Microscopic quasifission dynamics of the ${}^{54}\text{Cr}+{}^{243}\text{Am}$ reaction

Abstract

Synthesizing superheavy elements (SHEs) like $Z=119$ is severely hindered by the dominant quasifission (QF) channel, which prevents compound nucleus formation. Understanding QF dynamics is thus essential for future experiments. We investigate the QF mechanisms in the $^{54}\text{Cr}+^{243}\text{Am}$ reaction, a key candidate system for SHE 119, emphasizing the roles of projectile orientation and incident energy. Calculations are performed using the fully microscopic time-dependent Hartree-Fock theory based on the Skyrme energy density functional. We conduct systematic simulations covering a broad set of initial orientations of the deformed $^{54}\text{Cr}$ and $^{243}\text{Am}$ nuclei, alongside a finely spaced range of incident energies extending from below to well above the Coulomb barrier. Our fixed-energy results show that projectile side collisions are governed by shell effects driving heavy and light fragments toward spherical $Z=82$ and deformed $N=52\text{--}56$ closures, respectively, whereas tip collisions exhibit weaker shell influence. These shell-dominated reactions are characterized by shorter interaction times, attributed to the enhanced rigidity of shell-stabilized fragments accelerating neck rupture. The energy dependence reveals a complex evolution where the system transitions from an octupole-stabilized regime ($Z \approx 88$) to a spherical shell-driven regime, with specific energy windows exhibiting suppressed shell influence. Our study demonstrates that the manifestation of shell effects in QF is a dynamical outcome sensitively dependent on both collision geometry and incident energy. Systematically probing this energy sensitivity is crucial for identifying optimal incident energies where the QF process exhibits suppressed shell influence, thereby potentially enhancing the fusion probability and improving the prospects for synthesizing new SHEs.

Figures (6)

• Figure 1: Internuclear potentials for the $^{54}$Cr + $^{243}$Am system calculated using the FHF approximation. Each curve corresponds to a specific orientation combination labeled as $(\theta_{\mathrm{P}}, \theta_{\mathrm{T}})$. The solid lines represent the projectile tip orientation ($\theta_{\mathrm{P}} = 0^\circ$), while the dashed lines correspond to the projectile side orientation ($\theta_{\mathrm{P}} = 90^\circ$). Curves of the same color share the same target orientation $\theta_{\mathrm{T}}$. The black arrow indicates the incident energy $E_{\text{c.m.}} = 251.9$ MeV selected for the dynamical simulations.
• Figure 2: Polar representation of the final fragment distributions for the ${}^{54}\text{Cr}+{}^{243}\text{Am}$ reaction. The radial coordinates correspond to (a) neutron number $N$, (b) proton number $Z$, while the angular coordinate represents the center-of-mass scattering angle. Blue and orange symbols denote the results for the tip and side orientations of the ${}^{54}\text{Cr}$ projectile, respectively. The dashed circular arcs indicate the positions of relevant shell gaps $N=52, 126$ and $Z=36,82$.
• Figure 3: Fragment yield distributions for the $^{54}\mathrm{Cr}+{}^{243}\mathrm{Am}$ reaction at $E_{\text{c.m.}} = 251.9$ MeV. The panels display the yields as a function of (a) neutron number $N$ and (b) proton number $Z$. The shaded regions represent the weighted contributions from the ${}^{54}\text{Cr}$ tip (blue) and side (orange) orientations, integrated over all contributing impact parameters and nine ${}^{243}\text{Am}$ target orientations. The red solid lines denote the total angle-integrated yields. Vertical dashed lines mark the positions of relevant shell gaps at $N=52, 126$ and $Z=36, 82$.
• Figure 4: Fragment (a) proton number Z and (b) neutron number N as a function of contact time for the ${}^{54}\text{Cr}+{}^{243}\text{Am}$ reaction at $E_{\text{c.m.}} = 251.9$ MeV. The results for the projectile tip (blue points) and side (orange points) orientations are shown separately. Horizontal dashed lines indicate the positions of key spherical and deformed shell gaps.
• Figure 5: Statistical analysis of the contact time and event count versus the heavy fragment proton number $Z$ for the $^{54}\text{Cr} + {}^{243}\text{Am}$ reaction. The solid points refer to the average contact time $t_{\text{ave}}$ (left axis) calculated for each bin, with error bars corresponding to the standard deviation. The histogram displays the unweighted count of raw TDHF events $N_{\text{event}}$ (right axis). The data are grouped into bins of width $\Delta Z = 2$ centered on even proton numbers. The dashed line serves as a guide to the eye.