Most probable fission configurations and symmetric-asymmetric fission competition in Hg isotopes

Abstract The most probable fission fragment configurations $(A_L, A_H)$, $(Z_L, Z_H)$, and $(N_L, N_H)$, together with the relative contributions of symmetric and asymmetric fission channels, are systematically studied over a range of excitation energies for $^{178,180,182,183,184,190,192,194,195,196,198}\mathrm{Hg}^*$ isotopes using the dynamical cluster-decay model. The model uses updated bulk and neutron-proton asymmetry coefficients of the mass formula, adjusted to the AME2020 and/or FRDM(2012) mass evaluations, along with the temperature-dependent surface energy coefficient and nuclear deformations. The calculations show a consistent preference for charge-asymmetric configurations near the deformed shell closures $(Z_L, Z_H = 36, 44)$ at lower excitation energies throughout the isotopic chain. The neutron partition $(N_L, N_H)$ shows weaker isotopic dependence in the light fragment than in the heavy fragment, with the calculated light-fragment neutron number remaining near $N_L \approx 50$, while the heavy fragment accommodates a larger fraction of the isotopic neutron excess. For the lighter isotopes, $^{178-184}\mathrm{Hg}^{*}$, the asymmetric channel contribution is dominant at low excitation energies, associated with the deformed shell closures $(Z_L, Z_H = 36, 44)$ and at higher excitation energy the symmetric-fission channel contribution dominates, which is associated with $(Z_L= Z_H = 40)$. The transition toward predominantly symmetric fission occurs at an excitation energy of approximately 50 MeV. For the heavier isotopes $^{190-198}\mathrm{Hg}^{*}$, the asymmetric fission channel contribution likewise decreases with excitation energy, while the symmetric fission channel contribution increases, resulting in a near balance between the two fission channels at higher excitation energies. The increase in the calculated symmetric-fission channel cross-section with excitation energy is consistent with the increasing trend observed in the available experimental cross-sections.