- Email: [email protected]
Studies of the mass distribution in fission of uranium induced by 170 MeV protons
J inorg, nu¢l. Chem.. 1974. Vol. 36, pp. 245 ~249, Pergamon Press. Printed in Great Britain.
STUDIES OF THE MASS D I S T R I B U T I O N IN FISSION OF U R A N I U M I N D U C E D BY 170 MeV PROTONS* I. HALDORSEN, A. C. PAPPAS and M. SKARESTAD Department of Chemistry, University of Oslo, Blindern, Norway
( Received 20 April 1973)
Cumulative and independent fission yields have been determined radiochemically for some long-lived tin and antimony isotopes formed in the reaction U(170 MeV p, f). By use of charge dispersion curves established earlier, chain yields in the mass range 117 .<_ A < 126 were calculated. The mass distribution curve has been drawn in the same mass range-and proves that a valley occurs in this region. Thus it is confirmed that the mass distribution curve is neither symmetric, nor has it a broad flat maximum.
Abstract
INTRODUCTION
EXPERIMENTAL
FIssioN o f natural uranium induced by 170 MeV protons has been extensively studied by the Oslo Uppsala group [ 1-7], both concerning mass distribution, charge distribution and dispersion, neutron-emission, etc. Using these results and data from excitation curves[8-10], the mass distribution is well established by Pappas and Hageb¢[6] for masses A = 65-115 and A = 127-164, while no data are yet available below A = 65 and above A = 164. In the region A ~ 115130 the shape of the curve is not known, although radiochemical measurements on the borders of this region [6] seem to indicate a valley. This mass range is of particular interest in understanding how different fission components contribute in building up the overall mass yield curve[6]. As first indicated by Rudstam and Pappas[4] in an analytical treatment of this process based on a model of two different fission components, symmetric and asymmetric, the overall mass distribution should be neither single peaked nor reflection symmetric, and should show a small valley in the region A ~ 120-130. Pappas and Hageb~[6, 7] have further suggested that the curve can be resolved into several fission components, and such an interpretation can be helped by establishing the exact shape of the mass distribution curve in the actual mass region, The present work therefore reports on formation cross-sections (fission yields) for eight long-lived nuclides of tin and antimony and a calculation of six chain yields in the mass range 117 < A_< 126.
* Work based on a thesis by M. Skarestad. 245
Target and irradiations High purity uranium foils were irradiated in the circulating beam of the synchro-cyclotron at the Gustaf Werner Institute in Uppsala, with protons of 170 MeV maximum energy {157 MeV at maximum intensity), for periods of four or five hours. The targets were then transported to the University of Oslo for radiochemical studies.
Fission yield monitor Fission yields were all determined relative to 99Mo rather
than 89Sr used previously[2-5] because of the more convenient half life 66.7 hr. In addition its decay characteristics as well as fission yield (56 + 6) mbarn[l, 6] are now well established.
Chemical procedures After irradiation the target was dissolved in a mixture of conc. HC1 and conc. HNO3 containing 1-10 mg of the appropriate carriers. All procedures were such as to ensure complete chemical exchange between added carrier and fission product nuclides of same element. Molybdenum. Mo was isolated by a procedure clue to Stevenson et al.[11], but slightly modified for needs of the present work. The method is based on anion-exchange separation using Dowex 1, 100 200 mesh, as resin. The column length was reduced to a tenth of that used by Stevenson er al.. because of the smaller amount of carrier (l rag) used. The procedure is rapid, and tested to give an overall decontamination factor better than 105. Chemical yields ( ~ 80 o..,) were determined by a spectrophotometric method based on formation of the molybdenumlV)-thiocyanatecomplex [ 12].
I. HALDORSEN,A. C. PAPPASand M. SKARESTAD
246
Tin. The general procedure was similar to that developed by Lawrence et al.[13] for fission product Sn in thermal neutron induced fission. In high energy induced fission the method was shown to give contaminated samples due to a quite different yield pattern, and therefore modifications were required. The final procedure consists essentially of two main steps:0) Anion-exchange separation using Dowex-1, 100 200 mesh, as resin. Sn was absorbed from 0.9 M HC1 and eluted with 1-8 M HC104, and (ii) H2S precipitation in dilute HC11pH 1) with fluoride as complexing agent. This step gives good decontamination from many interfering elements. Individual decontamination factors for Te, Sb, Cd, In and Mo are of the order 108-109. This procedure, which takes three to four hours, results in high overall decontamination factor (> 105). A gravimetric method[14] based on N-benzoylphenylhydroxylamine as a precipitating agent was used for chemical yield ( ~ 50~o) determination. Antimony. This element was isolated from the fission product mixture about three months after irradiation, which allowed complete decay of the 9.6 d ~2S~Snto 2.7 yr 125Sb. Since by this time all short-lived activities had decayed, Sb was easily separated by HzS precipitation using fluoride complexing agent and HI to reduce Sb(V) to Sb(III). Te was then removed by H2S precipitation in conc. HC1, and finally Sb reduced to metal using CrCI2. The precipitate was washed and mounted, and chemical yield ( ~ 90 %) measured by direct weighing of the metallic Sb. Activity measurements The decay characteristics of the nuclides studied in this work are given in Table 1. A 50 cm 3 Ge(Li) detector was used to measure 7 radiations, and the single spectra stored using a 4096-channel analyser. The spectra were computer-analysed using a programme developed by Gunnink[15], modified[161 for the CDC-3300 computer at Oslo University. The fl emission from the Sn and the Mo samples was followed using an end window gas flow proportional counter which had been calibrated for efficiency against a 4nil counter. Standard corrections for absorption and scattering effects were carried out. The complex decay curves of the Sn samples were followed for four to five months and resolved Table 1. Nuclear properties used in measurements of nuclides studied
Nuclide
Half-life
ll3gSn lw"Sn 121sSn 123sSn
115d 14.0 d 27.3 hr 129.3 d
x25gSn
9.65 d
i24~Sb ~2SSb 126gSb 99Mo
60.2d 2-77 yr 12.4 d 66.7hr
Radiation followed (Er keV)
Abundance (%)
? 392 7 158 fl7 1089 /3? 1067 /37 60 x 3 7 428 7 665 y 181
64.4 87.0 100 0.60 + 0.05* 100 9.45 _+ 0.80* 100 98.2 29.6 100 6.80 + 0.50*
3-
lOO
*Aburidances determined in the present work. Other values found in [18, 19].
into three components, viz. 121gSn, 123gSn and i25gSn, using a least-squares computer programme[17]. The component 123gSn was determined after complete decay of i25gSn and chemical removal of lzSSb. The absolute abundances of the t' rays used for detection of 123gSn, 125gSn and 99Mo were determined by comparison of the decay rates in the efficiency calibrated 13proportional counter with those in the Ge(Li) detector. The results are shown in Table 1. These determinations of absolute abundances were carried out because of the wide spread in previously reported values[18, 19]. It is essential to have accurate absolute abundances, since inaccurate values give rise to large errors in calculated cross-sections. RESULTS AND DISCUSSION The fission yields determined in the present study are given in Table 2 as averages of 2-3 independent measurements. The errors quoted are those based on uncertainties in activity measurements, chemical yield determination and counting efficiency. The yield given for 113Sn (corrected for the b r a n c h e d decay of 113mSn) can be regarded as independent yield. This since the c o n t r i b u t i o n from i i 3 s b must be negligibly small whatever charge distribution and dispersion govern this mass region. (The distance from any reasonable most probable primary charge Zp must be at least 4 Z-units for 113Sb, cfr. also Fig. 2.) The yield of ~~7mSn is also independent, since neither ~a7In n o r i17Sb populate the 117Sn isomeric state[13]. It is essential, in order to arrive at the cumulative cross-section for the mass chains A = 117, 121, 123, 124, 125 and 126, to have knowledge of the charge dispersion a n d distribution parameters valid in this mass region. Pappas and Hagebqi[6] have previously firmly established b o t h the full width at half m a x i m u m ( F W H M ) of the charge dispersion curve a n d Zv's position in the mass range 127 < A < 134. The charge dispersion curve is found to be Gaussian with F W H M of ~ 2.8 Z-units. Additional data indicate that the same F W H M - v a l u e is valid in the whole mass range A ~ 100-140. This formed their basis for establishing the N v / Z p ratio as a function of mass n u m b e r in fission of u r a n i u m induced by 170 MeV protons, The validity of the charge dispersion curve a n d the Nv/Zv mass dependence has found support in several more recent studies[20-23] and seems now to be firmly established[24]. The parameters in [6] have Table 2. Measured yields Nuclide
Type of yield*
Yield (mb)
lX3Sn lw"Sn x2igSn 123~Sn 12~gSb 12SgSn 1255b 126gSb
I I C C I C C I
(1.0 +_ 0.2). 10 -2 1.96 +_ 0.20 23 + 2 19.6 ___ 1.5 9.9 + 1.0 10.2 + 0.9 18"8 + 2"0 9-8 + 0.9
* I : Independent yield ; C : Cumulative yield.
Mass distribution in U(170 MeV p,/)
247
Finally, the ratio o
cr,,(12SSb) and crcl123~Sn)have been measured (Table 2), it is possible to estimate crc(~23"Sn). The same approach can be used to estimate oc( 125'nSn).The yields estimated along these lines are in satisfactory agreement with the known fawmring of high-spin isomers in tission. It is evident from Tables 2 and 3 that only in the case of ~17Sn the estimated yield can be as high as 50 per cent of the total isotopic yield. In all other cases the estimated yields represent a minor correction in the total yields. Therefore, the errors in the final isotopic yields are not very sensitive to the assumptions made concerning isomeric yield ratios. Results obtained along these lines are given in Table 4. The chain yields in column 2 are based on yield measured for one isomer only, while those in column 3 are based on measured or estimated total isotopic yields. The nuclides of interest mostly lie close to the maximum of the charge dispersion curve, which renders the calculation of total chain yields from the measured independent yields more accurate {Table 31, An exception is 113Sn which is situated as far as 3.6 Z-units from the maximum (Zpl, making total chain yield estimate for A = 113 very uncertain, and was therefore not carried through. The chain yield for A = 125, when estimated from the measured yield of ~2SSb, lies slightly below the general trend in the mass yield curve, plotted in Fig. 1. A similar phenomenon is also noticed by Miller[33] who measured excitation functions for antimony isotopes, and found that the er~(*25Sb) in the whole energy range from 30-60 MeV is low with respect to the cumulative yields of the other antimony isotopes. Miller suggests that an explanation might be the possible existence of an isomer, e.g. ~25"Sb. The present authors have no firm evidence of such an isomer, although it may be of significance that this phenomenon has been observed in two independent studies. The mass yield curve in Fig. 1 gives firm evidence for a valley in the region 100 < A < 130, and confirms that the mass distribution curve shows a well defined structure, i.e. is neither symmetric nor has it a broad flat maximum. It is also evident that the asymmetric fission component makes a substantial contribution, even at
Table 3. Estimated yields
Table 4. Chain yields
therefore been used with confidence in the present calculations. A difficulty which arises in the t i n - a n t i m o n y region is due to the existence of many isomers. Unless each isomer is measured, the yields will only represent a fraction of the corresponding isotopic yield. At present isomer yield ratios in this region are known only for 1248b {,f,/rtt - 15.8 -k 1.6) and 126Sb (m/g = 4.4 _+ 0.4) from U ( 170 MeV p, J) [25]. Ratios for the other nuclides of interest must therefore be estimated in order to obtain the total isotopic yields. It is, however, fairly well established in high energy (_> 150 MeV) induced fission that high spin isomers are strongly favoured over low spin isomers[20, 21, 25-327.* On this basis one may conservatively assume following "lower limit" equation:
oi!117"Snlll,,'2--)l/oi[liTgSn(l12+)l
> 2
where cr~ is the independent yield of the species (~r is used to represent the cumulative yield). Using this assumption, the calculated "upper limit" independent yield for the stable ~1;~Sn in Table 3 is obtained. The independent yield of ~2~"Sn was estimated, assuming that : ti)
ai[li'mSn(ll!2-)~,,ai[llVgSn(1/2+)l ai[12 l,.Sn( 11/2 - )]/o'i 112 t gSn(3/2 +
)].
This gives together with the ratio : (ill k = 6i[llv"+gSnl,ai[lal"+gSn] as determined from the charge dispersion curve in Fig. 2 using appropriate Zp values from [6] : (iii)
k = o'i[llVrnSn]itTi~121mSn].
Since ai[ 1, v,.Snl has been measured (Table 2), tri[ 12a,.Sn ] may be calculated. Since the beta decay of ~21In feeds only the ground state of 12aSn~18], the cumulative yields for 121an is given by: o-c[121Sn] = o-i[121mSn7 + o'e[121gSn].
Nuclide
Type of yield*
Yield [mb}
1 ~ 7~Sn
I I C I C I
< 0.98 5.7 _+ 1.5 3.0 + 0.9 0.60 ± 0.05+ 2.3 ± 0.7 1-9 ± 0.2)
~21"Sn 123~Sn tz~"Sb i25"Sn lz6"Sb
*/:Independent yield: C: Cumulative yield. ? Based on measured isomeric yield ratios for these nuclides as given in r25J. * l'he
,,>nlv
known exception is for t2%b.
Mass number
Lower limit of chain yield {mbarn)*
Chain yield Imbarn)
117 121 123 124 125
21.8 ± 3.5 27-8 ± 3.8 30.2 ± 4-2 (35.3 + 4-81 30.2 +_ 3.7 (i25gSn)
126
136.5 _+ 4.5~
32.7 +_:5.5 34.5 ± 4.6 34.8 +: 5.l 35.3 +: 4.8 36.8 --: 6.1 !27.3 ± 3.8r~ 3(,.5 ~: 4.5
* The values given in this column are based on measured yields, without taking into account contributions from isolners. ) Based on ~2sSb, see text.
248
I. HALDORSEN,
i
100
i
i
J
i
i
i
A.
C.
PAPPAS
and
M.
SKARESTAD
seem to lead predominantly to a symmetric fission component centred around A ~ 105. In conclusion there is a substantial contribution at all energies from asymmetric division of nuclei. Thus the valleys in the mass distributions from the different asymmetric fission components are partly drowned by the heavy wing from the symmetric fission components. The overall result from these competitions will therefore be a valley as determined in the present work.
i
@ 10
o~ >,
Acknowledgements--The authors wish to acknowledge discussions with Drs. J. Alstad and E. Hageb¢ and are grateful to the Norwegian Research Council for Science and Humanities for financial support.
c o
REFERENCES
i 60
i 80
I
i 100 mass
I
I 120
I
I 140
I 160
number
Fig. 1. Mass yield curve for fission of uranium induced by 170 MeV protons. The full drawn curve as given in [6]. O, This work. 170 MeV, as indicated by Rudstam and Pappas [4]. This condition is also in agreement with more recent kinetic energy measurements of fission fragments from uranium bombarded with 155 MeV protons as reported by Galin et al.[34]. These authors obtain about 30 per cent asymmetric component at this energy, and it looks [24] as if the asymmetric fission component would vanish only at high (GeV) bombarding energies. The shape of the mass distribution in Fig. 1 with a light and heavy peak centred around A ~ 105 and A ~ 132, respectively, a shoulder at A = 90-95 and a valley centred around A ~ 120 gives substantial support to the description of fission of uranium induced by 170 MeV protons as proposed by Pappas and Hageb¢ [6]. Their schematic representation of the participation of the different fission components in building up the overall mass yield curve is based on a large spectrum of experimental information including charge distribution, charge dispersion and neutron emission. With reference to [6], the deposition energy spectrum as calculated by Metropolis et al.[-35] and results from angular correlation studies as reported by Remsberg et a/.[36], the "energy origin" of the different fission components, characteristics and contribution to the overall mass distribution seems to be the following one: The products in the light mass region represent mainly the light peak (A ~ 80) from an asymmetric fission component associated with high deposition energy events (i.e. low average mass of apparent fissioning nucleus), while the products in the heavy mass region represent mainly the heavy peak (A ~ 135) from an asymmetric fission component associated with low deposition energy events (i.e. high average mass of apparent fissioning nucleus). High excitation energies
1. A. Kjelberg and A. C. Pappas, Nucl. Phys. 1,332 (1956). 2. P. Aagaard, G. Andersson, J. O. Burgman and A. C. Pappas, J. inorg, nucl. Chem. 5, 105 (1957). 3. A. C. Pappas and J. Alstad, J. inorg, nucl. Chem. 17, 195 (1961). 4. G. Rudstam and A. C. Pappas, Nucl. Phys. 22, 468 (1961). 5. E. Hageb~, A. C. Pappas and P. Aagaard, J. inorg, nucL Chem. 26, 1639 (1964). 6. A. C. Pappas and E. Hageb~b, J. inorg, nucl. Chem. 28, 1769 (1965). 7. A. C. Pappas, Z. Naturf 21a, 995 (1966). 8. H. G. Hicks and R. S. Gilbert, Phys. Rev. 100, 1286 (1955). 9. M. Lindner and R. M. Osborne, Phys. Rev. 94, 1323 (1954). 10. G. Friedlander, L. Friedman, B. Gordon and L. Yaffe, Phys. Rev. 129, 1809 (1963). 11. P. Stevenson, H. G. Hicks and C. K. Levy, UCRL-4377, 24, 1954. 12. C. E. Crouthamel and C. E. Johnson, Analyt. Chem. 26, 1284 (1954). 13. F. O. Lawrence, W. R. Danoels andD. C. Hoffman, J. inorg, nucl. Chem. 28, 2477 (1966). 14. D. Ryan and G. D. Lurwick, Can. J. Chem. 31, 9 (1953). 15. R. Gunnink, UCID-15140, 1967. 16. O. Scheidemann, Internal Report I, Nucl. Chem. Div., Department of Chemistry, University of Oslo, 1972. 17. B. Sundvoll, Private communication. 18. C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes, 6th edn. John Wiley, London (1967). 19. Nuclear Data Tables. Academic Press, New York (1970). 20. J. A. Panontin and N. T. Porile, J. inorg, nucl. Chem. 30, 2017 (1968). 21. E. Hageb¢, J. inorg, nucl. Chem. 32, 2489 (1970). 22. J. J. Hogan and N. Sugarman, Phys. Rev. 182, 1210 (1968). 23. J. A. Panontin and N. Sugarman, J. inorg, nucl. Chem. 34, 1485 (1972). 24. A. C. Pappas, J. Alstad and E. Hageb~b, In Inorganic Nuclear Chemistry Series One. Vol. 8: Radiochemistry, Chapter 8, p. 321. MTP International Review of Science, Butterworths, London (1972). 25. E. Hageb~b, J. inorg, nucl. Chem. 29, 2578 (1967). 26. D. W. Seegmiller, Report UCRL-10850, 1963. 27. R. L. Kiefer, Report UCRL-11049, 1963. 28. C. T. Bishop, H. K. Vonach and J. R. Huizenga, Nucl. Phys. 60, 241 (1964).
Mass distribution in U(170 MeV p,f) 29. E. HagebO, J. inorg, nucl. Chem. 27, 927 (1965). 30. R. Vandenbosch, L. Haskin and J. C. N o r m a n , Phys. Rec. 137, B1134 (1965). 31. N. D. Dudey and T. T. Sugihara, Phys. Rer. 139, B896 (1965). 32. W. B. Waltersand and J. B. Hummel, Phys. Rev. 150, 867 (1966). 33. k . D . Miller, Ph.D. Thesis, McGill University, Montreal, Canada, 1970. 34. J. Galin, M. Lefort, 1. Peter and X. Tarrago, Nucl. Phys. A134, 513 (1969). 35. N. Metropolis, R. Bivens, M. Strom, A. Turkevich, J. M. Miller and G. Friedlander, Phys. Rev. 110, 185 ( 19581. 36. L. P. Remsberg, F. Plasil, J. B. C u m m i n g and M . L . Perlman, Phys. Ret,. 187, 1597 (1969).
249
10 0
lO 1
/
_~ ~, lo-2 =_
/
/
/
w
"S = °1o -3
APPENDIX
The mass distribution curve in Fig. 1 can be used to estimate the fractional chain yield of t13Sn from the measured independent yield in Table 2. With a chain yield of 50 m b a fractional yield of (2.0 + 0.5). 10 -4 is found. The result as plotted in Fig. 2 seems to indicate that the charge dispersion curve in this mass region may fall off rather rapidly at the wings.
-4
3
I
I
I
I
-2
~1
0
1
Z-
t 4
Zp
Fig. 2. Charge dispersion curve valid in the mass region A = 105 to A = 140. The full drawn curve as given in [6]. ©, This work.
STUDIES OF THE MASS D I S T R I B U T I O N IN FISSION OF U R A N I U M I N D U C E D BY 170 MeV PROTONS* I. HALDORSEN, A. C. PAPPAS and M. SKARESTAD Department of Chemistry, University of Oslo, Blindern, Norway
( Received 20 April 1973)
Cumulative and independent fission yields have been determined radiochemically for some long-lived tin and antimony isotopes formed in the reaction U(170 MeV p, f). By use of charge dispersion curves established earlier, chain yields in the mass range 117 .<_ A < 126 were calculated. The mass distribution curve has been drawn in the same mass range-and proves that a valley occurs in this region. Thus it is confirmed that the mass distribution curve is neither symmetric, nor has it a broad flat maximum.
Abstract
INTRODUCTION
EXPERIMENTAL
FIssioN o f natural uranium induced by 170 MeV protons has been extensively studied by the Oslo Uppsala group [ 1-7], both concerning mass distribution, charge distribution and dispersion, neutron-emission, etc. Using these results and data from excitation curves[8-10], the mass distribution is well established by Pappas and Hageb¢[6] for masses A = 65-115 and A = 127-164, while no data are yet available below A = 65 and above A = 164. In the region A ~ 115130 the shape of the curve is not known, although radiochemical measurements on the borders of this region [6] seem to indicate a valley. This mass range is of particular interest in understanding how different fission components contribute in building up the overall mass yield curve[6]. As first indicated by Rudstam and Pappas[4] in an analytical treatment of this process based on a model of two different fission components, symmetric and asymmetric, the overall mass distribution should be neither single peaked nor reflection symmetric, and should show a small valley in the region A ~ 120-130. Pappas and Hageb~[6, 7] have further suggested that the curve can be resolved into several fission components, and such an interpretation can be helped by establishing the exact shape of the mass distribution curve in the actual mass region, The present work therefore reports on formation cross-sections (fission yields) for eight long-lived nuclides of tin and antimony and a calculation of six chain yields in the mass range 117 < A_< 126.
* Work based on a thesis by M. Skarestad. 245
Target and irradiations High purity uranium foils were irradiated in the circulating beam of the synchro-cyclotron at the Gustaf Werner Institute in Uppsala, with protons of 170 MeV maximum energy {157 MeV at maximum intensity), for periods of four or five hours. The targets were then transported to the University of Oslo for radiochemical studies.
Fission yield monitor Fission yields were all determined relative to 99Mo rather
than 89Sr used previously[2-5] because of the more convenient half life 66.7 hr. In addition its decay characteristics as well as fission yield (56 + 6) mbarn[l, 6] are now well established.
Chemical procedures After irradiation the target was dissolved in a mixture of conc. HC1 and conc. HNO3 containing 1-10 mg of the appropriate carriers. All procedures were such as to ensure complete chemical exchange between added carrier and fission product nuclides of same element. Molybdenum. Mo was isolated by a procedure clue to Stevenson et al.[11], but slightly modified for needs of the present work. The method is based on anion-exchange separation using Dowex 1, 100 200 mesh, as resin. The column length was reduced to a tenth of that used by Stevenson er al.. because of the smaller amount of carrier (l rag) used. The procedure is rapid, and tested to give an overall decontamination factor better than 105. Chemical yields ( ~ 80 o..,) were determined by a spectrophotometric method based on formation of the molybdenumlV)-thiocyanatecomplex [ 12].
I. HALDORSEN,A. C. PAPPASand M. SKARESTAD
246
Tin. The general procedure was similar to that developed by Lawrence et al.[13] for fission product Sn in thermal neutron induced fission. In high energy induced fission the method was shown to give contaminated samples due to a quite different yield pattern, and therefore modifications were required. The final procedure consists essentially of two main steps:0) Anion-exchange separation using Dowex-1, 100 200 mesh, as resin. Sn was absorbed from 0.9 M HC1 and eluted with 1-8 M HC104, and (ii) H2S precipitation in dilute HC11pH 1) with fluoride as complexing agent. This step gives good decontamination from many interfering elements. Individual decontamination factors for Te, Sb, Cd, In and Mo are of the order 108-109. This procedure, which takes three to four hours, results in high overall decontamination factor (> 105). A gravimetric method[14] based on N-benzoylphenylhydroxylamine as a precipitating agent was used for chemical yield ( ~ 50~o) determination. Antimony. This element was isolated from the fission product mixture about three months after irradiation, which allowed complete decay of the 9.6 d ~2S~Snto 2.7 yr 125Sb. Since by this time all short-lived activities had decayed, Sb was easily separated by HzS precipitation using fluoride complexing agent and HI to reduce Sb(V) to Sb(III). Te was then removed by H2S precipitation in conc. HC1, and finally Sb reduced to metal using CrCI2. The precipitate was washed and mounted, and chemical yield ( ~ 90 %) measured by direct weighing of the metallic Sb. Activity measurements The decay characteristics of the nuclides studied in this work are given in Table 1. A 50 cm 3 Ge(Li) detector was used to measure 7 radiations, and the single spectra stored using a 4096-channel analyser. The spectra were computer-analysed using a programme developed by Gunnink[15], modified[161 for the CDC-3300 computer at Oslo University. The fl emission from the Sn and the Mo samples was followed using an end window gas flow proportional counter which had been calibrated for efficiency against a 4nil counter. Standard corrections for absorption and scattering effects were carried out. The complex decay curves of the Sn samples were followed for four to five months and resolved Table 1. Nuclear properties used in measurements of nuclides studied
Nuclide
Half-life
ll3gSn lw"Sn 121sSn 123sSn
115d 14.0 d 27.3 hr 129.3 d
x25gSn
9.65 d
i24~Sb ~2SSb 126gSb 99Mo
60.2d 2-77 yr 12.4 d 66.7hr
Radiation followed (Er keV)
Abundance (%)
? 392 7 158 fl7 1089 /3? 1067 /37 60 x 3 7 428 7 665 y 181
64.4 87.0 100 0.60 + 0.05* 100 9.45 _+ 0.80* 100 98.2 29.6 100 6.80 + 0.50*
3-
lOO
*Aburidances determined in the present work. Other values found in [18, 19].
into three components, viz. 121gSn, 123gSn and i25gSn, using a least-squares computer programme[17]. The component 123gSn was determined after complete decay of i25gSn and chemical removal of lzSSb. The absolute abundances of the t' rays used for detection of 123gSn, 125gSn and 99Mo were determined by comparison of the decay rates in the efficiency calibrated 13proportional counter with those in the Ge(Li) detector. The results are shown in Table 1. These determinations of absolute abundances were carried out because of the wide spread in previously reported values[18, 19]. It is essential to have accurate absolute abundances, since inaccurate values give rise to large errors in calculated cross-sections. RESULTS AND DISCUSSION The fission yields determined in the present study are given in Table 2 as averages of 2-3 independent measurements. The errors quoted are those based on uncertainties in activity measurements, chemical yield determination and counting efficiency. The yield given for 113Sn (corrected for the b r a n c h e d decay of 113mSn) can be regarded as independent yield. This since the c o n t r i b u t i o n from i i 3 s b must be negligibly small whatever charge distribution and dispersion govern this mass region. (The distance from any reasonable most probable primary charge Zp must be at least 4 Z-units for 113Sb, cfr. also Fig. 2.) The yield of ~~7mSn is also independent, since neither ~a7In n o r i17Sb populate the 117Sn isomeric state[13]. It is essential, in order to arrive at the cumulative cross-section for the mass chains A = 117, 121, 123, 124, 125 and 126, to have knowledge of the charge dispersion a n d distribution parameters valid in this mass region. Pappas and Hagebqi[6] have previously firmly established b o t h the full width at half m a x i m u m ( F W H M ) of the charge dispersion curve a n d Zv's position in the mass range 127 < A < 134. The charge dispersion curve is found to be Gaussian with F W H M of ~ 2.8 Z-units. Additional data indicate that the same F W H M - v a l u e is valid in the whole mass range A ~ 100-140. This formed their basis for establishing the N v / Z p ratio as a function of mass n u m b e r in fission of u r a n i u m induced by 170 MeV protons, The validity of the charge dispersion curve a n d the Nv/Zv mass dependence has found support in several more recent studies[20-23] and seems now to be firmly established[24]. The parameters in [6] have Table 2. Measured yields Nuclide
Type of yield*
Yield (mb)
lX3Sn lw"Sn x2igSn 123~Sn 12~gSb 12SgSn 1255b 126gSb
I I C C I C C I
(1.0 +_ 0.2). 10 -2 1.96 +_ 0.20 23 + 2 19.6 ___ 1.5 9.9 + 1.0 10.2 + 0.9 18"8 + 2"0 9-8 + 0.9
* I : Independent yield ; C : Cumulative yield.
Mass distribution in U(170 MeV p,/)
247
Finally, the ratio o
cr,,(12SSb) and crcl123~Sn)have been measured (Table 2), it is possible to estimate crc(~23"Sn). The same approach can be used to estimate oc( 125'nSn).The yields estimated along these lines are in satisfactory agreement with the known fawmring of high-spin isomers in tission. It is evident from Tables 2 and 3 that only in the case of ~17Sn the estimated yield can be as high as 50 per cent of the total isotopic yield. In all other cases the estimated yields represent a minor correction in the total yields. Therefore, the errors in the final isotopic yields are not very sensitive to the assumptions made concerning isomeric yield ratios. Results obtained along these lines are given in Table 4. The chain yields in column 2 are based on yield measured for one isomer only, while those in column 3 are based on measured or estimated total isotopic yields. The nuclides of interest mostly lie close to the maximum of the charge dispersion curve, which renders the calculation of total chain yields from the measured independent yields more accurate {Table 31, An exception is 113Sn which is situated as far as 3.6 Z-units from the maximum (Zpl, making total chain yield estimate for A = 113 very uncertain, and was therefore not carried through. The chain yield for A = 125, when estimated from the measured yield of ~2SSb, lies slightly below the general trend in the mass yield curve, plotted in Fig. 1. A similar phenomenon is also noticed by Miller[33] who measured excitation functions for antimony isotopes, and found that the er~(*25Sb) in the whole energy range from 30-60 MeV is low with respect to the cumulative yields of the other antimony isotopes. Miller suggests that an explanation might be the possible existence of an isomer, e.g. ~25"Sb. The present authors have no firm evidence of such an isomer, although it may be of significance that this phenomenon has been observed in two independent studies. The mass yield curve in Fig. 1 gives firm evidence for a valley in the region 100 < A < 130, and confirms that the mass distribution curve shows a well defined structure, i.e. is neither symmetric nor has it a broad flat maximum. It is also evident that the asymmetric fission component makes a substantial contribution, even at
Table 3. Estimated yields
Table 4. Chain yields
therefore been used with confidence in the present calculations. A difficulty which arises in the t i n - a n t i m o n y region is due to the existence of many isomers. Unless each isomer is measured, the yields will only represent a fraction of the corresponding isotopic yield. At present isomer yield ratios in this region are known only for 1248b {,f,/rtt - 15.8 -k 1.6) and 126Sb (m/g = 4.4 _+ 0.4) from U ( 170 MeV p, J) [25]. Ratios for the other nuclides of interest must therefore be estimated in order to obtain the total isotopic yields. It is, however, fairly well established in high energy (_> 150 MeV) induced fission that high spin isomers are strongly favoured over low spin isomers[20, 21, 25-327.* On this basis one may conservatively assume following "lower limit" equation:
oi!117"Snlll,,'2--)l/oi[liTgSn(l12+)l
> 2
where cr~ is the independent yield of the species (~r is used to represent the cumulative yield). Using this assumption, the calculated "upper limit" independent yield for the stable ~1;~Sn in Table 3 is obtained. The independent yield of ~2~"Sn was estimated, assuming that : ti)
ai[li'mSn(ll!2-)~,,ai[llVgSn(1/2+)l ai[12 l,.Sn( 11/2 - )]/o'i 112 t gSn(3/2 +
)].
This gives together with the ratio : (ill k = 6i[llv"+gSnl,ai[lal"+gSn] as determined from the charge dispersion curve in Fig. 2 using appropriate Zp values from [6] : (iii)
k = o'i[llVrnSn]itTi~121mSn].
Since ai[ 1, v,.Snl has been measured (Table 2), tri[ 12a,.Sn ] may be calculated. Since the beta decay of ~21In feeds only the ground state of 12aSn~18], the cumulative yields for 121an is given by: o-c[121Sn] = o-i[121mSn7 + o'e[121gSn].
Nuclide
Type of yield*
Yield [mb}
1 ~ 7~Sn
I I C I C I
< 0.98 5.7 _+ 1.5 3.0 + 0.9 0.60 ± 0.05+ 2.3 ± 0.7 1-9 ± 0.2)
~21"Sn 123~Sn tz~"Sb i25"Sn lz6"Sb
*/:Independent yield: C: Cumulative yield. ? Based on measured isomeric yield ratios for these nuclides as given in r25J. * l'he
,,>nlv
known exception is for t2%b.
Mass number
Lower limit of chain yield {mbarn)*
Chain yield Imbarn)
117 121 123 124 125
21.8 ± 3.5 27-8 ± 3.8 30.2 ± 4-2 (35.3 + 4-81 30.2 +_ 3.7 (i25gSn)
126
136.5 _+ 4.5~
32.7 +_:5.5 34.5 ± 4.6 34.8 +: 5.l 35.3 +: 4.8 36.8 --: 6.1 !27.3 ± 3.8r~ 3(,.5 ~: 4.5
* The values given in this column are based on measured yields, without taking into account contributions from isolners. ) Based on ~2sSb, see text.
248
I. HALDORSEN,
i
100
i
i
J
i
i
i
A.
C.
PAPPAS
and
M.
SKARESTAD
seem to lead predominantly to a symmetric fission component centred around A ~ 105. In conclusion there is a substantial contribution at all energies from asymmetric division of nuclei. Thus the valleys in the mass distributions from the different asymmetric fission components are partly drowned by the heavy wing from the symmetric fission components. The overall result from these competitions will therefore be a valley as determined in the present work.
i
@ 10
o~ >,
Acknowledgements--The authors wish to acknowledge discussions with Drs. J. Alstad and E. Hageb¢ and are grateful to the Norwegian Research Council for Science and Humanities for financial support.
c o
REFERENCES
i 60
i 80
I
i 100 mass
I
I 120
I
I 140
I 160
number
Fig. 1. Mass yield curve for fission of uranium induced by 170 MeV protons. The full drawn curve as given in [6]. O, This work. 170 MeV, as indicated by Rudstam and Pappas [4]. This condition is also in agreement with more recent kinetic energy measurements of fission fragments from uranium bombarded with 155 MeV protons as reported by Galin et al.[34]. These authors obtain about 30 per cent asymmetric component at this energy, and it looks [24] as if the asymmetric fission component would vanish only at high (GeV) bombarding energies. The shape of the mass distribution in Fig. 1 with a light and heavy peak centred around A ~ 105 and A ~ 132, respectively, a shoulder at A = 90-95 and a valley centred around A ~ 120 gives substantial support to the description of fission of uranium induced by 170 MeV protons as proposed by Pappas and Hageb¢ [6]. Their schematic representation of the participation of the different fission components in building up the overall mass yield curve is based on a large spectrum of experimental information including charge distribution, charge dispersion and neutron emission. With reference to [6], the deposition energy spectrum as calculated by Metropolis et al.[-35] and results from angular correlation studies as reported by Remsberg et a/.[36], the "energy origin" of the different fission components, characteristics and contribution to the overall mass distribution seems to be the following one: The products in the light mass region represent mainly the light peak (A ~ 80) from an asymmetric fission component associated with high deposition energy events (i.e. low average mass of apparent fissioning nucleus), while the products in the heavy mass region represent mainly the heavy peak (A ~ 135) from an asymmetric fission component associated with low deposition energy events (i.e. high average mass of apparent fissioning nucleus). High excitation energies
1. A. Kjelberg and A. C. Pappas, Nucl. Phys. 1,332 (1956). 2. P. Aagaard, G. Andersson, J. O. Burgman and A. C. Pappas, J. inorg, nucl. Chem. 5, 105 (1957). 3. A. C. Pappas and J. Alstad, J. inorg, nucl. Chem. 17, 195 (1961). 4. G. Rudstam and A. C. Pappas, Nucl. Phys. 22, 468 (1961). 5. E. Hageb~, A. C. Pappas and P. Aagaard, J. inorg, nucL Chem. 26, 1639 (1964). 6. A. C. Pappas and E. Hageb~b, J. inorg, nucl. Chem. 28, 1769 (1965). 7. A. C. Pappas, Z. Naturf 21a, 995 (1966). 8. H. G. Hicks and R. S. Gilbert, Phys. Rev. 100, 1286 (1955). 9. M. Lindner and R. M. Osborne, Phys. Rev. 94, 1323 (1954). 10. G. Friedlander, L. Friedman, B. Gordon and L. Yaffe, Phys. Rev. 129, 1809 (1963). 11. P. Stevenson, H. G. Hicks and C. K. Levy, UCRL-4377, 24, 1954. 12. C. E. Crouthamel and C. E. Johnson, Analyt. Chem. 26, 1284 (1954). 13. F. O. Lawrence, W. R. Danoels andD. C. Hoffman, J. inorg, nucl. Chem. 28, 2477 (1966). 14. D. Ryan and G. D. Lurwick, Can. J. Chem. 31, 9 (1953). 15. R. Gunnink, UCID-15140, 1967. 16. O. Scheidemann, Internal Report I, Nucl. Chem. Div., Department of Chemistry, University of Oslo, 1972. 17. B. Sundvoll, Private communication. 18. C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes, 6th edn. John Wiley, London (1967). 19. Nuclear Data Tables. Academic Press, New York (1970). 20. J. A. Panontin and N. T. Porile, J. inorg, nucl. Chem. 30, 2017 (1968). 21. E. Hageb¢, J. inorg, nucl. Chem. 32, 2489 (1970). 22. J. J. Hogan and N. Sugarman, Phys. Rev. 182, 1210 (1968). 23. J. A. Panontin and N. Sugarman, J. inorg, nucl. Chem. 34, 1485 (1972). 24. A. C. Pappas, J. Alstad and E. Hageb~b, In Inorganic Nuclear Chemistry Series One. Vol. 8: Radiochemistry, Chapter 8, p. 321. MTP International Review of Science, Butterworths, London (1972). 25. E. Hageb~b, J. inorg, nucl. Chem. 29, 2578 (1967). 26. D. W. Seegmiller, Report UCRL-10850, 1963. 27. R. L. Kiefer, Report UCRL-11049, 1963. 28. C. T. Bishop, H. K. Vonach and J. R. Huizenga, Nucl. Phys. 60, 241 (1964).
Mass distribution in U(170 MeV p,f) 29. E. HagebO, J. inorg, nucl. Chem. 27, 927 (1965). 30. R. Vandenbosch, L. Haskin and J. C. N o r m a n , Phys. Rec. 137, B1134 (1965). 31. N. D. Dudey and T. T. Sugihara, Phys. Rer. 139, B896 (1965). 32. W. B. Waltersand and J. B. Hummel, Phys. Rev. 150, 867 (1966). 33. k . D . Miller, Ph.D. Thesis, McGill University, Montreal, Canada, 1970. 34. J. Galin, M. Lefort, 1. Peter and X. Tarrago, Nucl. Phys. A134, 513 (1969). 35. N. Metropolis, R. Bivens, M. Strom, A. Turkevich, J. M. Miller and G. Friedlander, Phys. Rev. 110, 185 ( 19581. 36. L. P. Remsberg, F. Plasil, J. B. C u m m i n g and M . L . Perlman, Phys. Ret,. 187, 1597 (1969).
249
10 0
lO 1
/
_~ ~, lo-2 =_
/
/
/
w
"S = °1o -3
APPENDIX
The mass distribution curve in Fig. 1 can be used to estimate the fractional chain yield of t13Sn from the measured independent yield in Table 2. With a chain yield of 50 m b a fractional yield of (2.0 + 0.5). 10 -4 is found. The result as plotted in Fig. 2 seems to indicate that the charge dispersion curve in this mass region may fall off rather rapidly at the wings.
-4
3
I
I
I
I
-2
~1
0
1
Z-
t 4
Zp
Fig. 2. Charge dispersion curve valid in the mass region A = 105 to A = 140. The full drawn curve as given in [6]. ©, This work.