REEXAMINATION OF THE NUCLEAR ORIENTATION MODEL OF QUASI-FISSION REACTIONS TO EXPLAIN ANOMALOUS FRAGMENT ANISOTROPIES AT SUBBARRIER ENERGIES

In the present work we have measured fission fragment angular distribu tions and fusion excitation functions for the systems C-12, O-16 + Th- 232 at energies around the Coulomb barrier. These results along with t he results of our earlier measurements on F-19 + Th-232 system were an alyzed following Hinde et al. [phys. Rev. Lett. 74, 1295 (1995)] where it was pointed out that the anomalous fission fragment angular distri butions seen in heavy ion reactions below the Coulomb barrier can be d ue to possible occurrence of quasifission events caused by the interac tion of the projectile with the tip of the deformed target nucleus. Th e present data do not show the expected saturation behavior of the fis sion fragment anisotropy at sub-barrier energies for the C-12 + Th-232 system which exhibits different behavior of the fission fragment anis otropy in the sub-barrier region as compared to the other two systems. The present measurements using different projectiles on the same targ et nucleus do not seem to conform to the expected behavior of the quas ifission mechanism to be caused by the interaction of the projectile w ith the tip of the deformed target nucleus.