Reexamination of the nuclear orientation model of quasifission reactions to explain anomalous fragment anisotropies at sub-barrier energies.

Abstract

In the present work we have measured fission fragment angular distributions and fusion excitation functions for the systems $^{12}\mathrm{C}$, $^{16}\mathrm{O}$+$^{232}\mathrm{Th}$ at energies around the Coulomb barrier. These results along with the results of our earlier measurements on $^{19}\mathrm{F}$ + $^{232}\mathrm{Th}$ system were analyzed following Hinde et al. [Phys. Rev. Lett. 74, 1295 (1995)] where it was pointed out that the anomalous fission fragment angular distributions seen in heavy ion reactions below the Coulomb barrier can be due to possible occurrence of quasifission events caused by the interaction of the projectile with the tip of the deformed target nucleus. The present data do not show the expected saturation behavior of the fission fragment anisotropy at sub-barrier energies for the $^{12}\mathrm{C}$+ $^{232}\mathrm{Th}$ system which exhibits different behavior of the fission fragment anisotropy in the sub-barrier region as compared to the other two systems. The present measurements using different projectiles on the same target nucleus do not seem to conform to the expected behavior of the quasifission mechanism to be caused by the interaction of the projectile with the tip of the deformed target nucleus.