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

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.