- Email: [email protected]
Fast radiochemical isolation of fission product arsenic and the independent yield of arsenic-78 in thermal neutron fission of uranium-235
J. Inorg. Nucl. Chem., 1959, Vol. 11. pp. I73 to 180. Pergamon Press Ltd. Printed in Northern Ireland
FAST RADIOCHEMICAL ISOLATION OF FISSION PRODUCT ARSENIC AND THE INDEPENDENT YIELD OF ARSENIC-78 IN THERMAL NEUTRON FISSION OF URANIUM-235*~ A. KJELBERG.* a n d A. C. PAPPAS Department of Chemistry, University of dslo, Blindern, Norway (Received 2 February 1959)
Abstract--The previously reported independent yield of the 91.0 ruin :SAs in thermal neutron induced fission of z3~U is much higher than predicted from the now accepted charge distribution curve. The vield has therefore been redetermined using a radiochemical method for rapid isolation of arsenic from fission mixture where special precautions are taken in order to ensure a good and fast separation from its precursor germanium. The value thus obtained (1.7 = 0.5) × 10-' per cent is about one order of magnitude lower than the old value and fits the charge distribution curve. I.
THE C H A R G E D I S T R I B U T I O N 1N F I S S I O N
THr experimental study o f charge distribution in nuclear fission is based on r a d i o chemical d e t e r m i n a t i o n s o f i n d e p e n d e n t yields o f fission nuclides f r o m one or m o r e fission p r o d u c t decay chains. A study o f all nuclides in individual chains is impossible due to t o o short half-lives o f the e a r l y m e m b e r s . One is therefore forced to d e t e r m i n e i n d e p e n d e n t yields o f suitable nuclides in chains o f different mass numbers. T h e m o s t p r o b a b l e m o d e o f division o f nuclear charge is then evaluated by correlating the m e a s u r e d d a t a with semiempirical or theoretical relationships. On this basis it has been shown IL2) that in t h e r m a l neutron fission, the division results in equal d i s p l a c e m e n t f r o m stability o f the m o s t p r o b a b l e p r i m a r y charge for b o t h the light a n d the heavy fragment. In other words the effective chain lengths are the same for c o m p l e m e n t a r y fragments:
(Z~ -- Z.)ji~,~ = (Z• -- Z~)~a,.y
(1)
where Z ~ a n d Z~ are the charges (not necessary integral) c o r r e s p o n d i n g respectively to m a x i m u m stability and the m o s t p r o b a b l e primary, one, b o t h for the mass n u m b e r A. This relationship was first a d v a n c e d by GLENDENIN e t aL la) a n d later modified by PAPPAS '2'4) to account for the effects o f closed shells in the nuclear stability curve and the fact that the fission neutrons are emitted from the fragments after fission. * Work supported by the Royal Norwegian Council for Scientific and Industrial Research. + A short preliminary note of this work appeared in Tidsskrift fop" K/emi. Bergvesen og Metallurgi, 17 129 '(1957). ~. Present adress: Department of Chemistry, McGil! University, Montreal, Quebec, Canada. (1) L. E. GLENDENIN, The Distributaion of Nuclear Charge in Fission. Technical Report 35. Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge (1949). ~21A. C. PAPPAS,Proceedings of the hlternational Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 7, p. 19. United Nations, New York (2956). f3) L. E. GLENDENIN, C. D. CORYELL a n d R . R. EDWARDS, Radiochemical Studies:
The Fission Products
(Edited by C. D. CORVELLand N. SU6ARMAN), Plutonium Project Record, N N E S , Div. IV, Vol. 9, Paper 52. McGraw-Hill, New York (1951). ~4~A. C. PAPPAS,A Radiochemieal Study of Fission Yields attd the Effect of Closed Shells in Fission. American Report AECU-2806, also Inaugural Dissertation, University of Oslo (1953). 173 1
174
A. KJELBERG and A. C. PAPPAS
The charge distribution curve giving the fractional chain yield (relative probability) of a primary fragment (Z, A) as function of distance (Z -- Z~) from the most probable primary charge (Z~) is assumed identical for all mass chains and approximately Gaussian. Only when the charge division takes place as given by the modified theory of equal charge displacement I~) are these assumptions substantiated by experimental evidence in thermal and low energy fission. A few nuclides, however, have measured yields which differ from the ones expected from the charge distribution curve by more than the uncertainities in the measurements and in the assignments of Z~ would indicate. All these data but one or perhaps two cart be explained on the basis of enchanced independent yield due to: preference for closed shell nuclides in the primary fission act, ~5'e~prompt neutron emission from the 83 neutron species, Is) and spread in multiplicity of the evaporated neutrons. ~5) An explanation, however, still remains to be given to the high independent yield found by SU6AItMAN~7) for the 91.0 min 7SAs during a study of the genetics of the 7SGe JSAs fission chain. The measured yield is about tenfold higher than the modified theory of equal charge displacement would predict. If correct this value constitutes a challenge to the theory and it was therefore found necessary to test it by a redetermination. II.
PREVIOUS
STUDY
O F 7SAs
The experimental approach used by StlCARMAN~7) was based on irradiation of uranyl salts with thermal neutrons for 10 rain in the Los Alamos Homogeneous Reactor, followed by an isolation of germanium and arsenic from the fission mixture about 40 min later. This was performed by distilling off the germanium as tetrachloride and thereupon the arsenic as trichloride. During the subsequent study of the half-lives of ~SGe and VSAshe found an apparent independent yield of :SAs. This yield was determined by comparing the activity of the 91.0 rain :SAs in the arsenic sample with that formed by decay of 7SGe in tl~e germanium sample. The independent yield was found to be about 10 per cent of the chain yield for mass 78, or (1.8 4- 0-6) x 10-3 per cent. The main points of uncertainty in this determination are the exact timing and the possibility for an incomplete removal of germanium from the fission product mixture by the distillation. This, however, seems to, have been carefully considered by SUGARMAN,who estimated the limits of uncertainty due to timing and also seems to have taken precautions to ensure the absence of germanium in the residue after the distillation. It is, however, difficult to assess the absolute accuracy in the determination since the method used is essentially to determine the difference between the first 7SAs milking and a later one from the fission product solution, the more as the first milking was performed after appreciable decay of the precursor (about 30 per cent). III.
EXPERIMENTAL
WORK
In fission only a small fraction of 7SAs is formed independently whereas the majority arises from decay of 7SGe and its precursors. Thus in a determination of the independent yield of VaAs, special precautions must be taken in order to minimize the correction due to parent decay. A refined method ¢s~ A. C. PAPPAS, in Discussion of some Current Problems in Nuclear Fission. Held at the Institute of Theoretical Physics, Copenhagen, February 1958. (Notes by E. P. STEINBERGp. 21/. 16~ A. C. WAHL, J. Inorg. Nucl. Chem. 6, 263 (1958). ~v~N. SUGARMAN,Phys. Rev. 89, 570 (1953).
Fast radiochemical isolation of fission product ,rsenic
175
~vould therefore approach the problem by using the shortest possible irradiation and decay times. Further, in order to get the timing exact, a very fast and clean arsenic-germanium separation is required. A. Irradiation techniques
The target material consisted of 1-2 g of finely ground uranyl nitrate hexahydrate p.a. enclosed in a plastic capsule. This was irradiated for 2-3 min in one of the isoto-~e channels in the JEEP Heavy Water Reactor at JENER, Kjeller, at a neutron flux of approximate / 1 × 10 TM neutrons cm -a sec -1 The transportation of the irradiated target from the reactor to the laboratory was performed in 2-3 sec by using a " r a b b i t " device. B. Fast radiochemical isolation o f arsenic
Separations of trace amounts of elements by means of solvent extractions offer the possibilities for both fast and specific isolation. The optimum conditions for using solvent extraction in the present problem were therefore investigated and a radiochemical method developed by which it is possible to isolate arsenic 1-5-2 min after the end of the irradiation with high chemical yield, and high radiochemical purity with respect to its precursor germanium. Decontamination from the fission products following arsenic is then performed. It is well known that arsenic and germanium are extractable with organic solvents from hydrohalogenic acids. FlSrr~R et al. ~s~ have shown that germanium can be separated from arsenic by oxidizing the latter to the pentavalent state and extracting germanium with carbon tetrachloride from hydrochloric acid. For the present study, however, it would be preferable to separate arsenic from germanium by adapting a method developed by PRESrWOOD.~'~ He extracts arsenic with chloroform from hydriodic acid. The degree of separation and optimum conditions, however, are not given in his paper. The extractability as a function of the hydriodic acid concentration was therefore studied and also the possibility of using hydrochloric or hydrobromic acid and organic solvents other than chloroform. The 26.8 hr 76As and 12 hr 77Ge* were used as tracers and the degree of extraction measured in a "well-type" thallium activated sodium iodide crystal. The results disclosed that use of benzene is preferable both to chloroform and to carbon tetrachloride. Benzene not only increases the extractability but the separation of the organic and aqueous phases is sharper and faster especially at higher acid concentrations. A 10 sec extraction time proved to be sufficient and the degree of extraction was found to be independent of the concentration of the cation in the range studied i.e. from 10 -2 to 10-5 mole. The latter was verified during this study by BRINK et al. ~1°~for the range from 10-1 to 10-s mole. The extraction properties with benzene from different acids at room temperature are summarized in Fig. 1. In this are also included the results found by BRINK et al. for extraction of arsenic with hydriodic acid. The agreement between these two independent studies is good. The present one shows furthermore that by using hydrobromic acid it will not be possible to combine a good separation with a high chemical yield without a careful adjustment of the acid concentration, which must be avoided in fast work. The extraction from 3'0 N hydriodic acid into benzene, however, is very promising. After only 10 sec shaking of equal volumes more than 98 per cent of the trivalent arsenic is found in the organic phase while less than 5 per cent of the germanium is co-extracted. In a subsequent back-extraction with water, all arsenic will be transferred immediately to the aqueous phase while the back-extraction of germanium is very slow, much less than 0'2 per cent of the original germanium is found in the aqueous phase after 30 sec. By repeating the extraction cycle one finds within less than 2 rain a ratio between arsenic and germanium at least l0 s. The independent yield of 78As is expected to be from 1 to 10 per cent of the chain yield. This procedure should thus result in an 78As sample with a decontamination factor sufficient with respect to germanium for the present work. * Germanium tracer produced by neutron irradiation of germanium dioxide will after a while contain some 39 hr 77As formed by decay. By working fast, however, the amount of~TAs was kept negligible and could easily be accounted for. 18~W. FISCHER, W. HARRE, W. FREr-SE and K. G. HACKS'rEtrY,Z. Angew. Chem. 66, 165 (1954). ~9~ R. J. PRESrWOOD, see J. KLEINBERC, American Report LA-1721 (rev), 80 (1954). c~o) G. O. BR1NK, P. KArALAS, R. A. SHARP, E. L. WEiss and J. W. IRVINE, JR., J. Amer. Chem. Soc. 79, 1303 (1957).
176
A.K.JELBERG a n d
A.
C.
PAPPAS
The exchange between fission product arsenic and added carrier, however, might be slow. c11~ In order to ensure complete exchange the arsenic carrier is added in the pentavalent state and then reduced in the presence of fission product arsenic to the trivalent state by hydriodic acid. Tin and antimony were found to be extractable under these conditions. This would be no limitation except for the fact that their decay product tellurium will be partly carried in the subsequent precipitation of arsenic as sulphide and as metal. Therefore purification steps must be included in order to achieve further decontamination. The details of the procedure finally developed is given in the Appendix. i
tO0
I
I ,
I
I
#.o.~
i
i
i
i
!
!
:.,~ip-- - 7~r.~o...~
.0
if'-.
oll
,,,4(
13
/
" ./.,-,:._...:,..:, 7 0
I
li
,
, i
I
I0 I~ acid
Flo. l.--Extraction of As (III) and Ge (IV) with benzene from HBr and HI, equal phase volumes. 1: As/HI, 2: As/HBr, 3: Ge/HI, 4: Ge/HBr, Open circles: Data from BRINKet al. cl°l for As/HI. C. Fission monitor
12.80 day 14°Ba was used as internal monitor with a fission yield 6.44 per cent cx2~ obtained by averaging the results of the most reliable and careful measurements available. Barium was isolated from the fission mixture (the combined aqueous phase and wash solution after the extraction of arsenic) according to the radiochemical procedure given by GLENDENZN~3~ but slightly modified to fit the present conditions. The large content of hydriodic acid was first removed by evaporation to dryness. D. Counting techniques
The arsenic metal sample was collected on filter paper with defined sample area. This was mounted in a standard way, counted with a Tracerlab TGC-2 Geiger-Mueller Tube under standard conditions ditions, 14~ and the decay rates converted to absolute disintegration rates as previously described by PAPPAS."~ The arsenic and barium samples were counted in the same position, thus no geometry factor needed to be considered. IV.
RESULTS AND DISCUSSION
Fig. 2 shows the analysis of a typical/~-decay curve of the arsenic samples. These curves are easily analysed into two constituents with half-lives 38.5 ± 0.5 hr and 91 ± 2 rain o v e r m o r e t h a n f o u r half-lives, c o r r e s p o n d i n g to the 38.8 h r 77As a n d the 91.0 roan 7SAs. A s in SUGARMAN'S w o r k n o e v i d e n c e was f o u n d f o r t h e ,--40 rain arsenic r e p o r t e d b y BRIGHTSEN et al., ~14) t h u s also c o n f i r m i n g the findings o f CUNINGHAME. (11") ~lx~j. G. CUNINGHAME,Phil. Mag. 44, 900 (1953). ~xzl S. KATCOFr, Nucleonics 16, 78 (1958). ~13~L. E. GLENDENIN, Radiochemical Studies: The Fission Products (Edited by C. D. CORVELL and N. SUGARMAN),Plutonium Project Record, NNES, Div. IV, Vol. 9, Paper 288. McGraw-Hill, New York (1951). (14) R . A . B R I G H T E S E N , K . S H U R E , C . F I S H E R a n d
C. D. CORYELL,
Phys. Rev.
81,218
(1951).
Fast radiochemical isolation of fission product arsenic
177
For the calculation of the independent fission yield of 78As()'2 i) o n e has the following equation (2), where indices 1, 2, and 3 respectively refer to 78Ge,7%s and 14°Ba; ' ~ -- D~' " 1 ~ --
D~
1 . ) ' a (1 - -
I 21 -- )'z (1
3'1
e-~'2r)e -z~T
21 - -
22
- - ~eJ'lT)e-'~lt i-
" (-1 - -
(2)
e-Z2T)e-Z2t.J
In this equation the second term corrects for the amount of 7SAs formed by decay
1
2
:~
~,
5
6
days
~ i i Ii I ~ * i ! 1 i i t i , ! 1,103
,
.
\
38.t. ~
lO: ~
102
2
Z,
6
8 " 10
12
hours
Fro. 2.--Decay of arsenic sample obtained by isolation from fission product mixture as described in the text. A: Gross decay showing 77As B: Analysis of A showing ~aAs (C).
of the parent 7SGe during irradiation and the amount formed previous to the arsenicgermanium separation. Furthermore: D2 ~ is the disintegration rate of 7SAs at separation time t from the parent nuclide. D3 ~ is the disintegration rate at saturation for the internal monitor, i.e. 14aBa. T is the irradiation time. t is the decay time from the end of the irradiation to the separation of arsenic from germanium. y is the cumulative fission yield, 6.44 per cent for a4°Ba and 1.8 × 10-2 per cent for 78Ge.(r) The yield of the latter has also been measured by STEINBERGand ENGELKEMEIR (1~) who found a value about 10 per cent higher than that of SUGARMAN. This is, however, based on a value of 2.1 hr for the half-life of rSGe. ~15~E. P. STEINBERG and D. W. ENGELKEMEIR, Radiochemical Studies." The Fission Products (Edited by C. D. CORYELL and N. SU6ARMAN), Plutonium Project Record, NNES, Div. IV, Vol. 9, Paper 54. McGraw-Hill, New York (19511.
178
A. KdELBERGand A. C. PAPPAS
2 is the decay constant. The half-fife of 7SAs is 91.0 min and that for 7SGe 86 rnin. Due to the short irradiation and decay times in these experiments relative to the half-lives concerned, equation (2) can be approximated by: •
Y2~= ~
1
• Ya ~ T - - Yt21t
(3)
It can easily be shown that the use of equation (3) instead of (2) under the conditions of the present experiments introduces an error of less than two per cent in the final value of y2 ~. The results of three independent runs are given in Table 1. TABLE 1.--INDEPENDENT FISSION YIELD OF TeAs
Run
Disintegration rate of 78As, D2t (d/min) Disintegration rate of l*°Ba at saturation D~~ (d/rain) Irradiation time T (rain) Decay time t (rain)
1080 1"33 × 109 2"07
1320 1'3 × 109 2'00
1"47
1"50
1030 6.23 × 10u 2.95 2.08
Independent yield of 78As 10-* per cent
l'10 4- 0"4
2"12 4- 1"0
1.7s --I-0.5
The errors assigned are estimated on the basis of uncertainties in chemical yield, in analysis of decay curves, in timing, etc. According to this determination the independent yield of 78As in thermal neutron fission of ~:~U is (1.7 -+- 0.5) × 10-4 per cent. Thus the independent yield of 7SAs determined in the present work is about a factor ten below the value reported by SUGAR~N. As an explantion for this large difference is not straight forward it is of special interest to discuss the uncertainties that may be introduced by the chemical procedure used: 1. If a significant amount of germanium is co-extracted the yield will be too high. 2. Incomplete exchange between added carrier and fission product arsenic would result in too low yield. Owing to the way in which exchange is promoted, i.e. carrier added in the pentavalent state and reduced to trivalent just before the extraction, this point ought to be of minor influence. A further check in the validity of the present determination is given by an estimate of the fission yield of the 54 sec 77raGe obtained from the 77As content in the samples (77gGe has a half-life of 11-3 hr and 86 per cent of 77raGe decays directly to 77As). We find (5 ! 1) X 10- 3 per cent which is in agreement with 4.4 x 10-3 per cent obtained as a difference between the fission yields of 77As and 77gGe given by SUGARMAN (7). F r o m the work of.STEINBERGand ENGELKEMEIRtxS) a value of 5.4 × 10- a per cent can be deduced. At present no explanation can thus be given for the discrepancy between the previous determination of the independent yield of 78As and the present one. The cumulative yield o f the fission product decay chain with mass number 78 is
Fast radiochemical isolation of fission product arsenic
179
0.018 per cent tTI in thermal fission of 23~U. This gives a fractional chain yield for 7SAs of (9 -4- 3) × 10-3. According to PAPPASt~l Z~ = 30.9 ± 0.1" is the most probable primary charge for this chain. The chain position ( Z - Z~) of 83As 7s is therefore 2.1 ± 0-1. Using the charge distribution curve given by PAPPAS'~) this corresponds to a
,0
fractional yield of ~
-4-.
1°1 x
10-~.
Taking STEINBER~ and GLENDENIN'S
\
approach ~17~ in evaluating Z~ one finds that the measured fractional chain yield is slightly on the low side. Using the approach chosen by WAHL~6~one finds that the measured fractional yield of 78As gives a slightly higher value of Z~ than a straight line extrapolation of his values for S2Br, a6Rb and 89Kr would give. The net result is that the measured value is slightly on the high side. In the region of interest, i.e. around mass 78, shell or mirror shell effects are not present and all these three approaches differ therefore only in finer details in the evaluation of the most probable primary charge for chain 78 in fission. They are all based on the modified theory of equal charge displacement. The value measured in the present work for the independent yield of 7SAs is thus in good agreement with this theory, and the deviation due to 78As from the charge distribution curve given by this theory is removed. Concerning the old value for the independent yield of 7SAs one should presumably be permitted to conclude that this must be in error. APPENDIX Fast chemistry for isolation o f fission produc~ ~ arsenic In order to save time all extractions are performed in separatory funnels to which the proper solutions are added previous to the experiment. Irradiated UO2CNO3)~6H.,O (2.0 g) is dissolved (without heating) in 6"5 ml H 2 0 to which is added 2"0 ml (20 mg) barium carrier and 2'0 ml (20 mg) arsenic (V) carrier and three drops conc. HCI. The solution is transferred to a separatory funnel containing 20 ml benzene and 4"5 ml 10"2 N HI (commercial 67 per cent) is added and the funnel is immediately shaken for I0 sec. The exact time of phase separation is noted as time for the isolation from the germanium parent. The aqueous phase together with a 10 ml 3'0 N HI wash solution are collected for subsequent barium determination. The arsenic is back-extracted with 6.5 ml H 2 0 for 30 sec and the aqueous phase is washed with 10 ml benzene for 5 sec. The extraction and back-extraction is repeated. This part of the procedure takes about 1-5-2 rain. Approximately 1 hr later 1.0 ml (10 rng) tellurium (IV) carrier and 3.5 ml conc. HC1 is added to the water phase. The solution is heated nearly to boiling and reduced with SO2. After filtering the supernatant solution is boiled to drive offexcess SO2. The solution is cooled and 20 ml conc. HC added. Arsenic sulphide is precipitated with H2S, centrifuged off, decanted and washed with HCI 6 N solution saturated with H2S. The arsenic sulphide is dissolved in 0'5 ml conc. HC1, 0-5 ml conc. HNO3, and 1 "0 ml HCIO4, heated on a water bath for 5 rain and thereafter heated to strong fuming. After cooling, extraction and back-extraction, precipitation as sulphide and dissolution is performed as already described. The final arsenic solution is diluted to 20 ml with 6 N HCI. Metallic arsenic is precipitated by adding 100 mg KI and 1 g NaH2PO2 and heating on waterbath for coagulation. The coagulate is washed with hot H20, a slurry made in alcohol and As transferred to the filter paper, washed with alcohol and ether, and dried at 110°C for 5 min, weighed and mounted for counting. * The influence of the 64 proton subshell~-'1is not considered in the present study as the evaluation of proton subshells in the mass region 150--190 needs further experimental data& s' ~6) C. D. CORWLL, Annual report, Laboratory for Nuclear Science, Massachusetts Institute of Technology U.S.A. (1957). ~17~E. P. STEINBERGand L. E. GLENDENIN,Proceedings of the International Conference on the Peaceful Uses qfAtomic Energy, Geneva 1955, Vol. 7, p. 3. United Nations, New York (1956).
180
Acknowledgements--Theauthors
A. KaELB~RO and A. C. PAPPAS
wish to express their thanks to Dr. G. RANDERS,director of the Joint Establishment for Nuclear Energy Research (Kjeller), for his kind permission to use the reactor for this investigation, and to Dr. E. S~LAr,rD and his group for valuable assistance. The authors are indebted to the Royal Norwegian Council for Scientific and Industrial Research for financial aid and one of us (A. K.) thanks the Nansen foundation in Norway for additional supporting grants.
FAST RADIOCHEMICAL ISOLATION OF FISSION PRODUCT ARSENIC AND THE INDEPENDENT YIELD OF ARSENIC-78 IN THERMAL NEUTRON FISSION OF URANIUM-235*~ A. KJELBERG.* a n d A. C. PAPPAS Department of Chemistry, University of dslo, Blindern, Norway (Received 2 February 1959)
Abstract--The previously reported independent yield of the 91.0 ruin :SAs in thermal neutron induced fission of z3~U is much higher than predicted from the now accepted charge distribution curve. The vield has therefore been redetermined using a radiochemical method for rapid isolation of arsenic from fission mixture where special precautions are taken in order to ensure a good and fast separation from its precursor germanium. The value thus obtained (1.7 = 0.5) × 10-' per cent is about one order of magnitude lower than the old value and fits the charge distribution curve. I.
THE C H A R G E D I S T R I B U T I O N 1N F I S S I O N
THr experimental study o f charge distribution in nuclear fission is based on r a d i o chemical d e t e r m i n a t i o n s o f i n d e p e n d e n t yields o f fission nuclides f r o m one or m o r e fission p r o d u c t decay chains. A study o f all nuclides in individual chains is impossible due to t o o short half-lives o f the e a r l y m e m b e r s . One is therefore forced to d e t e r m i n e i n d e p e n d e n t yields o f suitable nuclides in chains o f different mass numbers. T h e m o s t p r o b a b l e m o d e o f division o f nuclear charge is then evaluated by correlating the m e a s u r e d d a t a with semiempirical or theoretical relationships. On this basis it has been shown IL2) that in t h e r m a l neutron fission, the division results in equal d i s p l a c e m e n t f r o m stability o f the m o s t p r o b a b l e p r i m a r y charge for b o t h the light a n d the heavy fragment. In other words the effective chain lengths are the same for c o m p l e m e n t a r y fragments:
(Z~ -- Z.)ji~,~ = (Z• -- Z~)~a,.y
(1)
where Z ~ a n d Z~ are the charges (not necessary integral) c o r r e s p o n d i n g respectively to m a x i m u m stability and the m o s t p r o b a b l e primary, one, b o t h for the mass n u m b e r A. This relationship was first a d v a n c e d by GLENDENIN e t aL la) a n d later modified by PAPPAS '2'4) to account for the effects o f closed shells in the nuclear stability curve and the fact that the fission neutrons are emitted from the fragments after fission. * Work supported by the Royal Norwegian Council for Scientific and Industrial Research. + A short preliminary note of this work appeared in Tidsskrift fop" K/emi. Bergvesen og Metallurgi, 17 129 '(1957). ~. Present adress: Department of Chemistry, McGil! University, Montreal, Quebec, Canada. (1) L. E. GLENDENIN, The Distributaion of Nuclear Charge in Fission. Technical Report 35. Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge (1949). ~21A. C. PAPPAS,Proceedings of the hlternational Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 7, p. 19. United Nations, New York (2956). f3) L. E. GLENDENIN, C. D. CORYELL a n d R . R. EDWARDS, Radiochemical Studies:
The Fission Products
(Edited by C. D. CORVELLand N. SU6ARMAN), Plutonium Project Record, N N E S , Div. IV, Vol. 9, Paper 52. McGraw-Hill, New York (1951). ~4~A. C. PAPPAS,A Radiochemieal Study of Fission Yields attd the Effect of Closed Shells in Fission. American Report AECU-2806, also Inaugural Dissertation, University of Oslo (1953). 173 1
174
A. KJELBERG and A. C. PAPPAS
The charge distribution curve giving the fractional chain yield (relative probability) of a primary fragment (Z, A) as function of distance (Z -- Z~) from the most probable primary charge (Z~) is assumed identical for all mass chains and approximately Gaussian. Only when the charge division takes place as given by the modified theory of equal charge displacement I~) are these assumptions substantiated by experimental evidence in thermal and low energy fission. A few nuclides, however, have measured yields which differ from the ones expected from the charge distribution curve by more than the uncertainities in the measurements and in the assignments of Z~ would indicate. All these data but one or perhaps two cart be explained on the basis of enchanced independent yield due to: preference for closed shell nuclides in the primary fission act, ~5'e~prompt neutron emission from the 83 neutron species, Is) and spread in multiplicity of the evaporated neutrons. ~5) An explanation, however, still remains to be given to the high independent yield found by SU6AItMAN~7) for the 91.0 min 7SAs during a study of the genetics of the 7SGe JSAs fission chain. The measured yield is about tenfold higher than the modified theory of equal charge displacement would predict. If correct this value constitutes a challenge to the theory and it was therefore found necessary to test it by a redetermination. II.
PREVIOUS
STUDY
O F 7SAs
The experimental approach used by StlCARMAN~7) was based on irradiation of uranyl salts with thermal neutrons for 10 rain in the Los Alamos Homogeneous Reactor, followed by an isolation of germanium and arsenic from the fission mixture about 40 min later. This was performed by distilling off the germanium as tetrachloride and thereupon the arsenic as trichloride. During the subsequent study of the half-lives of ~SGe and VSAshe found an apparent independent yield of :SAs. This yield was determined by comparing the activity of the 91.0 rain :SAs in the arsenic sample with that formed by decay of 7SGe in tl~e germanium sample. The independent yield was found to be about 10 per cent of the chain yield for mass 78, or (1.8 4- 0-6) x 10-3 per cent. The main points of uncertainty in this determination are the exact timing and the possibility for an incomplete removal of germanium from the fission product mixture by the distillation. This, however, seems to, have been carefully considered by SUGARMAN,who estimated the limits of uncertainty due to timing and also seems to have taken precautions to ensure the absence of germanium in the residue after the distillation. It is, however, difficult to assess the absolute accuracy in the determination since the method used is essentially to determine the difference between the first 7SAs milking and a later one from the fission product solution, the more as the first milking was performed after appreciable decay of the precursor (about 30 per cent). III.
EXPERIMENTAL
WORK
In fission only a small fraction of 7SAs is formed independently whereas the majority arises from decay of 7SGe and its precursors. Thus in a determination of the independent yield of VaAs, special precautions must be taken in order to minimize the correction due to parent decay. A refined method ¢s~ A. C. PAPPAS, in Discussion of some Current Problems in Nuclear Fission. Held at the Institute of Theoretical Physics, Copenhagen, February 1958. (Notes by E. P. STEINBERGp. 21/. 16~ A. C. WAHL, J. Inorg. Nucl. Chem. 6, 263 (1958). ~v~N. SUGARMAN,Phys. Rev. 89, 570 (1953).
Fast radiochemical isolation of fission product ,rsenic
175
~vould therefore approach the problem by using the shortest possible irradiation and decay times. Further, in order to get the timing exact, a very fast and clean arsenic-germanium separation is required. A. Irradiation techniques
The target material consisted of 1-2 g of finely ground uranyl nitrate hexahydrate p.a. enclosed in a plastic capsule. This was irradiated for 2-3 min in one of the isoto-~e channels in the JEEP Heavy Water Reactor at JENER, Kjeller, at a neutron flux of approximate / 1 × 10 TM neutrons cm -a sec -1 The transportation of the irradiated target from the reactor to the laboratory was performed in 2-3 sec by using a " r a b b i t " device. B. Fast radiochemical isolation o f arsenic
Separations of trace amounts of elements by means of solvent extractions offer the possibilities for both fast and specific isolation. The optimum conditions for using solvent extraction in the present problem were therefore investigated and a radiochemical method developed by which it is possible to isolate arsenic 1-5-2 min after the end of the irradiation with high chemical yield, and high radiochemical purity with respect to its precursor germanium. Decontamination from the fission products following arsenic is then performed. It is well known that arsenic and germanium are extractable with organic solvents from hydrohalogenic acids. FlSrr~R et al. ~s~ have shown that germanium can be separated from arsenic by oxidizing the latter to the pentavalent state and extracting germanium with carbon tetrachloride from hydrochloric acid. For the present study, however, it would be preferable to separate arsenic from germanium by adapting a method developed by PRESrWOOD.~'~ He extracts arsenic with chloroform from hydriodic acid. The degree of separation and optimum conditions, however, are not given in his paper. The extractability as a function of the hydriodic acid concentration was therefore studied and also the possibility of using hydrochloric or hydrobromic acid and organic solvents other than chloroform. The 26.8 hr 76As and 12 hr 77Ge* were used as tracers and the degree of extraction measured in a "well-type" thallium activated sodium iodide crystal. The results disclosed that use of benzene is preferable both to chloroform and to carbon tetrachloride. Benzene not only increases the extractability but the separation of the organic and aqueous phases is sharper and faster especially at higher acid concentrations. A 10 sec extraction time proved to be sufficient and the degree of extraction was found to be independent of the concentration of the cation in the range studied i.e. from 10 -2 to 10-5 mole. The latter was verified during this study by BRINK et al. ~1°~for the range from 10-1 to 10-s mole. The extraction properties with benzene from different acids at room temperature are summarized in Fig. 1. In this are also included the results found by BRINK et al. for extraction of arsenic with hydriodic acid. The agreement between these two independent studies is good. The present one shows furthermore that by using hydrobromic acid it will not be possible to combine a good separation with a high chemical yield without a careful adjustment of the acid concentration, which must be avoided in fast work. The extraction from 3'0 N hydriodic acid into benzene, however, is very promising. After only 10 sec shaking of equal volumes more than 98 per cent of the trivalent arsenic is found in the organic phase while less than 5 per cent of the germanium is co-extracted. In a subsequent back-extraction with water, all arsenic will be transferred immediately to the aqueous phase while the back-extraction of germanium is very slow, much less than 0'2 per cent of the original germanium is found in the aqueous phase after 30 sec. By repeating the extraction cycle one finds within less than 2 rain a ratio between arsenic and germanium at least l0 s. The independent yield of 78As is expected to be from 1 to 10 per cent of the chain yield. This procedure should thus result in an 78As sample with a decontamination factor sufficient with respect to germanium for the present work. * Germanium tracer produced by neutron irradiation of germanium dioxide will after a while contain some 39 hr 77As formed by decay. By working fast, however, the amount of~TAs was kept negligible and could easily be accounted for. 18~W. FISCHER, W. HARRE, W. FREr-SE and K. G. HACKS'rEtrY,Z. Angew. Chem. 66, 165 (1954). ~9~ R. J. PRESrWOOD, see J. KLEINBERC, American Report LA-1721 (rev), 80 (1954). c~o) G. O. BR1NK, P. KArALAS, R. A. SHARP, E. L. WEiss and J. W. IRVINE, JR., J. Amer. Chem. Soc. 79, 1303 (1957).
176
A.K.JELBERG a n d
A.
C.
PAPPAS
The exchange between fission product arsenic and added carrier, however, might be slow. c11~ In order to ensure complete exchange the arsenic carrier is added in the pentavalent state and then reduced in the presence of fission product arsenic to the trivalent state by hydriodic acid. Tin and antimony were found to be extractable under these conditions. This would be no limitation except for the fact that their decay product tellurium will be partly carried in the subsequent precipitation of arsenic as sulphide and as metal. Therefore purification steps must be included in order to achieve further decontamination. The details of the procedure finally developed is given in the Appendix. i
tO0
I
I ,
I
I
#.o.~
i
i
i
i
!
!
:.,~ip-- - 7~r.~o...~
.0
if'-.
oll
,,,4(
13
/
" ./.,-,:._...:,..:, 7 0
I
li
,
, i
I
I0 I~ acid
Flo. l.--Extraction of As (III) and Ge (IV) with benzene from HBr and HI, equal phase volumes. 1: As/HI, 2: As/HBr, 3: Ge/HI, 4: Ge/HBr, Open circles: Data from BRINKet al. cl°l for As/HI. C. Fission monitor
12.80 day 14°Ba was used as internal monitor with a fission yield 6.44 per cent cx2~ obtained by averaging the results of the most reliable and careful measurements available. Barium was isolated from the fission mixture (the combined aqueous phase and wash solution after the extraction of arsenic) according to the radiochemical procedure given by GLENDENZN~3~ but slightly modified to fit the present conditions. The large content of hydriodic acid was first removed by evaporation to dryness. D. Counting techniques
The arsenic metal sample was collected on filter paper with defined sample area. This was mounted in a standard way, counted with a Tracerlab TGC-2 Geiger-Mueller Tube under standard conditions ditions, 14~ and the decay rates converted to absolute disintegration rates as previously described by PAPPAS."~ The arsenic and barium samples were counted in the same position, thus no geometry factor needed to be considered. IV.
RESULTS AND DISCUSSION
Fig. 2 shows the analysis of a typical/~-decay curve of the arsenic samples. These curves are easily analysed into two constituents with half-lives 38.5 ± 0.5 hr and 91 ± 2 rain o v e r m o r e t h a n f o u r half-lives, c o r r e s p o n d i n g to the 38.8 h r 77As a n d the 91.0 roan 7SAs. A s in SUGARMAN'S w o r k n o e v i d e n c e was f o u n d f o r t h e ,--40 rain arsenic r e p o r t e d b y BRIGHTSEN et al., ~14) t h u s also c o n f i r m i n g the findings o f CUNINGHAME. (11") ~lx~j. G. CUNINGHAME,Phil. Mag. 44, 900 (1953). ~xzl S. KATCOFr, Nucleonics 16, 78 (1958). ~13~L. E. GLENDENIN, Radiochemical Studies: The Fission Products (Edited by C. D. CORVELL and N. SUGARMAN),Plutonium Project Record, NNES, Div. IV, Vol. 9, Paper 288. McGraw-Hill, New York (1951). (14) R . A . B R I G H T E S E N , K . S H U R E , C . F I S H E R a n d
C. D. CORYELL,
Phys. Rev.
81,218
(1951).
Fast radiochemical isolation of fission product arsenic
177
For the calculation of the independent fission yield of 78As()'2 i) o n e has the following equation (2), where indices 1, 2, and 3 respectively refer to 78Ge,7%s and 14°Ba; ' ~ -- D~' " 1 ~ --
D~
1 . ) ' a (1 - -
I 21 -- )'z (1
3'1
e-~'2r)e -z~T
21 - -
22
- - ~eJ'lT)e-'~lt i-
" (-1 - -
(2)
e-Z2T)e-Z2t.J
In this equation the second term corrects for the amount of 7SAs formed by decay
1
2
:~
~,
5
6
days
~ i i Ii I ~ * i ! 1 i i t i , ! 1,103
,
.
\
38.t. ~
lO: ~
102
2
Z,
6
8 " 10
12
hours
Fro. 2.--Decay of arsenic sample obtained by isolation from fission product mixture as described in the text. A: Gross decay showing 77As B: Analysis of A showing ~aAs (C).
of the parent 7SGe during irradiation and the amount formed previous to the arsenicgermanium separation. Furthermore: D2 ~ is the disintegration rate of 7SAs at separation time t from the parent nuclide. D3 ~ is the disintegration rate at saturation for the internal monitor, i.e. 14aBa. T is the irradiation time. t is the decay time from the end of the irradiation to the separation of arsenic from germanium. y is the cumulative fission yield, 6.44 per cent for a4°Ba and 1.8 × 10-2 per cent for 78Ge.(r) The yield of the latter has also been measured by STEINBERGand ENGELKEMEIR (1~) who found a value about 10 per cent higher than that of SUGARMAN. This is, however, based on a value of 2.1 hr for the half-life of rSGe. ~15~E. P. STEINBERG and D. W. ENGELKEMEIR, Radiochemical Studies." The Fission Products (Edited by C. D. CORYELL and N. SU6ARMAN), Plutonium Project Record, NNES, Div. IV, Vol. 9, Paper 54. McGraw-Hill, New York (19511.
178
A. KdELBERGand A. C. PAPPAS
2 is the decay constant. The half-fife of 7SAs is 91.0 min and that for 7SGe 86 rnin. Due to the short irradiation and decay times in these experiments relative to the half-lives concerned, equation (2) can be approximated by: •
Y2~= ~
1
• Ya ~ T - - Yt21t
(3)
It can easily be shown that the use of equation (3) instead of (2) under the conditions of the present experiments introduces an error of less than two per cent in the final value of y2 ~. The results of three independent runs are given in Table 1. TABLE 1.--INDEPENDENT FISSION YIELD OF TeAs
Run
Disintegration rate of 78As, D2t (d/min) Disintegration rate of l*°Ba at saturation D~~ (d/rain) Irradiation time T (rain) Decay time t (rain)
1080 1"33 × 109 2"07
1320 1'3 × 109 2'00
1"47
1"50
1030 6.23 × 10u 2.95 2.08
Independent yield of 78As 10-* per cent
l'10 4- 0"4
2"12 4- 1"0
1.7s --I-0.5
The errors assigned are estimated on the basis of uncertainties in chemical yield, in analysis of decay curves, in timing, etc. According to this determination the independent yield of 78As in thermal neutron fission of ~:~U is (1.7 -+- 0.5) × 10-4 per cent. Thus the independent yield of 7SAs determined in the present work is about a factor ten below the value reported by SUGAR~N. As an explantion for this large difference is not straight forward it is of special interest to discuss the uncertainties that may be introduced by the chemical procedure used: 1. If a significant amount of germanium is co-extracted the yield will be too high. 2. Incomplete exchange between added carrier and fission product arsenic would result in too low yield. Owing to the way in which exchange is promoted, i.e. carrier added in the pentavalent state and reduced to trivalent just before the extraction, this point ought to be of minor influence. A further check in the validity of the present determination is given by an estimate of the fission yield of the 54 sec 77raGe obtained from the 77As content in the samples (77gGe has a half-life of 11-3 hr and 86 per cent of 77raGe decays directly to 77As). We find (5 ! 1) X 10- 3 per cent which is in agreement with 4.4 x 10-3 per cent obtained as a difference between the fission yields of 77As and 77gGe given by SUGARMAN (7). F r o m the work of.STEINBERGand ENGELKEMEIRtxS) a value of 5.4 × 10- a per cent can be deduced. At present no explanation can thus be given for the discrepancy between the previous determination of the independent yield of 78As and the present one. The cumulative yield o f the fission product decay chain with mass number 78 is
Fast radiochemical isolation of fission product arsenic
179
0.018 per cent tTI in thermal fission of 23~U. This gives a fractional chain yield for 7SAs of (9 -4- 3) × 10-3. According to PAPPASt~l Z~ = 30.9 ± 0.1" is the most probable primary charge for this chain. The chain position ( Z - Z~) of 83As 7s is therefore 2.1 ± 0-1. Using the charge distribution curve given by PAPPAS'~) this corresponds to a
,0
fractional yield of ~
-4-.
1°1 x
10-~.
Taking STEINBER~ and GLENDENIN'S
\
approach ~17~ in evaluating Z~ one finds that the measured fractional chain yield is slightly on the low side. Using the approach chosen by WAHL~6~one finds that the measured fractional yield of 78As gives a slightly higher value of Z~ than a straight line extrapolation of his values for S2Br, a6Rb and 89Kr would give. The net result is that the measured value is slightly on the high side. In the region of interest, i.e. around mass 78, shell or mirror shell effects are not present and all these three approaches differ therefore only in finer details in the evaluation of the most probable primary charge for chain 78 in fission. They are all based on the modified theory of equal charge displacement. The value measured in the present work for the independent yield of 7SAs is thus in good agreement with this theory, and the deviation due to 78As from the charge distribution curve given by this theory is removed. Concerning the old value for the independent yield of 7SAs one should presumably be permitted to conclude that this must be in error. APPENDIX Fast chemistry for isolation o f fission produc~ ~ arsenic In order to save time all extractions are performed in separatory funnels to which the proper solutions are added previous to the experiment. Irradiated UO2CNO3)~6H.,O (2.0 g) is dissolved (without heating) in 6"5 ml H 2 0 to which is added 2"0 ml (20 mg) barium carrier and 2'0 ml (20 mg) arsenic (V) carrier and three drops conc. HCI. The solution is transferred to a separatory funnel containing 20 ml benzene and 4"5 ml 10"2 N HI (commercial 67 per cent) is added and the funnel is immediately shaken for I0 sec. The exact time of phase separation is noted as time for the isolation from the germanium parent. The aqueous phase together with a 10 ml 3'0 N HI wash solution are collected for subsequent barium determination. The arsenic is back-extracted with 6.5 ml H 2 0 for 30 sec and the aqueous phase is washed with 10 ml benzene for 5 sec. The extraction and back-extraction is repeated. This part of the procedure takes about 1-5-2 rain. Approximately 1 hr later 1.0 ml (10 rng) tellurium (IV) carrier and 3.5 ml conc. HC1 is added to the water phase. The solution is heated nearly to boiling and reduced with SO2. After filtering the supernatant solution is boiled to drive offexcess SO2. The solution is cooled and 20 ml conc. HC added. Arsenic sulphide is precipitated with H2S, centrifuged off, decanted and washed with HCI 6 N solution saturated with H2S. The arsenic sulphide is dissolved in 0'5 ml conc. HC1, 0-5 ml conc. HNO3, and 1 "0 ml HCIO4, heated on a water bath for 5 rain and thereafter heated to strong fuming. After cooling, extraction and back-extraction, precipitation as sulphide and dissolution is performed as already described. The final arsenic solution is diluted to 20 ml with 6 N HCI. Metallic arsenic is precipitated by adding 100 mg KI and 1 g NaH2PO2 and heating on waterbath for coagulation. The coagulate is washed with hot H20, a slurry made in alcohol and As transferred to the filter paper, washed with alcohol and ether, and dried at 110°C for 5 min, weighed and mounted for counting. * The influence of the 64 proton subshell~-'1is not considered in the present study as the evaluation of proton subshells in the mass region 150--190 needs further experimental data& s' ~6) C. D. CORWLL, Annual report, Laboratory for Nuclear Science, Massachusetts Institute of Technology U.S.A. (1957). ~17~E. P. STEINBERGand L. E. GLENDENIN,Proceedings of the International Conference on the Peaceful Uses qfAtomic Energy, Geneva 1955, Vol. 7, p. 3. United Nations, New York (1956).
180
Acknowledgements--Theauthors
A. KaELB~RO and A. C. PAPPAS
wish to express their thanks to Dr. G. RANDERS,director of the Joint Establishment for Nuclear Energy Research (Kjeller), for his kind permission to use the reactor for this investigation, and to Dr. E. S~LAr,rD and his group for valuable assistance. The authors are indebted to the Royal Norwegian Council for Scientific and Industrial Research for financial aid and one of us (A. K.) thanks the Nansen foundation in Norway for additional supporting grants.