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

TL;DR

The paper investigates shell effects in quasifission forming $^{180}$Hg and their connection to fission in the preactinide region. It combines static fission-path calculations from constrained Hartree–Fock–Bogoliubov theory with dynamic time‑dependent Hartree–Fock simulations of quasifission for three entrance channels that form $^{180}$Hg, analyzing $Q_{20}$ and $Q_{30}$ and comparing fragment masses and total kinetic energies. The results show that shell effects hinder mass equilibration between prefragments, biasing toward the $80/100$ split, and reveal how the PES ridge and valley shape the reaction trajectories; notably, the $^{68}$Zn+$^{112}$Sn quasifission exhibits a fission-like path and TKE close to the fission data, aided by an elongated light fragment that aligns with proton-shell stabilization. The study highlights that dynamical calculations are essential to understand preactinide fission, where PES topography alone does not fully determine fragment outcomes, and it links quasifission pathways to specific fission channels via surface features on the PES.

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.

Figures (6)

• Figure 1: The heavy-fragment mass as a function of the prefragments contact time $\tau$. The gray band represents the experimental data of the most probable heavy-fragment mass $100(1)$ in ${^{180}\rm{Hg}}$ fission Andreyev2010PRL. The letters "s" and "t" denote the initial orientations "side" and "tip", respectively.
• Figure 2: Total kinetic energy of fragments as a function of heavy-fragment mass. The letters "s" and "t" denote the initial orientations "side" and "tip", respectively. The contact time $\tau$ for each reaction is indicated by the face color of each point. The black curve is given by Viola systematic Hinde1987NPA. The experimental data of fission fragments TKE are taken from Ref. Elseviers2013PRC.
• Figure 3: (a) Quasifission trajectories of all entrance channels superimposed on the ${^{180}\rm{Hg}}$ potential-energy surface. (b)(c) The enlarged views highlighting the trajectories of the ${^{68}\rm{Zn}} + {^{112}\rm{Sn}}$ reaction. The black dashed line and the colored solid line represent the entrance and the contact stage of reaction, respectively (see text for details). The stars indicate the contact point of the projectile and target. The letters "s" and "t" denote the initial orientations "side" and "tip", respectively.
• Figure 4: The trajectories of quasifission listed in Tab. \ref{['tab:compare']} and the static ${^{180}\rm{Hg}}$ fission path on top of ${^{180}\rm{Hg}}$ potential-energy surface. The stars indicate the contact point of the projectile and target.
• Figure 5: The prefragment masses as a function of the dinuclear system $Q_{20}$ for the quasifission events listed in Tab. \ref{['tab:compare']} and for the fission of $^{180}\rm{Hg}$. The stars indicate the contact points of the projectile and target for each quasifission. The gray bands indicate the mass regions around $80\pm1$ and $100\pm1$.