Measurement of the Spin and Magnetic Moment of ³¹Mg: Evidence for a Strongly Deformed Intruder Ground State

Unambiguous values of the spin and magnetic moment of ³¹Mg are obtained by combining the results of a hyperfine-structure measurement and a β-NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear g factor and spin I=1/2, the magnetic moment μ(³¹Mg)=-0.88355(15)μ_N is deduced. A revised level scheme of ³¹Mg (Z=12, N=19) with ground state spin/parity I^π=1/2⁺ is presented, revealing the coexistence of 1p-1h and 2p-2h intruder states below 500 keV. Advanced shell-model calculations and the Nilsson model suggest that the I^π=1/2⁺ ground state is a strongly prolate deformed intruder state. This result plays a key role for the understanding of nuclear structure changes due to the disappearance of the N=20 shell gap in neutron-rich nuclei.