6533b7d3fe1ef96bd125fef5

RESEARCH PRODUCT

Measurement of the Spin and Magnetic Moment ofMg31: Evidence for a Strongly Deformed Intruder Ground State

subject

description

Unambiguous values of the spin and magnetic moment of $^{31}\mathrm{M}\mathrm{g}$ are obtained by combining the results of a hyperfine-structure measurement and a $\ensuremath{\beta}$-NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear $g$ factor and spin $I=1/2$, the magnetic moment $\ensuremath{\mu}(^{31}\mathrm{M}\mathrm{g})=\ensuremath{-}0.88355(15){\ensuremath{\mu}}_{N}$ is deduced. A revised level scheme of $^{31}\mathrm{M}\mathrm{g}$ ($Z=12$, $N=19$) with ground state spin/parity ${I}^{\ensuremath{\pi}}=1/{2}^{+}$ 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 ${I}^{\ensuremath{\pi}}=1/{2}^{+}$ 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.