On the origin of the anomalous behaviour of 2+ excitation energies in the neutron-rich Cd isotopes

TL;DR

This paper investigates the unusual 2+ excitation energies in neutron-rich Cd isotopes near N=82, revealing that low energies in 128Cd are due to its doubly magic oblate shape, challenging previous shell quenching hypotheses.

Contribution

The study applies modern beyond mean field methods with the Gogny force to explain the anomalous 2+ energies across Cd isotopes, emphasizing shape effects over shell quenching.

Findings

Low 2+ energy in 128Cd is linked to its oblate doubly magic shape.

Shape coexistence influences excitation energies in neutron-rich Cd isotopes.

The results challenge the idea of shell quenching near N=82 in these isotopes.

Abstract

Recent experimental results obtained using $\beta$ decay and isomer spectroscopy indicate an unusual behaviour of the energies of the first excited 2$^{+}$ states in neutron-rich Cd isotopes approaching the N=82 shell closure. To explain the unexpected trend, changes of the nuclear structure far-off stability have been suggested, namely a quenching of the N=82 shell gap already in $^{130}$Cd, only two proton holes away from doubly magic $^{132}$Sn. We study the behaviour of the 2$^+$ energies in the Cd isotopes from N=50 to N=82, i.e. across the entire span of a major neutron shell using modern beyond mean field techniques and the Gogny force. We demonstrate that the observed low 2$^+$ excitation energy in $^{128}$Cd close to the N=82 shell closure is a consequence of the doubly magic character of this nucleus for oblate deformation favoring thereby prolate configurations rather than spherical ones.