The land of deformation south of $^{68}$Ni

TL;DR

This study uses large-scale shell model calculations to analyze shape transitions and collectivity in neutron-rich nuclei around N=40, revealing early deformation in chromium isotopes and detailed shell evolution insights.

Contribution

It provides a comprehensive theoretical analysis of shape changes and collectivity in N=40 nuclei using an advanced shell model with realistic interactions, predicting deformation onset.

Findings

Good agreement with experimental data

Deformation starts at N=38 in chromium isotopes

Shell evolution explains shape changes

Abstract

We study the development of collectivity in the neutron-rich nuclei around $N=40$, where experimental and theoretical evidences suggest a rapid shape change from the spherical to the rotational regime, in analogy to what happens at the {\it island of inversion} surrounding $^{31}$Na. Theoretical calculations are performed within the interacting shell model framework in a large valence space, based on a $^{48}$Ca core which encompasses the full $pf$ shell for the protons and the $0f_{5/2}$, $1p_{3/2}$, $1p_{1/2}$, $0g_{9/2}$ and $1d_{5/2}$ orbits for the neutrons. The effective interaction is based on a G-matrix obtained from a realistic nucleon-nucleon potential whose monopole part is corrected empirically to produce effective single particle energies compatible with the experimental data. We find a good agreement between the theoretical results and the available experimental data. We predict the onset of deformation at different neutron numbers for the various isotopic chains. The maximum collectivity occurs in the chromium isotopes, where the large deformation regime starts already at $N=38$. The shell evolution responsible for the observed shape changes is discussed in detail, in parallel to the situation in the $N=20$ region.