Impact of shape coexistence on the symmetric to asymmetric fission mode transition in Th isotopes

TL;DR

This study uses advanced microscopic models to analyze how shape coexistence influences the transition from symmetric to asymmetric fission modes in thorium isotopes, revealing a strong correlation with the evolution of fission fragment shapes and energies.

Contribution

It introduces a fully microscopic framework linking shape coexistence and fission mode transition in Th isotopes, highlighting the role of light fragment deformation energies and shell effects.

Findings

Fission fragment charge distributions match experimental data.

A rapid transition from symmetric to asymmetric fission occurs between A=222 and 234.

Light fragment shape coexistence significantly influences fission mode transition.

Abstract

We study the evolution of fission modes along the Th isotopic chain using a microscopic framework combining the time-dependent generator coordinate method and finite-temperature covariant density functional theory. Theoretical fission fragment charge distributions agree well with experiments, and reveal a rapid symmetric-to-asymmetric transition from $A=222$ to 234. By analyzing the collective potential energy surfaces and time evolution of collective probability density distributions, we demonstrate that this fission mode transition is strongly correlated with the rapidly deepening asymmetric fission valley $-$ a phenomenon driven by the reduction of deformation energies of both the heavy and light fragments formed in the asymmetric fission valley. Further analysis attributes the decrease of light-fragment deformation energies to the onset of a coexisting large-deformed minimum in neutron-rich Kr and Sr isotopes (dominated isotopes for light asymmetric peak), which arises from a deformed proton $Z=38$ shell closure near $\beta_2\approx0.46$. Notably, we identify, for the first time, the pivotal role of the light fragment and its shape coexistence structure on the fission mode transition in Th isotopes in a fully microscopic framework.