Shape coexistence near theN=38shell gap: Magnetic moment of the 981.6 keVJπ=8+level inAs72

Abstract

We report on the first determination of the magnetic moment of the 981.1 keV, ${J}^{\ensuremath{\pi}}={8}^{+}$ level in $^{72}\mathrm{As}$, a nucleus that belongs to the $A\ensuremath{\approx}70$ mass region dominated by rapidly changing deformations and shapes. The ${8}^{+}$ level is the bandhead of a collective sequence of positive parity levels coexisting with low-spin and medium-spin spherical shell-model states. The magnetic moment was measured by the time-integral perturbed angular distributions method to be $\ensuremath{\mu}=\ensuremath{-}(4.272\ifmmode\pm\else\textpm\fi{}0.280){\ensuremath{\mu}}_{N}$. This value is in disagreement with the presumed $[\ensuremath{\pi}(1{g}_{9/2}),\ensuremath{\nu}(1{g}_{9/2})]$ configuration and points to a more complex configuration involving two neutrons in the $1{g}_{9/2} $orbital.