Laser and β-NMR spectroscopy on neutron-rich magnesium isotopes

Ground-state properties of neutron-rich 29, 31Mg have been recently measured at ISOLDE/CERN in the context of shell structure far from stability. By combining the results of β-NMR and hyperfine-structure measurements unambiguous values of the nuclear spin and magnetic moment of 31Mg are obtained. Iπ = 1/2+ and μ = - 0.88355(15)μN 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”.