PRECISE FISSION FRAGMENT ANISOTROPIES FOR THE C-12-232 REACTION - SUPPORTING THE NUCLEAR ORIENTATION DEPENDENCE OF QUASI-FISSION( TH)

Recently measured fission fragment angular anisotropies for the reacti on C-12 + Th-232 show unusual behavior compared to other reactions of C-12, O-16, and F-19 on actinide targets at energies below the fusion barrier. As a result, doubts have been raised about the hypothesis tha t large sub-barrier anisotropies are due to the occurrence of quasifis sion when the projectiles collide with the tips of the deformed actini de target nuclei. To investigate this inconsistency, fission fragment angular distributions for this reaction were measured to high precisio n 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 anisotro pies for full momentum transfer fission were determined, and the fusio n barrier distribution was extracted. These data support the interpret ation 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.