We report on the first determination of the nuclear ground-state spin of ${}^{33}\mathrm{Mg}$, $I=3/2$, and its magnetic moment, $\mu =-0.7456(5){\mu }_N$, by combining laser spectroscopy with nuclear magnetic resonance techniques. These values are inconsistent with an earlier suggested 1 particle-1 hole configuration and provide evidence for a 2 particle-2 hole intruder ground state with negative parity. The results are in agreement with an odd-neutron occupation of the $3/2[321]$ Nilsson orbital at a large prolate deformation. The discussion emphasizes the need of further theoretical and experimental investigation of the island of inversion, a region previously thought to be well understood.

• Received 29 July 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.212501