In-beam gamma-ray spectroscopy at the proton dripline: 23Al

TL;DR

This study reports the first in-beam gamma-ray spectroscopy of 23Al at the proton dripline, revealing a key gamma transition from a core-excited state, advancing understanding of nuclear structure near the proton dripline.

Contribution

It presents the first in-beam gamma-ray spectroscopy of 23Al using two different reactions, identifying a core-excited state and its decay, which was not previously observed.

Findings

Detected a 1616 keV gamma-ray transition in 23Al.

Identified the transition as from a core-excited 7/2+ state to the 5/2+ ground state.

Proposed the state structure as a combination of 22Mg core and a proton in d5/2 orbital.

Abstract

We report on the first in-beam $\gamma$-ray spectroscopy of \nuc{23}{Al} using two different reactions at intermediate beam energies: inelastic scattering off \nuc{9}{Be} and heavy-ion induced one-proton pickup, \nuc{9}{Be}(\nuc{22}{Mg},\nuc{23}{Al}$+\gamma$)X, at 75.1 MeV/nucleon. A $\gamma$-ray transition at 1616(8) keV -- exceeding the proton separation energy by 1494 keV -- was observed in both reactions. From shell model and proton decay calculations we argue that this $\gamma$-ray decay proceeds from the core-excited $7/2^+$ state to the $5/2^+$ ground state of \nuc{23}{Al}. The proposed nature of this state, $[\nuc{22}{Mg}(2^+_1) \otimes \pi d_{5/2}]_{7/2+}$, is consistent with the presence of a gamma-branch and with the population of this state in the two reactions.