Towards the Limits of Existence of Nuclear Structure: Observation and First spectroscopy of the Isotope ${}^{31}$K by measuring its three-proton Decay

The most-remote from stability isotope ${}^{31}$K, which is located four atomic mass units beyond the proton drip line, has been observed. It is unbound in respect to three-proton ($3p$) emission, and its decays have been detected in flight by measuring trajectories of all decay products using micro-strip detectors. The $3p$-emission processes have been studied by means of angular correlations ${}^{28}$S+3$p$ and the respective decay vertexes. The energies of the previously-unknown ground and excited states of ${}^{31}$K have been determined. This provides its $3p$ separation-energy value $S_{3p}$ of $-4.6(2)$ MeV. Upper half-life limits of 10 ps of the observed ${}^{31}$K states have been derived from distributions of the measured decay vertexes.