Precise fission fragment anisotropies for the12C+232Th reaction: Supporting the nuclear orientation dependence of quasifission

Abstract

Recently measured fission fragment angular anisotropies for the reaction ${}^{12}$C + ${}^{232}$Th show unusual behavior compared to other reactions of ${}^{12}$C, ${}^{16}$O, and ${}^{19}$F on actinide targets at energies below the fusion barrier. As a result, doubts have been raised about the hypothesis that large sub-barrier anisotropies are due to the occurrence of quasifission when the projectiles collide with the tips of the deformed actinide target nuclei. To investigate this inconsistency, fission fragment angular distributions for this reaction were measured to high precision in the bombarding energy range 57--75 MeV. Fission following transfer reactions was identified and rejected at all angles using the deduced velocity vector of the fissioning nuclei. Cross sections and anisotropies for full momentum transfer fission were determined, and the fusion barrier distribution was extracted. These data support the interpretation that the dependence of the competition between quasifission and fusion fission on the orientation of the deformed actinide nucleus can explain the large sub-barrier anisotropies.