VOLUME 52, NUMBER 5 PHYSICAL REVIEW LETTERS 30 JANUARY 1984 Fission Anisotropics and Sequential Fission Back et al. 1 have reported measurements of fission angular distributions for the reactions 16 0 + 238 U and 32 S + 208 Pb. As the observed anisotropics for 32 S + 208 Pb are much greater than those for 16 0+ 238 U, they conclude that this constitutes an experimental signature for "quasifission." Fission for 16 0+ 238 U after transfer reactions could also lead to its smaller anisotropies. Rel- evant experimental information exists from studies of fusion versus transfer 2 and of correlated fission fragments. 3 For 166-MeV 16 0 + 238 U and 209 Bi, "non-compound-nucleus" fission comprises 30% and 0%, respectively, and corresponds to very low linear (and angular) momentum transfer. The low fissility of 209 Bi shields it from sequential fission as one can expect for 32 S + 208 Pb. For lower incident energies the projectilelike particle is deflected to larger angles and folding- angle measurements become less useful. For 102-MeV 16 O+ 208 Pb, nonfusion reactions constitute 27% of the nuclear reaction cross section. 2 We estimate perturbations to the anisotropy for 16 0 + 238 U via the results in Fig. 1. Fusion fission for 209 Bi is well described by a critical radius, r crit =0.95 fm, 5 with which we estimate the fusion-fission contribution (a cf ) for 16 0+ 238 U; we assign the difference to fission after transfer. The value of the maximum spin for the fissile nuclei is 7 m « (o cf /irX 2 ) 1/2 , and the fission anisotropy is related to I m 2 /8 eff T. In Table I we give corrected values for # 0 /£ eff calculated as follows 6 : (a) from the measured anisotropy and the corrected value of I m and (b) with an additional correction assuming isotropy for nonfusion fission. We expect reality to lie between (a) and (b). This range for 16 0 + 238 U includes the values for 32 S + 208 Pb; thus the comparison does not give a clear signature for quasifission. Experiments are re- quired that veto sequential fission. 7 In addition the "standard fission theory" is inapplicable for high-spin systems 6 ; therefore the comparison to the rotating-liquid-drop model is not pertinent. Louis C. Vaz John M. Alexander Department of Chemistry State University of New York at Stony Brook Stony Brook, New York 11794 Received 25 April 1983 PACS numbers: 25.70.Jj, 25.85.Ge 5000 16 0+ 209 Bi i6 0 + 209Bi _ a sikkeland 0 Viola 0 + " ° u \ • Back V Vaz 190 2I0 Eam.(MeV) FIG. 1. Fission excitation functions for 16 O + 209 Bi, 238 U (Refs. 1,4, and 6). Solid curves were calculated for complete fusion (Ref. 5), and dashed for a fusion barrier 2 MeV lower. 1 B. B. Back et ah, Phys. Rev. Lett. 5(3, 818 (1983). 2 F. Videbsek et al., Phys. Rev. C 15, 954 (1977). 3 T. Sikkeland and V. E. Viola, Jr., in Proceedings of the Third Conference on Reactions between Complex Nuclei, Asilomar, California, 1963 (Univ. of California Press,Berkeley, 1963), p. 232, and Phys. Rev. 125, 1350 (1962). 4 V. E. Viola and T. Sikkeland, Phys. Rev. 128, 767 (1962), and 135, B669 (1964). 5 J. Galin^ a L, Phys. Rev. C 9, 1018 (1974). The r crit scheme applies for cr cf ^800 mb. 6 L. C. Vaz and J. M. Alexander, Z. Phys. A 3121, 163 (1983), and Phys. Rep. 97C , 1 (1983). 7 K. T. Lesko^ a L, Phys. Rev. C 27, 2999 (1983). TABLE I. Reanalysis for 16 0 + 238 U. E lab (MeV) 110 130 148 (mb) 760 1046 1180 In, m M 41 53 60 (a) 0.82 0.77 0.82 V^eff (b) 1.88 1.85 1.70 396 © 1984 The American Physical Society

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Page 1: Fission Anisotropies and Sequential Fission

VOLUME 52, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 30 JANUARY 1984

Fission Anisotropics and Sequential Fission

Back et al.1 have reported measurements of fission angular distributions for the reactions 160 +238U and 32S +208Pb. As the observed anisotropics for 32S +208Pb are much greater than those for 160+238U, they conclude that this constitutes an experimental signature for "quasifission." Fission for 160+238U after transfer reactions could also lead to its smaller anisotropies. Relevant experimental information exists from studies of fusion versus transfer2 and of correlated fission fragments.3 For 166-MeV 160 + 238U and 209Bi, "non-compound-nucleus" fission comprises 30% and 0%, respectively, and corresponds to very low linear (and angular) momentum t ransfer. The low fissility of 209Bi shields it from se quential fission as one can expect for 32S +208Pb. For lower incident energies the projectilelike particle is deflected to larger angles and folding-angle measurements become less useful. For 102-MeV 16O+208Pb, nonfusion reactions constitute 27% of the nuclear reaction cross section.2

We estimate perturbations to the anisotropy for 160 + 238U via the results in Fig. 1. Fusion fission for 209Bi is well described by a critical

radius, rc r i t =0.95 fm,5 with which we estimate the fusion-fission contribution (acf) for 160+238U; we assign the difference to fission after transfer. The value of the maximum spin for the fissile nuclei is 7m« (ocf/irX2)1/2, and the fission anisotropy is related to Im

2/8effT. In Table I we give corrected values for # 0 /£ e f f calculated as follows6: (a) from the measured anisotropy and the corrected value of Im and (b) with an additional correction assuming isotropy for nonfusion fission. We expect reality to lie between (a) and (b). This range for 160 + 238U includes the values for 32S + 208Pb; thus the comparison does not give a clear signature for quasifission. Experiments are r e quired that veto sequential fission.7 In addition the "standard fission theory" is inapplicable for high-spin systems6; therefore the comparison to the rotating-liquid-drop model is not pertinent.

Louis C. Vaz John M. Alexander

Depar tment of Chemis t ry State Univers i ty of New York at Stony Brook Stony Brook, New York 11794

Received 25 April 1983 PACS n u m b e r s : 25 .70 .J j , 25.85.Ge

5 0 0 0

1 6 0+ 2 0 9 Bi

i 6 0 + 209Bi _ a sikkeland

0 Viola 0 + " ° u \ • Back

V Vaz

190 2 I0

Eam.(MeV)

FIG. 1. F i s s i o n excitat ion functions for 16O + 2 0 9Bi, 238U (Refs. 1,4, and 6). Solid c u r v e s w e r e calculated for comple te fusion (Ref. 5), and dashed for a fusion b a r r i e r 2 MeV lower .

1 B. B . Back et ah, P h y s . Rev. Let t . 5(3, 818 (1983). 2 F . Videbsek et al., P h y s . Rev. C 15, 954 (1977). 3 T . Sikkeland and V. E. Viola, J r . , in Proceedings of

the Third Conference on Reactions between Complex Nuclei, Asilomar, California, 1963 (Univ. of Cal ifornia P r e s s , B e r k e l e y , 1963), p . 232, and P h y s . Rev. 125, 1350 (1962).

4V. E. Viola and T. Sikkeland, P h y s . Rev. 128, 767 (1962), and 135, B669 (1964).

5 J . G a l i n ^ aL, P h y s . Rev. C 9, 1018 (1974). The r c r i t s cheme appl ies for crcf ^ 8 0 0 mb .

6 L. C. Vaz and J . M. Alexander , Z . P h y s . A 3121, 163 (1983), and P h y s . Rep. 97C , 1 (1983).

7K. T . L e s k o ^ aL, P h y s . Rev. C 27 , 2999 (1983).

TABLE I. Reanalys is for 1 60 + 238U.

E lab (MeV)

110 130 148

(mb)

760 1046 1180

In, m

M 41 53 60

(a)