Competition between normal and intruder states inside the “island of inversion”

Abstract

The ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of the exotic $^{30}\mathrm{Ne}$ ($N=20$) is reported. For the first time, the low-energy level structure of the $N=19$, $^{30}\mathrm{Na}$ (${T}_{Z}=4$), is obtained from \ensuremath{\beta}-delayed \ensuremath{\gamma} spectroscopy using fragment-$\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences. The level structure clearly displays ``inversion,'' i.e., intruder states with mainly $2p2h$ configurations displacing the normal states to higher excitation energies. The good agreement in excitation energies and the weak and electromagnetic decay patterns with Monte Carlo shell model calculations with the SDPF-M interaction in the $\mathit{sdpf}$ valence space confirms the small ${d}_{3/2}\text{\ensuremath{-}}{f}_{7/2}$ shell gap. The relative position of the normal dominant and intruder dominant excited states provides valuable information to understand better the $N=20$ shell gap.