Shell effects in quasifission toward $^{180} \mathrm{Hg}$: insights into fission asymmetric modes

Abstract

Background: Experiment of $^{180}\mathrm{Hg}$ fission revealed a possible ``new asymmetric fission mode'' in the preactinide region, posing challenges to current fission theory. Similarity on shell effects are observed between fission and quasifission, providing possibility for widely exploring the topography of fission potential-energy surface (PES). Purpose: We aim to investigate the shell effects in the quasifission forming $^{180}\mathrm{Hg}$ and to explore their connection with the $^{180}\mathrm{Hg}$ fission. Method: $^{68}\mathrm{Zn}+^{112}\mathrm{Sn}$, $^{74}\mathrm{Se}+^{106}\mathrm{Pd}$, and $^{80}\mathrm{Kr}+^{100}\mathrm{Ru}$ central collisions at different energies and projectile orientations are calculated using the Skyrme time-dependent Hartree-Fock approach. The static fission properties are calculated with the constrained Hartree-Fock-Bogoliubov method and compared with the quasifission results. Results: Shell effects are found to hinder mass equilibration between the prefragments, enhancing the production of fragments near the $80/100$ mass split. By comparing the quasifission trajectories with the PES in the $(Q_{20}, Q_{30})$ space, the role of PES ridge in forming fragments is identified. The presence of asymmetric valley causes the $ {^{68}\mathrm{Zn}} + {^{112}\mathrm{Sn}} $ quasifission exhibits prefragment mass equilibration process and scission-point configuration similar to those of fission. The elongated light fragment is found to be a key factor in reproducing the experimental fission total kinetic energies. Conclusions: By using quasifission dynamics as a probe of the fission pathway, the present calculations help clarify the specific influence of the PES topography. This highlights the importance of dynamical calculations for preactinide fission, where the manifestation of shell effects is not intuitively evident from the PES.