Refined topology of the N = 20 island of inversion with high precision mass measurements of $^{31-33}$Na and $^{31-35}$Mg

TL;DR

This paper reports high-precision mass measurements of sodium and magnesium isotopes near the N=20 island of inversion, revealing insights into shell evolution and discovering a new isomer, with implications for nuclear structure beyond the dripline.

Contribution

It provides the most precise mass measurements to date for these isotopes and compares results with ab initio predictions, advancing understanding of shell evolution in exotic nuclei.

Findings

Mass of $^{33}$Na uncertainty reduced by over two orders of magnitude

Discovery of a millisecond isomer in $^{32}$Na

Shell strength increases significantly for $^{31}$Na

Abstract

Mass measurements of $^{31-33}$Na and $^{31-35}$Mg using the TITAN MR-TOF-MS at TRIUMF's ISAC facility are presented, with the uncertainty of the $^{33}$Na mass reduced by over two orders of magnitude. The excellent performance of the MR-TOF-MS has also allowed the discovery of a millisecond isomer in $^{32}$Na. The precision obtained shows that the binding energy of the normally closed N = 20 neutron shell reaches a minimum for $^{32}$Mg but increases significantly for $^{31}$Na, hinting at the possibility of enhanced shell strength toward the unbound $^{28}$O. We compare the results with new ab initio predictions that raise intriguing questions of nuclear structure beyond the dripline.