Shell effects in quasi-fission for calcium induced reactions forming thorium isotopes

TL;DR

This study investigates how shell effects influence quasi-fission in calcium-induced reactions forming thorium isotopes, revealing persistent asymmetric modes and barriers preventing symmetric fission in neutron-deficient systems.

Contribution

It demonstrates, using time-dependent Hartree-Fock calculations, that shell effects sustain asymmetric quasi-fission modes in neutron-deficient thorium isotopes, unlike typical fission behavior.

Findings

All reactions show mass equilibration stopping at Z~54.

No transition to symmetric modes in neutron-deficient systems.

Potential energy surfaces indicate a persistent asymmetric valley.

Abstract

Quantum shell effects induce an asymmetric fission mode in actinides, which disappears in neutron deficient isotopes. Quasi-fission, characterized by a significant mass transfer in heavy ion collisions at low-energies, is expected to be affected by similar shell effects. This is studied in 40-56Ca+176Yb reactions with the time-dependent Hartree-Fock approach. All reactions exhibit a mass equilibration process that stops when a heavy fragment with Z~54 protons is formed. Unlike the fission of thorium compound nuclei, quasi-fission does not exhibit a transition to symmetric modes in neutron deficient systems. This observation is interpreted in terms of potential energy surfaces that show a persistence of an asymmetric valley with an increasing barrier preventing its population in fission of the most neutron deficient thorium isotopes.