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

Abstract

The strength of the N=28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of Ar46-48, 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 SDPF-U phenomenological shell model interaction. Our results are also compared with 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 Ar46 is the transition between the closed-shell Ca48 and collective S44.