In-beam $\gamma$-ray spectroscopy of $^{32}$Mg via direct reactions

TL;DR

This study extends the level scheme of $^{32}$Mg using in-beam gamma-ray spectroscopy with direct reactions, providing new spin-parity assignments and comparing results with theoretical models to understand its structure within the island of inversion.

Contribution

It offers an updated level scheme for $^{32}$Mg with detailed spin-parity assignments, utilizing knockout reactions and reaction model calculations, advancing understanding of its microscopic structure.

Findings

Updated level scheme with negative and positive parity states

Good agreement with some nuclear models on certain features

Remaining discrepancies highlight the complexity of $^{32}$Mg's structure

Abstract

Background: The nucleus $^{32}$Mg ($N=20$ and $Z=12$) plays a central role in the so-called "island of inversion" where in the ground states $sd$-shell neutrons are promoted to the $fp$-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number $N=20$. Purpose: The primary goals of this work are to extend the level scheme of $^{32}$Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam $\gamma$-ray spectroscopy of $^{32}$Mg was performed using two direct-reaction probes, one-neutron (two-proton) knockout reactions on $^{33}$Mg ($^{34}$Si). 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 $^{32}$Mg, 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 $^{32}$Mg 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.