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)

0.82 0.77 0.82

V ^ e f f (b)

1.88 1.85 1.70

396 © 1984 T h e Amer ican Physical Society