In-beam $\gamma$-ray spectroscopy of $^{32}$Mg via direct reactions
Résumé
Background: The nucleus ( and ) plays a central role in the so-called “island of inversion,” where in the ground states -shell neutrons are promoted to the -shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number .
Purpose: The primary goals of this work are to extend the level scheme of , provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations.
Method: In-beam -ray spectroscopy of was performed using two direct-reaction probes: one-neutron (two-proton) knockout reactions on (). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions.
Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for , which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models.
Conclusions: In some of these models, different aspects of and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and, even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.