${}^{18}\mathrm{Mg}$ was observed, for the first time, by the invariant-mass reconstruction of ${}^{14}\mathrm{O}+4p$ events. The ground-state decay energy and width are $E_T=4.865(34)\mathrm{MeV}$ and $\mathrm{\Gamma }=115(100)\mathrm{keV}$, 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 ${}^{16}\mathrm{Ne}$. 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 ${}^{20}\mathrm{Mg}$, 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.

• Received 30 August 2021
• Revised 18 October 2021
• Accepted 16 November 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.262502