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

The most probable fission fragment configurations (AL, AH), (ZL, ZH), and (NL, NH), together with the relative contributions of symmetric and asymmetric fission channels, are systematically studied over a range of excitation energies for ^178,180,182,183,184,190,192,194,195,196,198Hg^* 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 (ZL, ZH = 36, 44) at lower excitation energies throughout the isotopic chain. The neutron partition (NL, NH) shows weaker isotopic dependence in the light fragment than in the heavy fragment, with the calculated light-fragment neutron number remaining near NL ≈ 50, while the heavy fragment accommodates a larger fraction of the isotopic neutron excess. For the lighter isotopes, ¹⁷⁸⁻¹⁸⁴Hg*, the asymmetric channel contribution is dominant at low excitation energies, associated with the deformed shell closures (ZL, ZH = 36, 44) and at higher excitation energy the symmetric-fission channel contribution dominates, which is associated with (ZL= ZH = 40). The transition toward predominantly symmetric fission occurs at an excitation energy of approximately 50 MeV. For the heavier isotopes ¹⁹⁰⁻¹⁹⁸Hg*, 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.