European Physical Journal A vol:25 issue:s01 pages:193-197
Ground-state properties of neutron-rich 2931 Mg have been recently measured at ISOLDE/CERN in the context of shell structure far from stability. By combining the results of beta-NMR and hyperfine-structure measurements unambiguous values of the nuclear spin and magnetic moment of 31 Mg are obtained. I-pi = 1/2(+) and mu = -0.88355(15)mu(N) can be explained only by an intruder ground state with at least 2p-2h excitations, revealing the weakening of the N = 20 shell gap in this nucleus. This result plays an important role in the understanding of the mechanism and boundaries of the so called "island of inversion".