Observation and spectroscopy of the proton-unbound nucleus Al21

We report on the observation of the previously unknown isotope ${}^{21}\mathrm{Al}$, the first unbound aluminum isotope located beyond the proton dripline. The ${}^{21}\mathrm{Al}$ nucleus decays by one-proton ($1p$) emission, and its in-flight decays were detected by tracking trajectories of the decay products with silicon microstrip detectors. The $1p$-emission processes were studied by analyzing the measured angular correlations of decay products ${}^{20}\mathrm{Mg}+p$. The $1p$-decay energies of the ground and low-lying excited states of ${}^{21}\mathrm{Al}$, its mass excess, and the proton separation energy value $S_p=-1.{15}_{-0.07}^{+0.10}$ MeV were determined.