rate of 131Ba(ABa) through the factor f A B ~ - Nv f
(3)
wherefis the )'-branching in the decay of 131Ba through the 496 KeV ),-ray. N / f was corrected back to the initial separation time: ABa /i I
-
-
e_a~ t
N~ -
-
(4)
fe_~x t
or
N~, f-
where
(5)
Ale_,h t
A1 = calculated lZlBa present at initial separation time t = decay time of 131Ba ( = growth time of 131Cs) 21 = decay constant of 131Ba
9"2200.496 400
~0 0.630
o
4.0
1.04 ' 0.9t0
0.1
0.0~
I 0
200 400 600 PULSE" HEIGHT
FIG. 1.--y-Ray spectrum of 13tBa. 20 KeV/channel.
f was obtained by substituting from Equation (2) the value for A 1 into Equation (5). From six determinations, each on a different day, f = 0.60 with a standard deviation of 0.03. Figure 1 shows the )'-ray spectrum of 131Ba obtained by use of the 3 in. × 3 in. NaI(Tl) crystal and the twenty-channel analyser. A considerably more active source 18~N. H. LAZAR,I.R.E. Transactions of the Professional Group on Nuclear Science, Vol. NS-5, #3, 138, December, 1958.
1082
W.S. LYON
was used to obtain these data than was used to measure the 0.496 KeV branching, since it was of interest here to determine intensities of the low abundant high energy v-rays. Successive graphical subtraction revealed v-rays as indicated in Fig, 1. Two different sources of 131Ba of different age were used to obtain these spectra; in each of the solutions a slight amount of 13aBa was present, as evidenced by the faint indication of a y-photopeak at 370 KeV. After decay of the a3tBa had proceeded for a number of months, the XaaBa contribution was corrected for by rerunning the y-ray TABLE1.--INTENSITIESOF ~.'-RAYSPRESENTIN 18XBaDECAY Relative intensities y-energy, (MeV) 0.130 0-220 0'380 0.496 0.600 0'630 0"820 0.910 1.04
y's/dis. LYON
BEOGS, AUGUST~
et al. tt~
0"73 0.55 0"36 1'00
0"56 0'42 0"28 1'00
0"03
0'09 0'005 0 019 0030
LYON 0'47 0"46 0"26 1'00 0-053
0'28 0"28 0'16 0'60 0'032
0"0057 0"0016 0.028
0"0034 0"0009 0-017
spectrum, matching the 370 KeV peak to a known t~Ba y-ray spectrum, drawing in this contribution to the lZXBa spectrum, and subtracting. Agreement between the y-ray intensities in the two sources was within a few per cent. The peak at 630 KeV is a composite of a true 600 KeV y-ray photopeak and a coincident-sum peak of 630 KeV, which arises from coincidence summing between the 130 KeV and the 496 KeV y-rays. This was established:. (1) by measurement and comparison of the y-ray spectrum obtained at 9.3 cm with that obtained at 20 cm; (2) by calculation of the contribution to the 630 KeV peak of summing in a manner similar to that described by LAZAR.~s) Both the previous techniques indicated the 630 KeV y-ray photopeak to be due to coincidence summing. Consequently, the observed photopeak intensities for both the 130 KeV and the 496 KeV y-ray have been corrected for the slight loss at 9.3 cm of photopeak pulses due to summing. These y-ray intensities are listed in Table 1, together with previously reported values; the intensities measured in this work were obtained using the method of LAZAR,~s) as previously described in the discussion o f the lZlBa determination. Absolute intensities are also shown in Table 1 and were obtained by multiplying the relative intensities in Table 1 by the experimentally determined absolute intensity value of 0.60 for the 496 KeV y-ray. Inasmuch as the irradiation time of the material and the absolute activities of the 131Ba la3Ba could be calculated, the ratio of the pile neutron activation cross-sections (~r13o/~13~) could be calculated. The activity of the l~Ba was calculated using the branching ratio of LANOEVIN/9) which has been verified in our laboratory and which assigns 75 per cent of the capture processes through the 370 KeV y-ray. The value obtained for this ratio ~130fir13~.is 1"36 -4- 0.08. BEGaS et aL ~1~ have reported a value of ~9)M. LANGEV1N,Ann. Phys. 1, 57 (1956).
Decay of ~31Ba-~a~Cs
1083
1.2 i 0.3 which was obtained using somewhat different assumptions regarding the decay schemes of 131Ba and tuBa. The y branchings for the lalBa y-rays used by BEGGSin the calculation of 131Ba activity lead to a value about 10 per cent higher than that obtained by use of data in this report. A larger difference arises in the lUBa activity calculations which was made by BEGGSet al. from a measurement of the 0.082 y-ray and use of an earlier 0cT value from HAYWARDet alJ 1°~ o f 3-5. More recent measurementstg, 11) indicate the 0~r to be ~-A-6. Measurements made in this laboratory confirm this latter figure. In addition, these authors used 7.5 years as the half-life of ~ B a . Published data from ORNL indicates the half-life of l~Ba to be 10.7 ± 0.2 yr. t~) When one calculates the ratio ~qa0/a132 by using the decay schemes and half-lives suggested in this paper, the value given by BEGGSet al. for the ratio ~1~0/~1z,,is found to be 1-50 4- 0.39; this agrees well with our value of 1.36 ± 0.08. The half-lives of both ia~Ba and 131Cs have been redetermined, 13~Ba by measurements on a high pressure gamma ionization chamber and l~XCs by graphical integration of the K X-ray photopeak obtained by use of the 3 in. × 3 in. NaI(T1) y-ray spectrometer. The half-life of XaXBais in agreement with previously published halflife of 12.0 days obtained at this laboratory, t~3~ The half-life for 131Cs was found to be 9.83 ~ 0.28 days. DISCUSSION
AND CONCLUSION
Nothing reported in this paper affects the level assignments made on the basis of coincidence and other nuclear measurements obtained in the past. Consequently, no discussion of this subject is relevant. The absolute y-branchings as listed in Table 1 should now make it possible to assay accurately ~31Ba by y-spectrometric methods. The reactor neutron cross-section for the reaction Xa°Ba(n,y)131Ba has been previously reported tm as 8.8 barns. Use of the ratio ~130/~ta2 - 1"36 yields a value for la2Ba(n, y)133Ba of 6-5 barns. ~10) R. W. HAYWARD,D. D. HOPPES and H. ERNST, Phys. Rev. 93, 916 (1954). ~m R. K. GUI'TA, S. JHA, M. C. JOSHr and B. K. MADAN, Nuovo Cimento 8, 48 (1958). (12) E. I. WYATT, S. A. R~YNOLDS, T. H. HANDLEY, W. S. LYON and H. A. PARKER,Nucl. Sci. Engng. 11, 74 (1961). tlS~ H. W. WRIGHT, E. I. WYATT, S. A. REYNOLDS,W. S. LYON and T. H. HANDLEY,Nucl. Sci. Enffng. 2, 247 (1957). ~a4~W. S. LYON, Nucl. Sci. Engng. 8, 378 (1960).