Mg18 was observed, for the first time, by the invariant-mass reconstruction of O14+4p events. The ground-state decay energy and width are ET=4.865(34) MeV and Γ=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 Ne16. 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 Mg20, 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.
First Observation of the Four-Proton Unbound Nucleus Mg 18 / Jin, Y.; Niu, C. Y.; Brown, K. W.; Li, Z. H.; Hua, H.; Anthony, A. K.; Barney, J.; Charity, R. J.; Crosby, J.; Dell'Aquila, D.; Elson, J. M.; Estee, J.; Ghazali, M.; Jhang, G.; Li, J. G.; Lynch, W. G.; Michel, N.; Sobotka, L. G.; Sweany, S.; Teh, F. C. E.; Thomas, A.; Tsang, C. Y.; Tsang, M. B.; Wang, S. M.; Wu, H. Y.; Yuan, C. X.; Zhu, K.. - In: PHYSICAL REVIEW LETTERS. - ISSN 0031-9007. - 127:26(2021), p. 262502. [10.1103/PhysRevLett.127.262502]