Laser and $\beta$-NMR spectroscopy on neutron-rich magnesium isotopes

Ground state properties of neutron-rich $^{29,31}$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. $\scriptstyle\textrm{I}$$^\pi$ = 1/2$^{+}$ and $\mu$ = -0.88355(15) $\mu\scriptstyle_\textrm{N}$ 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”.