Role of the scission-line structure in the formation of the fragment-mass distribution in ${}^{180}$Hg fission

Abstract

The fission of ${}^{180}\mathrm{Hg}$ is investigated by studying the stochastic evolution on the potential energy surface calculated within the multidimensionally constrained relativistic Hartree--Bogoliubov (MDC-RHB) model. The MDC-RHB model provides the zero-temperature potential-energy surface in the $({\beta }_{20},{\beta }_{30})$ plane and self-consistent density distributions, from which scission configurations are identified using a geometrical neck criterion. The evolution toward scission is described by a master equation on the same deformation mesh, with transition rates determined by deformation-dependent microscopic level densities. The level densities are obtained in a finite-temperature BCS treatment using the MDC-RHB single-particle spectra and zero-temperature pairing gaps as input. The effective barrier-top region is found to lie close to the scission line, making the detailed energetic structure of the scission boundary particularly important for the fragment distribution. The potential-energy landscape and scission-line structure strongly favor reflection-asymmetric configurations with pronounced prefragment deformations and lead to fragment-mass distributions centered close to the experimentally observed $A_{\mathrm{L}}/A_{\mathrm{H}}\simeq 80/100$ division. An additional constrained ${\beta }_{40}$ analysis indicates that fluctuations beyond the $({\beta }_{20},{\beta }_{30})$ collective space can generate an additional spread of the fragment masses. With increasing excitation energy over the range $E^{\ast }=34.0$--$65.5$ MeV, the symmetric component is enhanced, while the asymmetric fission mode remains dominant.