Suppression of collective spin excitations for high-Kstates in heavy ion induced fission reactions

Abstract

Spin distributions of fission fragments have been measured in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O},$ ${}^{19}\mathrm{F}{+}^{209}\mathrm{Bi},$ and ${}^{232}\mathrm{Th}$ reactions at near and above barrier energies. It is found that the collective spin modes are suppressed for fragment emission along $90\ifmmode^\circ\else\textdegree\fi{}$ in comparison to the forward-backward emission angles. The suppression factor exhibits a universal scaling with respect to the rotational frequency of the fissioning nucleus along the fission symmetry axis. The analysis of data from the present measurements, as well as those available from the literature, shows that the suppression of collective spin degrees of freedom has the form ${\mathrm{S}}_{\mathrm{coll}}(\ensuremath{\theta})=\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}E(\ensuremath{\theta})/T){\mathrm{S}}_{\mathrm{coll}}(0\ifmmode^\circ\else\textdegree\fi{}),$ where $\ensuremath{\Delta}E(\ensuremath{\theta})$ is the excitation energy in the tilting mode characterized by the K quantum number.