Shell effect driven fission modes in fragment mass and total kinetic energy distribution of Hg*192

Abstract

Background: Asymmetric fission in sublead nuclei, especially mercury and platinum isotopes, has generated keen interest in studying fission in $A<200$ mass region. The role of proton and neutron shells on asymmetric fission in these nuclei is still under test and requires more investigation.Purpose: This paper presents measurements aimed at studying the fission modes of $^{192}\mathrm{Hg}$ formed in the reaction $^{32}\mathrm{S}+^{160}\mathrm{Gd}$ in the excitation energy range 54--74 MeV.Method: Mass distributions have been determined from the fission fragments' (FF) time of flight (TOF). Two multiwire proportional chambers were placed symmetrically on opposite sides of the beam to cover the folding angle for symmetric fission. The measured TOF of the fission fragments was used to obtain their velocities, and after clean separation of compound nuclear fission events, the velocities were further used to obtain the fission fragment mass and total kinetic energy (TKE) distribution.Results: The fission fragment mass distributions at all beam energies were found to be characteristically flat topped, which deviates significantly from a single Gaussian behavior. The distinctive features of mass and energy distributions happen to be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by different total kinetic energies in varying mass regions. These asymmetric fission modes are a consequence of the stabilization role of proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36, $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 46, and $Z$ = 28/50. The most probable mass of light and heavy fragments are found to be around 86 and 106 u, respectively. The symmetric yield is found to increase with the excitation energy.Conclusions: The present study indicates a mixture of symmetric and asymmetric modes in the fission of $^{192}\mathrm{Hg}$ nuclei and points out the relevance of deformed proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36 and 46.