- Email: [email protected]
Thermal neutron fission cross-section of 229Th
J. laofg. Nutcl.Chem., 1960,V0/ 15, pp. I to$. ~ P n ~ L t d ,
THERMAL
NEUTRON
FISSION
CROSS-SECTION
O F Z~sTh*
J. E. G m v L ~ K. F. FLYNN and J. GRAY, JR. Argonne National Laboratory, Lemont, Illinois
(Receioed 21 October 1959) Abstract--The fission cross-section of r*rl'h for thermal neutrons has been determined by counting samples placed in the thermal column of the Argonne heterogeneous heavy-water reactor, CP-5 Comparison of the fission counting rates of nsTh samples with those of m U and nspu standards gave a cross-section value of 30.5 4- 3 barns.
THE thermal neutron fission cross-section of ~gTh has been .reported to be 45 barns within 25 per cent. tl) Preliminary work by the authors of this paper indicated the lower limit of the above value to be more nearly correct. The following is a description of an experiment designed to assign a more definitive value for this cross-section. EXPERIMENTAL
Chemical separation Thorium-229 was separated from its uranium parent, zuU, by adsorption from a 1 M H N O , solution onto a cation exchange column and subsequent elution by means of a 0.5 M ammonium citrate solution at a p H of 6.t Following the initial separation the thorium was subjected to an extensive chemical purification to assure removal of any other fissionable elements that may have been present. The citrate solution containing the thorium was evaporated with aqua regia to destroy the citrate and the thorium carried on a LaFs precipitate. This precipitate was converted to the hydroxide and dissolved in concentrated HC1 containing a trace of H N O , . This solution was then passed through a series of three Dowex A-I anion exchange columns to adsorb any uranium and/or plutonium. The thorium contained in the column eluant was co-precipitated with LaFs and converted to the hydroxide. Upon dissolution with concentrated HCI the thorium was extracted with a 10 per cent solution of mono-octyl phosphate in toluene. For further purification from uranium the organic solution was scrubbed three times with concentrated H a , once with a 1 M I-INO3-0'I M NaBrOs solution and finally two more times with HCI. c*~ The thorium was re-extracted with 4 M HF-0.5 M HC1 solution after dilution of the organic phase with tri-butyl phosphate. Following evaporation of the fluoride solution to near dryness the thorium was dissolved in concentrated HCI containing a trace of HNOa and again passed through a series of throe anion exchange columns. The eluant from the final column was evaporated just to dryness with HNOs and then taken up in 0-1 M HNOs. A portion of this solution was extracted into a 0.2 M thenoyl trifluoroacetone (TrA)--henzene solution. Sample no. 1 was prepared by deposition of the T I ' A solution onto a platinum disk ~ in. diameter and 0-005 in. thick. The disk was heated to redness in an induction heater to remove the organic deposit. The remainder of the thorium was further purified by repetition of some of the above procedures: a LaFs co-precipitation, conversion to the hydroxide and passage through another series of three anion exchange columns, co-precipitation with La(OH)s, dissolution with 0.1 M H N O , and extraction with a TrA-benzene solution. A second sample (no. 2) was prepared from this TFA * Based on work performed under the auspices of the U.S. Atomic Energy Commission. 1"The initial~ separation of thorium daughter--uranium parent was made several years prior to the present experiment. ~s~ i,~ M. H. STUVI~, A. Gmoa~., F. HAOEMANN,TID-5223 (Pt. 2) Paper No. 9.1 (1952). ~2~L. B. MAONUSSON. Private communication (1952). ts) D. F. PEPPARD,G. W. MASONand R. J. S l n o ~ , J. lnorg. Nucl. Chem. 10, 117 (1959). 1 1
2
J . E . GINDLER, K. F. FLYNN and J. GRAY, JR.
solution. Sample no. 3 was prepared by stripping the thorium from the TTA-benzene solution with nitric acid and volatilizing it onto a masked platinum disk.
Fission measurements The "-activity o f =srI'h in each sample was determined by "-pulse analysis and total ~,-counting. The number o f atoms of n r l ' h was calculated from its "-activity, an "-half-life o f 7340 years~'~ and the ,,-counter efficiency.* The results o f this calculation are given in Table 1. Standard samples o f nsPu and m U were prepared from stock solutions.t Mass spectrometric analyses o f these solutions are given in Table 2. The number o f atoms in each standard sample was calculated from the ,,-activity, the ,,-counter eff~iency* and the half-life o f the respective nuclide. A half-life of 24,360 years 'sj was used for nSPu and 1-626 × 106 years m for m U . These results are also given in Table 1. TABLB I.--DFI'ERI~HNATION OF ~ Sample
I 2 3
A t o m s of ss~I"h 6"70 6"70 6"03 6"03 2"92
X X X X X 2"92 ×
Net fissions (c.p.m.)
Standar I
I0zs 48,500 50,000 I0z' I014 172,000 I014 171,600 I0x4 211,500 10tt 218,000
FI~ION CRO~-$ECTION OF "=VI'h Net fissions (c.p.m.)
A t o m s of standard
s= U
13.85 X I0zs
sSgpU mU **spU tn U s3Spu
8.88 13.85 8-88 13.85
X I0zs X I0Is X 10ss X 10zs
8"88 X 10is
Approximate flux (neutronslcm-Ssec-1)
175,400 175,600 67,100 64,700 176,100 176,200
4.0 4-0 1.5 1.5 4-0 4.0
× X × × X ×
10u 10n 10xx 10al 1011 10~l
~='Th)
(barns) 30"1 30"2 31"1 31"3 30"0 30"2 Av. 30"5
TAei.e 2 . - - M ~ s SPECTROMETRIC ANALYSIS OF STOCK SOLUTIONS USED TO PREPARE FI_~ION COUNTING STANDARDS Solution
Nuclides
sssw
=asU satU su U ss6U nSpU s40pu ~lpu
=spu
Per cent composition by mass 98"33 0"127 0"0102 1'53 99-972 0"028
-t- 0-02 + 0.002 + 0.0004 + 0"02 ± 0.001 -4- 0.001
<0.0007
Fission measurements were made in the thermal colunm of the Argonne heavy-water pile, CP-5, using a double ionization chamber. The chamber permitted the counting o f two samples, one of thorium and a standard, simultaneously. The two samples were placed back-to-back in the chamber * Three total "-counters were used during the course of the experiment. The counting efficiency of one had been determined~s) to be 51.6 per cent for an UlAm sample evaporated upon a platinum disk. The other two counters were intercalibrated with this counter using a plutonium standard. This intercalibration gave efficiencies of 50.6 per cent for the counter used for tile thorium samples and 51-6 per cent for the counter used for both the m U and m P u standards. The extent to wluch this intercalibration is correct assuming the counting efficiency for plutonium and the other nuclides is the same as that for U~Am, determines in part the accuracy of the experiment. 1"The stock solutions and standard plates were prepared by F. T. H A o ~ m ~ . c4~ F. HAODt~,tCN,L. I. KATZIN, M. H. Szuvreg, O. T. S~dtmto and A. GmoRso, Phys. Re,= 79, 435 (1950) (s) W. C. B~N'rLV. Private communication (1957). ¢~) J. C. WAI.L~U~, Thesis, University of California Radiation Laboratory Report UCRL-1255 (1951) (unpublished). ~7) Yx. P. Doguct~Yev and I. S. Osleov, Atomnaya Energiya 6, 73, (1959).
Thermal neutron fission cross-section of ~°Th
3
perpendicular to the longitudinal axis of the thermal column such that the sample positioned nearest the core of the pile received a neutron flux 1.017 times greater than the other sample. Measurements were made at two flux positions, one of approximately 4 × 10~t neutrons cm-2sec-: and the other of approximately 1-5 x 10at neutrons cm-Zsec-~. The lower flux was used with the larger thorium sample (no. 2) to reduce the fission counting-rate to a level such that coincidence losses were less than 2 per cent. The thermal nature of the neutrons had been determined previously by PrAFF and SHELrON~s~ who measured the activities induced in bare and cadmium covered gold foils located at various positions in the column. The ratio of the activities of bare to covered gold foils was found to be 1.2 × 13s and 1.4 × l0s for the high and the low flux positions, respectively. Background counting-rates determined with a blank platinum disk in the chamber were small compared to those obtained with either the thorium samples or the ~3sUand ~3Wustandards. With the high neutron flux the background in chamber A was 270 c.p.m, and in B 133 c.p.m. With the low flux, chamber A had a background of 140 c.p.m, and B 74 c.p.m. The fission counting-rates corrected for coincidence loss, sample position, and background are given in Table 1. RESULTS The fission cross-section of Z29Th for thermal n e u t r o n s has been calculated from the relation O'rh :
F~h NT h
Nstaasta Fsta '
where N is the n u m b e r o f atoms of the fissioning nuclide a n d F is the fission c o u n t i n g rate with reference to t h o r i u m (Th) or s t a n d a r d (std). Fission cross-section values o f 527 b a r n s tg~ for z ~ U a n d 746 b a r n s tg~ for 2agPu were used. The latter cross-section was multiplied b y a factor o f 1.075 tl°l to correct for the fact that the ~ g P u fission cross-section is n o t 1/v in the thermal region a n d that the n e u t r o n d i s t r i b u t i o n is assumed to be Maxwellian. T a b l e 1 lists the calculated fission cross-section of ~2~l'h for thermal n e u t r o n s for each o f six measurements. T h e average value o f this cross-section is 30.5 b a r n s a n d has a precision of 0.5 b a r n . The absolute error is estimated to be less t h a n 10 per cent or 3 barns. This includes errors in c o u n t i n g because o f b a c k g r o u n d , coincidence loss, sample position a n d c o u n t e r efficiency as well as the statistical error, errors in the half-life d e t e r m i n a t i o n of the various nuclides, a n d errors in the fission cross-section of the standards. LEBEDEV a n d KALASHNIKOVA(11) have shown recently that slow n e u t r o n s with 0-5 eV to several tens o f electron volts have a greater effect o n the fission of 229Th t h a n they do o n 2~U. I n view of this it w o u l d be o f great interest to determine the fission cross-section of 22~I'h as a function of n e u t r o n energy.
Acknowledgements--The authors would like to thank Dr. L. B. MAGNUSSONfor the ~2~Thused in the experiment, Dr. F. T. HAOEML~ who furnished the standard samples, and Mr. D. J. HENDERSON who made the or-pulseanalyses. ta~ E. R. PVAFrand R. D. S~mLTON,Wright Air Development Command Technical Report 57-361, Vol.I, ASTIA Document No. AD 155789, Appendix III, pp. 707-729 (1958) (unpublished). ct~ D. J. HUGHESand R. B. SCHWARTZ,Neutron Cross-sections, Brookhaven National Laboratory Report BNL 325, (2nd Ed.), July, 1958 [Superintendent of Documents, U.S. Government Printing Office, Washington 25, D.C. (1958)]. tl0~ D. J. HUGHESand J. A. HAKVEY,Neutron Cross-sections, Brookhaven National Laboratory Report BNL-325, July, 1955 [Superintendentof Documents, U.S. Government Printing Office, Washington 25, D.C. (1955)I. ~n) V. I. LEn~D~V and V. I. K~LASm, aKOVA, Soviet Physics JETP 35, (8), 370 (1959).
THERMAL
NEUTRON
FISSION
CROSS-SECTION
O F Z~sTh*
J. E. G m v L ~ K. F. FLYNN and J. GRAY, JR. Argonne National Laboratory, Lemont, Illinois
(Receioed 21 October 1959) Abstract--The fission cross-section of r*rl'h for thermal neutrons has been determined by counting samples placed in the thermal column of the Argonne heterogeneous heavy-water reactor, CP-5 Comparison of the fission counting rates of nsTh samples with those of m U and nspu standards gave a cross-section value of 30.5 4- 3 barns.
THE thermal neutron fission cross-section of ~gTh has been .reported to be 45 barns within 25 per cent. tl) Preliminary work by the authors of this paper indicated the lower limit of the above value to be more nearly correct. The following is a description of an experiment designed to assign a more definitive value for this cross-section. EXPERIMENTAL
Chemical separation Thorium-229 was separated from its uranium parent, zuU, by adsorption from a 1 M H N O , solution onto a cation exchange column and subsequent elution by means of a 0.5 M ammonium citrate solution at a p H of 6.t Following the initial separation the thorium was subjected to an extensive chemical purification to assure removal of any other fissionable elements that may have been present. The citrate solution containing the thorium was evaporated with aqua regia to destroy the citrate and the thorium carried on a LaFs precipitate. This precipitate was converted to the hydroxide and dissolved in concentrated HC1 containing a trace of H N O , . This solution was then passed through a series of three Dowex A-I anion exchange columns to adsorb any uranium and/or plutonium. The thorium contained in the column eluant was co-precipitated with LaFs and converted to the hydroxide. Upon dissolution with concentrated HCI the thorium was extracted with a 10 per cent solution of mono-octyl phosphate in toluene. For further purification from uranium the organic solution was scrubbed three times with concentrated H a , once with a 1 M I-INO3-0'I M NaBrOs solution and finally two more times with HCI. c*~ The thorium was re-extracted with 4 M HF-0.5 M HC1 solution after dilution of the organic phase with tri-butyl phosphate. Following evaporation of the fluoride solution to near dryness the thorium was dissolved in concentrated HCI containing a trace of HNOa and again passed through a series of throe anion exchange columns. The eluant from the final column was evaporated just to dryness with HNOs and then taken up in 0-1 M HNOs. A portion of this solution was extracted into a 0.2 M thenoyl trifluoroacetone (TrA)--henzene solution. Sample no. 1 was prepared by deposition of the T I ' A solution onto a platinum disk ~ in. diameter and 0-005 in. thick. The disk was heated to redness in an induction heater to remove the organic deposit. The remainder of the thorium was further purified by repetition of some of the above procedures: a LaFs co-precipitation, conversion to the hydroxide and passage through another series of three anion exchange columns, co-precipitation with La(OH)s, dissolution with 0.1 M H N O , and extraction with a TrA-benzene solution. A second sample (no. 2) was prepared from this TFA * Based on work performed under the auspices of the U.S. Atomic Energy Commission. 1"The initial~ separation of thorium daughter--uranium parent was made several years prior to the present experiment. ~s~ i,~ M. H. STUVI~, A. Gmoa~., F. HAOEMANN,TID-5223 (Pt. 2) Paper No. 9.1 (1952). ~2~L. B. MAONUSSON. Private communication (1952). ts) D. F. PEPPARD,G. W. MASONand R. J. S l n o ~ , J. lnorg. Nucl. Chem. 10, 117 (1959). 1 1
2
J . E . GINDLER, K. F. FLYNN and J. GRAY, JR.
solution. Sample no. 3 was prepared by stripping the thorium from the TTA-benzene solution with nitric acid and volatilizing it onto a masked platinum disk.
Fission measurements The "-activity o f =srI'h in each sample was determined by "-pulse analysis and total ~,-counting. The number o f atoms of n r l ' h was calculated from its "-activity, an "-half-life o f 7340 years~'~ and the ,,-counter efficiency.* The results o f this calculation are given in Table 1. Standard samples o f nsPu and m U were prepared from stock solutions.t Mass spectrometric analyses o f these solutions are given in Table 2. The number o f atoms in each standard sample was calculated from the ,,-activity, the ,,-counter eff~iency* and the half-life o f the respective nuclide. A half-life of 24,360 years 'sj was used for nSPu and 1-626 × 106 years m for m U . These results are also given in Table 1. TABLB I.--DFI'ERI~HNATION OF ~ Sample
I 2 3
A t o m s of ss~I"h 6"70 6"70 6"03 6"03 2"92
X X X X X 2"92 ×
Net fissions (c.p.m.)
Standar I
I0zs 48,500 50,000 I0z' I014 172,000 I014 171,600 I0x4 211,500 10tt 218,000
FI~ION CRO~-$ECTION OF "=VI'h Net fissions (c.p.m.)
A t o m s of standard
s= U
13.85 X I0zs
sSgpU mU **spU tn U s3Spu
8.88 13.85 8-88 13.85
X I0zs X I0Is X 10ss X 10zs
8"88 X 10is
Approximate flux (neutronslcm-Ssec-1)
175,400 175,600 67,100 64,700 176,100 176,200
4.0 4-0 1.5 1.5 4-0 4.0
× X × × X ×
10u 10n 10xx 10al 1011 10~l
~='Th)
(barns) 30"1 30"2 31"1 31"3 30"0 30"2 Av. 30"5
TAei.e 2 . - - M ~ s SPECTROMETRIC ANALYSIS OF STOCK SOLUTIONS USED TO PREPARE FI_~ION COUNTING STANDARDS Solution
Nuclides
sssw
=asU satU su U ss6U nSpU s40pu ~lpu
=spu
Per cent composition by mass 98"33 0"127 0"0102 1'53 99-972 0"028
-t- 0-02 + 0.002 + 0.0004 + 0"02 ± 0.001 -4- 0.001
<0.0007
Fission measurements were made in the thermal colunm of the Argonne heavy-water pile, CP-5, using a double ionization chamber. The chamber permitted the counting o f two samples, one of thorium and a standard, simultaneously. The two samples were placed back-to-back in the chamber * Three total "-counters were used during the course of the experiment. The counting efficiency of one had been determined~s) to be 51.6 per cent for an UlAm sample evaporated upon a platinum disk. The other two counters were intercalibrated with this counter using a plutonium standard. This intercalibration gave efficiencies of 50.6 per cent for the counter used for tile thorium samples and 51-6 per cent for the counter used for both the m U and m P u standards. The extent to wluch this intercalibration is correct assuming the counting efficiency for plutonium and the other nuclides is the same as that for U~Am, determines in part the accuracy of the experiment. 1"The stock solutions and standard plates were prepared by F. T. H A o ~ m ~ . c4~ F. HAODt~,tCN,L. I. KATZIN, M. H. Szuvreg, O. T. S~dtmto and A. GmoRso, Phys. Re,= 79, 435 (1950) (s) W. C. B~N'rLV. Private communication (1957). ¢~) J. C. WAI.L~U~, Thesis, University of California Radiation Laboratory Report UCRL-1255 (1951) (unpublished). ~7) Yx. P. Doguct~Yev and I. S. Osleov, Atomnaya Energiya 6, 73, (1959).
Thermal neutron fission cross-section of ~°Th
3
perpendicular to the longitudinal axis of the thermal column such that the sample positioned nearest the core of the pile received a neutron flux 1.017 times greater than the other sample. Measurements were made at two flux positions, one of approximately 4 × 10~t neutrons cm-2sec-: and the other of approximately 1-5 x 10at neutrons cm-Zsec-~. The lower flux was used with the larger thorium sample (no. 2) to reduce the fission counting-rate to a level such that coincidence losses were less than 2 per cent. The thermal nature of the neutrons had been determined previously by PrAFF and SHELrON~s~ who measured the activities induced in bare and cadmium covered gold foils located at various positions in the column. The ratio of the activities of bare to covered gold foils was found to be 1.2 × 13s and 1.4 × l0s for the high and the low flux positions, respectively. Background counting-rates determined with a blank platinum disk in the chamber were small compared to those obtained with either the thorium samples or the ~3sUand ~3Wustandards. With the high neutron flux the background in chamber A was 270 c.p.m, and in B 133 c.p.m. With the low flux, chamber A had a background of 140 c.p.m, and B 74 c.p.m. The fission counting-rates corrected for coincidence loss, sample position, and background are given in Table 1. RESULTS The fission cross-section of Z29Th for thermal n e u t r o n s has been calculated from the relation O'rh :
F~h NT h
Nstaasta Fsta '
where N is the n u m b e r o f atoms of the fissioning nuclide a n d F is the fission c o u n t i n g rate with reference to t h o r i u m (Th) or s t a n d a r d (std). Fission cross-section values o f 527 b a r n s tg~ for z ~ U a n d 746 b a r n s tg~ for 2agPu were used. The latter cross-section was multiplied b y a factor o f 1.075 tl°l to correct for the fact that the ~ g P u fission cross-section is n o t 1/v in the thermal region a n d that the n e u t r o n d i s t r i b u t i o n is assumed to be Maxwellian. T a b l e 1 lists the calculated fission cross-section of ~2~l'h for thermal n e u t r o n s for each o f six measurements. T h e average value o f this cross-section is 30.5 b a r n s a n d has a precision of 0.5 b a r n . The absolute error is estimated to be less t h a n 10 per cent or 3 barns. This includes errors in c o u n t i n g because o f b a c k g r o u n d , coincidence loss, sample position a n d c o u n t e r efficiency as well as the statistical error, errors in the half-life d e t e r m i n a t i o n of the various nuclides, a n d errors in the fission cross-section of the standards. LEBEDEV a n d KALASHNIKOVA(11) have shown recently that slow n e u t r o n s with 0-5 eV to several tens o f electron volts have a greater effect o n the fission of 229Th t h a n they do o n 2~U. I n view of this it w o u l d be o f great interest to determine the fission cross-section of 22~I'h as a function of n e u t r o n energy.
Acknowledgements--The authors would like to thank Dr. L. B. MAGNUSSONfor the ~2~Thused in the experiment, Dr. F. T. HAOEML~ who furnished the standard samples, and Mr. D. J. HENDERSON who made the or-pulseanalyses. ta~ E. R. PVAFrand R. D. S~mLTON,Wright Air Development Command Technical Report 57-361, Vol.I, ASTIA Document No. AD 155789, Appendix III, pp. 707-729 (1958) (unpublished). ct~ D. J. HUGHESand R. B. SCHWARTZ,Neutron Cross-sections, Brookhaven National Laboratory Report BNL 325, (2nd Ed.), July, 1958 [Superintendent of Documents, U.S. Government Printing Office, Washington 25, D.C. (1958)]. tl0~ D. J. HUGHESand J. A. HAKVEY,Neutron Cross-sections, Brookhaven National Laboratory Report BNL-325, July, 1955 [Superintendentof Documents, U.S. Government Printing Office, Washington 25, D.C. (1955)I. ~n) V. I. LEn~D~V and V. I. K~LASm, aKOVA, Soviet Physics JETP 35, (8), 370 (1959).