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

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.