Erosion of N = 28 shell closure: Shape coexistence and monopole transition

TL;DR

This study investigates shape coexistence in neutron-rich N=28 isotones, revealing how shell gap quenching induces diverse nuclear shapes and affects monopole transition strengths, using advanced molecular dynamics simulations.

Contribution

It demonstrates the coexistence of different nuclear shapes in N=28 isotones and links this to monopole transition strengths without assuming predefined shapes.

Findings

Rigid shapes coexist in 40Mg and 42Si.

Large-amplitude collective motion in 44S.

Monopole transition strengths reflect shape coexistence.

Abstract

Background: In neutron-rich nuclei neighboring 42Si, the quenching of the N = 28 shell gap occurs and is expected to induce the shape coexistence in their excitation spectra. Purpose: We show that different nuclear shapes coexist in N = 28 isotones 40Mg, 42Si, and 44S, and investigate observables to probe it. Method: Antisymmetrized molecular dynamics with Gogny D1S density functional is applied to describe the shape coexistence phenomena without ad hoc assumption of the nuclear shape. Results: We find that rigid shapes with different deformations coexist in the ground and the first excited 0+ states of 40Mg and 42Si, while in 44S the states exhibit large-amplitude collective motion, which does not have any particular shape. These characteristics are reflected well in the monopole transition strengths. Conclusion: The quenching of the N = 28 shell gap leads to the unique shape coexistence in the N = 28 isotones, which can be probed by the monopole transition strengths.