Mg isotopes and the disappearance of magic N=20

Collinear laser spectroscopy and β-NMR spectroscopy with optical pumping were applied at ISOLDE/CERN to measure for the first time the magnetic moments of neutron-rich ²⁷Mg, ²⁹Mg, ³¹Mg and ³³Mg, along with the spins of the two latter. The magnetic moment of ²⁷Mg was derived from its hyperfine structure detected in UV fluorescent light, whereas the nuclear magnetic resonance observed in β-decay asymmetry from a polarised ensemble of nuclei gave the magnetic moment of ²⁹Mg. For ³¹Mg and ³³Mg, the hyperfine structure and nuclear magnetic resonance gave the spin and the magnetic moment. The preliminary results for ²⁷Mg and ²⁹Mg are consistent with a large neutron shell gap at N=20, whereas data on ³¹Mg show that for this nucleus N=20 is not a magic number, which is also the case for ³³Mg, based on preliminary analysis. Thus, the two latter isotopes belong to the “island of inversion.” Magdalena Kowalska for the IS 427 collaboration at ISOLDE/CERN.