Nigrogen-9 - a ground-state five-proton emitter at the edge of the chart of nuclides*

For light nuclei just beyond the proton drip line, one finds a region of both ground-state single- and two-proton emitters. Even further out from this drip line, a small number of three- and four-proton emitters have been observed.  At present there are only two known 4p emitters; ⁸C and ¹⁸Mg.   On the other side of the chart of nuclides, one 4n emitter (²⁸O) has been observed beyond the neutron drip line.  In the regions even further removed from both these drip lines one may find nuclides that decay by emitting even more nucleons. Such exotic states are expected to be located near the boundary of the chart of nuclides where the decay widths of such exotic states are so large so that they can be not be well differentiated from the rest of continuum. In this talk we will show strong evidence for the existence of ⁹N, with five unbound protons outside of an alpha-particle core.  This resonance was produced in the fragmentation of a secondary ¹³O beam provided by the National Superconducting Cyclotron Laboratory. The decay products were detected in a highly-pixelated array and the invariant masses of events with 5 protons and an alpha particle were determined. Great care must be taken in understanding the background in the invariant-mass distribution from detected 5p+α events where some of the protons are produced promptly in the fragmentation reaction and not from resonance decay. Evidence for one, or more likely two, ⁹N states have been found in the invariant-mass spectrum. These exotic nuclides are great testing grounds for nuclear-structure models that include the coupling of the continuum.  Nitrogen-9 was also studied theoretically in the Gamow shell model and  ½⁺ and ½^- states are predicted at the energies of the two likely  invariant-mass peaks in the experimental distribution.