Spin and Magnetic Moment of ³³Mg: Evidence for a Negative-Parity Intruder Ground State

We report on the first determination of the nuclear ground-state spin of ³³Mg, I=3/2, and its magnetic moment, μ=-0.7456(5) μ_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.