Examining the $N$ = 28 shell closure through high-precision mass measurements of $^{46-48}$Ar

TL;DR

This study uses high-precision mass measurements of neutron-rich argon isotopes to investigate the persistence of the N=28 shell closure, comparing experimental data with advanced theoretical models to understand nuclear structure in this region.

Contribution

The paper provides the most precise mass measurements of $^{46-48}$Ar to date and compares these with state-of-the-art theoretical calculations, offering nuanced insights into the N=28 shell closure.

Findings

Mass values are up to 90 times more precise than previous data.

Results suggest N=28 shell closure persists in argon but with nuanced interpretation.

$^{46}$Ar is identified as a transitional nucleus between closed-shell and collective behaviors.

Abstract

The strength of the $N$ = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46-48}$Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the $N$ = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the \emph{SDPF-U} phenomenological shell model interaction. Our results are also compared with \emph{ab initio} calculations using the Valence Space In-Medium Similarity Renormalization Group and the Self-Consistent Green's Function approaches. Both calculations provide a very good account of mass systematics at and around $Z$ = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that $^{46}$Ar is the transition between the closed-shell $^{48}$Ca and collective $^{44}$S.