Observation and Spectroscopy of the Proton-Unbound Nucleus Al21

We report on the observation of the previously unknown isotope Al-21, the first unbound aluminum isotope located beyond the proton dripline. The Al-21 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 Mg-20+p. The 1p-decay energies of the ground and low-lying excited states of Al-21, its mass excess, and the proton separation energy value S-p=-1.15(-0.07)(+0.10) MeV were determined.