Anomalous increase in width of fission-fragment mass distribution as a probe for onset of quasifission reactions in deformed target-projectile system at near and sub-barrier energies

Abstract

Fission-fragment mass distribution has been studied for the $^{16}\mathrm{O}+^{232}\mathrm{Th},^{209}\mathrm{Bi}$ systems over an energy range of $102.8--78.6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and $81.6--72.6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, respectively, in a laboratory frame. The variance of the mass distribution $({\ensuremath{\sigma}}_{m}^{2})$ for the $^{16}\mathrm{O}+^{209}\mathrm{Bi}$ system varies linearly with center of mass energy, while a significant anomalous behavior is found for the system $^{16}\mathrm{O}+^{232}\mathrm{Th}$. Coupled with our earlier observation for the system $^{19}\mathrm{F}+^{232}\mathrm{Th}$ [T. K. Ghosh et al., Phys. Rev. C 69, 031603(R) (2004)], we propose that the accurate measurement of mass distribution is a powerful tool to look for the onset of a nonstatistical reaction mechanism in heavy-ion-induced fission of deformed heavy nuclei.