Observation and spectroscopy of proton-unbound nucleus ${}^{21}$Al

We report on the observation of previously-unknown isotope ${}^{21}$Al, the first unbound aluminum isotope located beyond the proton dripline. The ${}^{21}$Al nucleus decays by one-proton (1p) emission, and its in-flight decays were detected by tracking trajectories of all decay products with micro-strip silicon detectors. The 1p-emission processes were studied by analyses of the measured angular correlations of decay products ${}^{20}$Mg+p. The 1p-decay energies of ground and low-lying excited states of ${}^{21}$Al, its mass excess and proton separation energy value $S_p$=$-1.1(1)$ MeV were determined.