Towards the Limits of Existence of Nuclear Structure: Observation and First Spectroscopy of the Isotope K-31 by Measuring Its Three-Proton Decay

The most remote isotope from the proton dripline (by 4 atomic mass units) has been observed: 31K. It is unbound with respect to three-proton (3p) emission, and its decays have been detected in flight by measuring the trajectories of all decay products using microstrip detectors. The 3p emission processes have been studied by the means of angular correlations of 28S+3p and the respective decay vertices. The energies of the previously unknown ground and excited states of 31K have been determined. This provides its 3p separation energy value S3p of −4.6(2)  MeV. Upper half-life limits of 10 ps of the observed 31K states have been derived from distributions of the measured decay vertices.