Fragment spin as a function of the mass asymmetry in heavy ion induced fission reactions

The total average spins of fission fragments were measured as a function of fragment mass in C-12, O-16 + Bi-209 and C-12, O-16+ Th-232 reactions at bombarding energies near and above the Coulomb barrier. The mass dependence of the fragment spin for the C-12, O-16+ Th-232 reactions is observed to be in marked contrast to that for the C-12, O-16+ Bi-209 reactions. The present results have been analyzed within the framework of the statistical model taking into account the finite relaxation times for the equilibration of the collective-spin modes. The observed features in C-12, O-16+ Th-232 reactions can be ascribed to incomplete equilibration of the collective-spin modes where the mass relaxation at extreme mass asymmetries terminates before full statistical equilibration of the collective-spin modes is reached. [S0556-2813(99)04110-2].