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

Abstract

The total average spins of fission fragments were measured as a function of fragment mass in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{209}\mathrm{Bi}$ and ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ reactions at bombarding energies near and above the Coulomb barrier. The mass dependence of the fragment spin for the ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ reactions is observed to be in marked contrast to that for the ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{209}\mathrm{Bi}$ 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 ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ 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.