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

• Published in: arXiv.org 2022-02
• Main Authors: Kitamura, N, Wimmer, K, Miyagi, T, Poves, A, Shimizu, N, Tostevin, J A, Bader, V M, Bancroft, C, Barofsky, D, Baugher, T, Bazin, D, Berryman, J S, Bildstein, V, Gade, A, Imai, N, Kröll, T, Langer, C, Lloyd, J, Lunderberg, E, Nowacki, F, Perdikakis, G, Recchia, F, Redpath, T, Saenz, S, Smalley, D, Stroberg, S R, Utsuno, Y, Weisshaar, D, Westerberg, A
• Format: Article
• Language: English
• Subjects: Gamma rays; Mathematical models; Nuclear structure; Nuclei (nuclear physics); Parity; Protons; Selectivity; Spectroscopy; Spectrum analysis
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2202.12820

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.