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 12C, 16O+209Bi and 12C, 16O+232Th reactions at bombarding energies near and above the Coulomb barrier. The mass dependence of the fragment spin for the 12C, 16O+232Th reactions is observed to be in marked contrast to that for the 12C, 16O+209Bi 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 12C, 16O+232Th 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.