Anisotropy of selected fission fragments for helium-ion-induced fission of Pb206 and Bi209

I.E.6: I 2.J

I

Nuclear Physics 58 (1964) 321 --327; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or m i c r o f i l m w i t h o u t written p e r m i s s i o n from the publisher

ANISOTROPY

OF SELECTED

FOR HELIUM-ION-INDUCED

FISSION

FISSION

FRAGMENTS

O F Pb 2°6 A N D Bi 2°9

K. F. FLYNN, L. E. G L E N D E N I N and J. R. H U I Z E N G A

Argonne National Laboratory, Argonne, Illinois t Received 20 April 1964 Abstract: Fission fragments from 42 MeV helium-ion-induced fission of Pb2°e and Bi~°' were collected in thin aluminium catcher foils at centre-of-mass angles 90 ° and 165 ° to the beam direction. Several elements were chemically separated from the foils and purified by standard radiochemical procedures. The differential cross section ratios W(165°)/W(90°) for ruthenium (RuX°5), palladium (Pd t°9,112), silver (Ag m, 112,n3), strontium (Sr sl, 92), molybdenum (MoSD and bromine (Br u) were measured. The average values of W(165°)/14/(90 °) determined in these experiments by radiochemieal techniques are 2.3 and 2.0 for Pb ~°+ and Bi2°', respectively, in excellent agreement with previous results obtained with solid state counters. Since second-chance fission (u, nf) is negligible for each of these targets, the magnitude of the fission fragment anisotropy for a particular mass split is a measure of the shape of its saddle configuration. Although the saddle shapes depend on the target nucleus, the different mass splits of a particular target appear to arise from a configuration with essentially a common shape. However, the anisotropy of Br sa from the fission of Bi2°~ was 10 ?/ooless than that of other fragments. This result is consistent with an increase in the effective moment of inertia of a saddle configuration containing a fragment with a 50 N shell. E

N U C L E A R FISSION Pb ~°~, Bi~°9 fission product yields at 90 ° and 165 °.

1. Introduction A study of the relationship between the mass division and angular distribution of fission f r a g m e n t s is u s e f u l in t h e e l u c i d a t i o n o f t h e fission p r o c e s s . P r e v i o u s e x p e r i menters have observed a correlation between mass division and angular distribution f o r p h o t o n - 1) a n d c h a r g e d p a r t i c l e - 2, a) i n d u c e d fission. H o w e v e r , t h e s e e x p e r i m e n t s w e r e p e r f o r m e d at e x c i t a t i o n e n e r g i e s w h e r e m u l t i p l e c h a n c e fission c o u l d o c c u r , a n d h e n c e t h e i n t e r p r e t a t i o n o f t h e s e r e s u l t s is n o t u n i q u e . F o r a fixed v a l u e o f t h e a n g u l a r m o m e n t u m , p r e s e n t t h e o r y 4, s) p r e d i c t s the a n g u l a r d i s t r i b u t i o n to b e d e t e r m i n e d b y t h e K d i s t r i b u t i o n at s o m e c r i t i c a l stage o f t h e fission p r o c e s s . I f the e x c i t a t i o n e n e r g y is sufficient, t h e K d i s t r i b u t i o n c a n be c h a r a c t e r i z e d b y a p a r a m e t e r Ko2 w h i c h is d e f i n e d b y

Kg = J e f f T / h 2. T h e effective m o m e n t o f i n t e r t i a J e f f is e q u a l t o J ± J l l / ( J . l . - - i l l ) ,

(1) where J± and

* This report is based on work performed under the auspices of the U.S. Atomic Energy Commission. An abstract of the preliminary results obtained, in this work was reported previously XT). 321

322

K.F.

rL~

et al.

JIt are the perpendicular and parallel moments of inertia, respectively, of the nucleus at the critical deformation where the K distribution is fixed. A relationship 3.6) has been shown to exist between fission fragment mass division and excitation energy for some fissioning systems by observing a marked increase in the ratio of an asymmetric to symmetric fission product near the onset of new thresholds for fission such as the (~, nf), (~, 2nf) reactions. Hence at excitation energies where multichance fission is possible, the larger anisotropies for asymmetric fission is explainable in terms of the above enhancement of asymmetric fission at the lower excitation energies. The average temperature in eq. (1) for asymmetric fission is lower and this result gives a smaller value of Ko2 and a larger anisotropy. From the above discussion it is obvious that it is more desirable to study the correlation between mass division and angular distribution for systems restricted to single chance fission. Such measurements have been reported for the U238(n,f) process with neutrons from the D(d, n)He 3 reaction 7) and the T(p, n)He 3 reaction 8). The neutrons from the above deuterium target had an energy of 3 MeV, although the energy ranged down to 0.2 MeV due to a thick target 7). A double ionization chamber was used to find the angular anisotropy and the ratio of the fission fragment masses. The differential fission cross section ratios W(0°)/W(90 °) were constant at about 1.2 for all mass ratios between 1.25 and 1.65 although smaller and larger mass ratios (.where the statistical errors were large) gave larger anisotropies 7). The neutron energies 8) from the tritium gas target which impinged on the photographic plates ranged from 1.23 to 1.73 Me¥. This experiment 8) gave constant W(0°)/W(90 °) ratios of 1.2 and 1.8 for fission fragments of all mass ratios for neutron energies greater than 1.62 MeV and less than 1.62 MeV, respectively. Furthermore, the latter experiment indicated a larger anisotropy for the very symmetric events (MI/Mz < 1.05). A similar experiment 9) has been reported for the Th232(n, f) process in which most of the fission events were induced by neutrons with energies of less than 6 MeV. Within experimental error the W(O°)/W(90 °) ratios for fragments of all mass ratios between 1.1 and 1.9 were constant at approximately 1.3. Sint~e all of these experiments suffer from experimental limitations, no quantitative conclusion about the correlation between mass division and angular distribution can be reached, although all the experiments indicate that the anisotropy is approximately independent of mass division. Due to the practical difficulty of obtaining sufficiently intense beams of 2 to 3 MeV neutrons to study single-chance fission of nuclei in the region of uranium, we have attempted to study the fission fragment mass-angle correlations of nuclei in the vicinity of lead by helium-ion bombardment. Pre~iSias measurements lo) in this laboratory have shown that 42-MeV helium ion bombardments of nuclei near lead give single-chance fission, i.e., (~, nf) cross sections are less than one percent of the (~t, f) cross sections. Hence in a study of the helium-ion-induced fission of these light targets, one is assured that the anisotropies of selected fission fragments are not being influenced by temperature effects due to multichance fission.

HELIUM-INDUCED-ION-FISSION

323

Interest in such angular distributions in the vicinity of lead arises also for maother reason. Experimental measurements 11) and theoretical calculations 12.13) indicate that the saddle configurations o f these nuclei are nearly identical with their scission configurations. The possibility exists, therefore, that nuclear shell structure in the final fragments may influence the angular distributions.

2. Experimental Procedures Targets of Bi 2°9 and Pb 2°6 (88 ~/o) of approximately 1 rag/fro 2 thickness were prepared by volatilizing the metals onto 2 rag/era 2 aluminium backing foils. The targets were placed in the centre of a 28 cm scattering chamber which was evacuated during each run. The helium-ion projectiles were accelerated to approximately 42 MeV in the Argonne 150 cm cyclotron. The beam was collimated such that the beam image on the target was defined within a 0.3 cm circle. Each run consisted o f about a 4 h bombardment with a helium-ion current on the target o f about 0.5/~A. Aluminium catcher foils of 12.9 cm 2 area were placed 10 cm from the target at centre-ofmass angles 90 ° and 165 ° to the beam direction, each catcher foil subtending a solid angle o f about 1 ~o of 4n. The aluminium catcher foils which were approximately 8 mg/cm 2 in thickness were usually covered with a thin aluminium foil of 0.2 mg/cm 2 in thickness. The normal to the target bisected the angle between the catcher foils such that the fragments arriving at each catcher foil traversed the same target thickness. After each run the catcher foils were dissolved and the fission product elements were isolated. All the samples except those used for the bromine determinations were dissolved in 30 ml of concentrated HCI containing the appropriate carriers (i.e. Ru, Pd, Ag, Sr, Mo), 1 ml o f fuming HNO3 and 1 drop of liquid bromine. This solution was evaporated to 6 N in HCI and the Mo was extracted into either. The aqueous phase was diluted to 0.5 N in HCI to precipitate AgCI. Palladium was then precipitated with dimethylglyoxime (DMG). The excess D M G was compeltely removed by extraction into chloroform, and the aqueous phase was evaporated to a low volume. Five ml of HCIO4 were added to this solution, and RuO4 was distilled into 6 N NaOH. The still residue was diluted with water, and SrCO3 was precipitated using excess NaOH-Na2CO3 solution to keep the A1 from the catcher foils in solution. Those samples used in the determination of bromine were dissolved in 6 N N a O H containing bromine and palladium carriers. The solution was acidified with HNO3, and bromine was extracted into chloroform using KMnO4 as the oxidizing agent. The excess KMnO4 in the aqueous phase was destroyed with H202, and P d D M G was precipitated. Each element was then further purified using established radiochemical procedures. The purified samples (typically about 30 mg on a 2 cm 2 filter paper disc) were weighed to determine chemical yields, mounted on cards, covered with 0.8 rag/era 2 rubber hydrochloride and counted in an end-window beta proportional counter.

K.F. FLYNN et al.

324

T h e c o u n t e r was e q u i p p e d with an anticoincidence shield which r e d u c e d the backg r o u n d to a b o u t 2 c o u n t s p e r m i n u t e (c/rain). T h e d e c a y o f each s a m p l e was followed in o r d e r to establish its r a d i o c h e m i c a l p u r i t y and, in the case o f A g , to separate the short-lived a n d the long-lived c o m p o n e n t s . T h e o b s e r v e d initial c o u n t i n g rates o f the samples r a n g e d f r o m a b o u t 500 c/rain d o w n t o a b o u t 10 c/rain. A l l a c c e p t a b l e s a m p l e s h a d a n initial c o u n t i n g rate o f at least five times b a c k g r o u n d .

3. Experimental Results T h e o b s e r v e d c o u n t i n g r a t e o f each s a m p l e was corrected for decay a n d for chemical yield. The r a t i o o f the c o r r e c t e d c o u n t i n g r a t e at 165 ° to the c o r r e c t e d c o u n t i n g r a t e at 90 ° for each i s o t o p e in each r u n is e q u a l t o the r a t i o o f the cross sections [W(165°)/ W(90°)] for t h a t p a r t i c u l a r i s o t o p e a n d t h e r e f o r e for t h a t p a r t i c u l a r m a s s split o f the fissioning nucleus. T h e c a l c u l a t i o n o f the m a s s r a t i o (MH/ML) was b a s e d o n the a s s u m p t i o n t h a t a n average o f three n e u t r o n s were e v a p o r a t e d per fission. H e n c e in the case o f Bi 2°9, s y m m e t r i c fission w o u l d occur at m a s s 105, a n d in the case o f P b 2°6 it w o u l d occur at m a s s 103.5. T h e results o f these calculations are given in tables 1 and 2. The t a b u l a t e d cross section r a t i o s have been c o r r e c t e d to the centre-of-mass system. The q u o t e d e r r o r s are the s t a n d a r d deviations. TABLE 1 Anisotropy in the 42 MeV helium-ion-induced fission of Bi2°' Fission product Ru I°5 pd109,u~ Mo a9 Agm AgaX~,u3 Sr'1,92 Bras

Number of determinations 2 7 2 3 4 4 5

W(165°)/W(90 °) Bi2°'(a, f)

Mass ratio

1.95 4- 0.06 2.004-0.05 1.994-0.06 1.964-0.09 2.03 4- 0.18 1.944-0.08 1.784-0.06

1.00 1.11 1.12 1.12 1.15 1.30 1.53

TABLE 2 Anisotropy in the 42 MeV helium-ion-induced fission of Pb 2°6 Fission product Ru 1°~ Mo ~9 Pc[1°9,112 Aga~2,Ha Sr91,92

Number of determinations

W(165°)/W(90 °) Pb~°6(ct, f)

Mass ratio

1 2 3 2 2

2.174-(0.1) 2.204-0,11 2.354-0.11 2.33±0.29 2.194-0.04

1.03 1.09 " 1.14 1.19 1.26

Plots o f the a n i s o t r o p y [W(165°)/W(90°)] as a function o f m a s s r a t i o for the 42 M e V h e l i u m - i o n - i n d u c e d fission o f P b z°6 a n d Bi z°9 are shown in fig. 1. T h e e r r o r

HELIUM-INDUCED-ION FISSION

325

flags indicate the standard error of the mean for each point. Within the limits o f experimental error the anisotropy is constant for both Pb 2°6 and Bi 2°9 from symmetry out to mass ratios of about 1.3. However, measurements at high asymmetry on Bi 2°9 (i.e. MH/ML = 1.53) showed a small but significant decrease in the anisotropy. I

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2.5 2.4 pb zoe

2.3 ; " 2.2

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i 209

1.8

1,7 1.6 1.0

I I.I

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Fig. 1. Anisotropy versus mass split for the 42 MeV helium-ion-induced fission of Pb ~°e and Bi 2°9.

Yields for the various fission products measured in these experiments were calculated f r o m the data using empirically determined correction factors and counting efficiencies. Final values were determined by normalizing the smooth mass-yield curves to 200 ~o. The calculated fission yields with estimated errors are given in table 3. Each value was based on an average of five to ten samples. The various runs were TABLE 3 Yields for the 42 MeV helium-ion-induced fission o f Pb 2°e and Bi ~°9 Fission yields (%) Isotope Br s3 Sr 9x Mo" R u a°n Pd 1°9 Ag tat Pd 11~ Ag 11~

Pb 2°e

4.4 %4-0.5 6.1%-4-0.6 8.0 %-4-0.8 7.8 %4-0.8 5.1 ~ ± 0 . 5 6.5 ~o±0.7 5.2 %:k0.5

Bi z~9 0.9 3.6 6.0 7.5 6.5 5.6 5.7 5.5

%4-0.1 %-4-0.3 %:k0.5 %q-0.5 %4-0.5 ~o-4-0.5 %±0.5 %±0.5

normalized to each other through Pd x°9. Mass yield curves for the 42 MeV heliumion-induced fission of Pb 2°6 and Bi 2°9 are plotted in figs. 2 and 3, respectively.

K.F. FLY'NN et al.

326

4. Discussion The observed full width at half m a x i m u m for the fission fragment m a s s yield curves for helium-ion-induced fission o f Pb 2°6 and Bi 2°9 are 25 and 27 m a s s units, respectively. These values are in substantial agreement with other radiochemical data 14) on Pb 2°6 and solid-state detector results 15) on Bi 2°9. I0

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Fig. 3. Mass distribution for the 42 MeV helium-ion-induced fission of Bi=°L The magnitude o f the fission fragment anisotropy W(165°)/W(90 °) is rather constant as a function o f the fission fragment m a s s for the targets o f Pb 2°6 and Bi 2°9 which were studied.

HELIUM-INDUCED-ION FISSION

327

F o r Br a3, however, where the m a s s r a t i o is 1.53, the a n i s o t r o p y for h e l i u m - i o n i n d u c e d fission o f Bi 2°9 is o b s e r v e d to be s o m e I0 percent smaller. This is a n interesting result because the fission p r o d u c t Br 83 is d e r i v e d m a i n l y f r o m p r i m a r y f r a g m e n t s with a 50 n e u t r o n shell (e.g., Br as, Se 84, ASS3). T h e d e p e n d e n c e o f J c r f on m a s s split has been c o m p u t e d for simple configurations. In the first c a l c u l a t i o n the s a d d l e shape is r e p r e s e n t e d b y t w o t o u c h i n g p r e l a t e s p h e r o ids with a r a t i o o f m a j o r axis c to m i n o r axis a o f 1.8. F o r a n increace in M 1 / M 2 f r o m 1 to 1.5, ~'cff increases b y a p p r o x i m a t e l y 6 p e r c e n t (for the f r a g m e n t s o f une q u a l m a s s it was a s s u m e d t h a t c/a r e m a i n e d c o n s t a n t at 1.8). T h e increase in Ko2 is directly p r o p o r t i o n a l to the increase in o¢,f e as can be seen f r o m eq. (1). F o r a 6 increase in Ko2, the a n i s o t r o p y I4I(165°)/W(90 °) decreases a b o u t 3 percent, an a m o u n t c o n s i d e r a b l y less t h a n t h a t observed. A s e c o n d calculation was p e r f o r m e d in which the saddle shape was a s s u m e d to be the s a m e as the scission shape. T h e scission shape o f A t 213 as a function o f M I / M 2 was c o m p u t e d f r o m kinetic energy d a t a b y V a n d e n b o s c h x6), a n d f r o m these d a t a the d e p e n d e n c e o f o¢,ff on M 1 / M 2 was c o m p u t e d . T h e resulting values o f the anisot r o p y as a function o f M t / M 2 for A t 2x3 fission are very similar to the e x p e r i m e n t a l values. H o w e v e r , the smaller a n i s o t r o p y for Br 83 c a n n o t be u n i q u e l y interpreted in terms o f an increase in J , ff. T h e possibility exists t h a t the increase in K 2 m a y be associated with a larger t e m p e r a t u r e (see eq. (1)) for the saddle configuration l e a d i n g to Br 83. T h e a u t h o r s wish to t h a n k D r . R. V a n d e n b o s c h for c o m p u t i n g the d e p e n d e n c e o f J e f f o n M1/M2 for his p r e d i c t e d scission shapes o f A t 213.

References 1) A. W. Fairhall, I. Halpern and E. J. Winhold, Phys. Rev. 94 (1954) 733 2) B. L. Cohen, W. H. Jones, G. H. McCormick and B. L. Ferrell, Phys. Roy. 94 (1954) 625; B. L. Cohen, B. L. Ferrell-Bryan, D. J. Coombe and M. K. Hullings, Phys. Roy. 98 (1955) 685 3) J. A. Coleman, University of Washingon, Thesis (1962) 4) A. Bohr, Prec. Int. Conf., on the Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 2, (United Nations, New York, 1956) p. 151 5) I. I-Ialpern and V. M. Strutinski, Prec. Int. Conf. on the Peaceful Used of the Atomic Energy, Geneva, 1958, Vol. 15 (United Nations, New York, 1959) p. 408 6) B. J. Bowles, F. Brown and J. P. Butler, Phys. Roy. 107 (1957) 751 7) I. A. Baranov, A. N. Protopopov and V. P. Eismont, JETP (Soviet Physics) 14 (1962) 713 8) J. H. Manley, Nuclear Physics 33 (1962) 70 9) B. D. Kuz'minov, L. S. Kutsaeva and I. I. Bondarenko, JETP (Soviet Physics) 15 (1962) 75 10) J. R. I-Iuizenga, R. Chaudhry and R. Vandenbosch, Phys. Rev. 126 (1962) 210 11) R. Chaudhry, R. Vandenbosch and J. R. Huizenga, Phys. Roy. 126 (1962) 220 12) S. Cohen and W. J. Swiatecki, University of California Radiation Laboratory Report, UCRL 10450 (1962) unpublished 13) V. M. Strutinski, N. Ya. Lyashchenko and N. A. Popov, Nuclear Physics 46 (1963) 639 14) E. F. Neuzil and A. W. FairhaU, Phys. Rev. 129 (1963) 2705 15) J. P. Unik and I. R. Huizenga, Phys. Rev. 134 (1964) B90 16) R. Vandenbosch, Nuclear Physics 46 (1963) 129 17) K. E. Flynn et al., Bull. Am. Phys. Soc. 7 (1962) 303