Fission modes of mercury isotopes

Background: Recent experiments on β-delayed fission in the mercury-lead region and the discovery of asymmetric fission in ¹⁸⁰Hg [A. N. Andreyev et al., Phys. Rev. Lett. 105, 252502 (2010)] have stimulated theoretical interest in the mechanism of fission in heavy nuclei.Purpose: We study fission modes and fusion valleys in ¹⁸⁰Hg and ¹⁹⁸Hg to reveal the role of shell effects in the prescission region and explain the experimentally observed fragment mass asymmetry and its variation with A.Methods: We use the self-consistent nuclear density functional theory employing Skyrme and Gogny energy density functionals.Results: The potential energy surfaces in multidimensional space of collective coordinates, including elongation, triaxiality, reflection-asymmetry, and necking, are calculated for ¹⁸⁰Hg and ¹⁹⁸Hg. The asymmetric fission valleys---well separated from fusion valleys associated with nearly spherical fragments---are found in both cases. The density distributions at scission configurations are studied and related to the experimentally observed mass splits.Conclusions: The energy density functionals SkM* and D1S give a very consistent description of the fission process in ¹⁸⁰Hg and ¹⁹⁸Hg. We predict a transition from asymmetric fission in ¹⁸⁰Hg toward a more symmetric distribution of fission fragments in ¹⁹⁸Hg. For ¹⁸⁰Hg, both models yield ¹⁰⁰Ru/⁸⁰Kr as the most probable split. For ¹⁹⁸Hg, the most likely split is ¹⁰⁸Ru/⁹⁰Kr in HFB-D1S and ¹¹⁰Ru/⁸⁸Kr in HFB-SkM*.