Towards the Limits of Existence of Nuclear Structure: Observation and First Spectroscopy of the Isotope 31K 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 (3 p ) emission, and its decays have been detected in flight by measuring the trajectories of all decay products using microstrip detectors. The 3 p emission processes have been studied by the means of angular correlations of ²⁸S +3 p and the respective decay vertices. The energies of the previously unknown ground and excited states of ³¹K have been determined. This provides its 3 p separation energy value S_{3 p} 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.