From $^{27}$Mg to ${33}$Mg: transition to the "Island of inversion"

Since introduced in nuclear physics, the concept of shell structure and magic numbers has governed our understanding of nuclear matter in atomic nuclei close to stability. Nuclear species with a magic number of protons and/or neutrons are known to be more stable and close to spherical in shape. Experimental studies of 30Ne, 31Na and 32Mg, systems with a closed neutron sd shell (N=20), provide evidence for a ground-state deformation in these nuclei. The observed phenomenon is understood as an inversion of the normal spherical ground-state configuration, expected according to the traditional shell model, with deformed states governed by particle-hole excitations over the N=20 shell gap. The properties of nuclei in this Island of inversion are of particular importance for the theoretical modeling of the region. The present doctoral dissertation reports on the first determination of the nuclear ground-state spins and magnetic moments of 31,33Mg, obtained by means of laser spectroscopy in combination with nuclear magnetic resonance techniques. Large-scale shell-model calculations are employed to draw conclusions for their nuclear structure through a comparison with the experimental results. The measured spins and negative magnetic moments can only be reproduced by allowing an excitation of two neutrons across the N=20 shell gap to the negative parity orbitals f7/2 and p3/2. Thus, the ground-state parity of 31Mg is positive and the one of 33Mg is negative. Discrepancies in the interpretation of former experimental studies are discussed and resolved based on the firm ground-state spin-parity assignments from this work. An alternative interpretation is given within the frame of the Nilsson and particle plus rotor models. The results are consistent with an odd-neutron occupation of the 1/2[200] (31Mg) and 3/2[321] (33Mg) Nilsson orbitals at large prolate deformations. Additionally, the hyperfine parameters of the stable 25Mg II in the D1 and D2 lines are obtained by means of fluorescence spectroscopy. The magnetic moments of 27,29Mg and an estimate of the quadrupole moment of 29Mg, extracted from the hyperfine structure of their singly-ionized atoms, are reported for the first time. All results are discussed in terms of nuclear-structure evolution towards the Island of inversion.