Single-neutron knockout from intermediate energy beams ofMg30,32: Mapping the transition into the “island of inversion”

Abstract

The breakdown of the $N=20$ magic number in the so-called island of inversion around $^{32}\mathrm{Mg}$ is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of $^{30}\mathrm{Mg}$ and $^{32}\mathrm{Mg}$, with the latter consisting nearly purely of 2p-2h $N=20$ cross-shell configurations. Single-neutron knockout experiments on $^{30,32}\mathrm{Mg}$ projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.