First Observation of the Four-Proton Unbound Nucleus Mg-18

Mg-18 was observed, for the first time, by the invariant-mass reconstruction of O-14 + 4p events. The ground-state decay energy and width are E-T = 4.865(34) MeV and Gamma = 115(100) keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of Ne-16. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2(+) state. As this energy exceeds that for the 2(+) state in Mg-20, this observation provides an argument for the demise of the N = 8 shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.