Excitation-energy dependence of fission in the mercury region

Background: Recent experiments on β-delayed fission reported an asymmetric mass yield in the neutron-deficient nucleus ¹⁸⁰Hg. Earlier experiments in the mass region $A=190--200$ close to the β-stability line, using the $(p,f)$ and (α,f) reactions, observed a more symmetric distribution of fission fragments. While the β-delayed fission of ¹⁸⁰Hg can be associated with relatively low excitation energy, this is not the case for light-ion reactions, which result in warm compound nuclei. The low-energy fission of 180,198Hg has been successfully described by theory in terms of strong shell effects in pre-scission configurations associated with dinuclear structures.Purpose: To elucidate the roles of proton and neutron numbers and excitation energy in determining symmetric- and asymmetric-fission yields, we compute and analyze the isentropic potential energy surfaces of 174,180,198Hg and 196,210Po.Methods: We use the finite-temperature superfluid nuclear density functional theory for excitation energies up to E*=30 MeV and zero angular momentum. For our theoretical framework, we consider the Skyrme energy density functional SkM* and a density-dependent pairing interaction.Results: For 174,180Hg, we predict fission pathways consistent with asymmetric fission at low excitation energies, with the symmetric-fission pathway opening very gradually as excitation energy is increased. For ¹⁹⁸Hg and ¹⁹⁶Po, we expect the nearly symmetric-fission channel to dominate. ²¹⁰Po shows a preference for a slightly asymmetric pathway at low energies, and a preference for a symmetric pathway at high energies.Conclusions: Our self-consistent theory suggests that excitation energy weakly affects the fission pattern of the nuclei considered. The transition from the asymmetric fission in the proton-rich nuclei to a more symmetric fission in the heavier isotopes is governed by the shell structure of pre-scission configurations.