Background: The island of inversion near the $N=20$ shell gap is home to nuclei with reordered single-particle energy levels compared with the spherical shell model. Studies of ${}^{31}\mathrm{Ne}$ have revealed that its ground state has a halo component characterized by a valence neutron orbiting a deformed ${}^{30}\mathrm{Ne}$ core. This lightly bound nucleus with a separation energy of only $S_n=170$ keV is expected to have excited states that are neutron unbound.

Purpose: The purpose of this experiment was to investigate the low-lying excited states in ${}^{31}\mathrm{Ne}$ that decay by the emission of a single neutron.

Methods: An 89 MeV/nucleon ${}^{33}\mathrm{Mg}$ beam impinged on a segmented Be reaction target. Neutron-unbound states in ${}^{31}\mathrm{Ne}$ were populated via a two-proton knockout reaction. The ${}^{30}\mathrm{Ne}$ fragment and associated neutron from the decay of ${}^{31}\mathrm{Ne}$ were detected by the MoNA-LISA-Sweeper experimental setup at the National Superconducting Cyclotron Laboratory. Invariant-mass spectroscopy was used to reconstruct the two-body decay energy (${}^{30}\mathrm{Ne}+n$).

Results: The two-body decay energy spectrum exhibits two features: a low-lying peak at $0.30\pm 0.17$ MeV and a broad enhancement at $1.50\pm 0.33$ MeV, each fit with an energy-dependent asymmetric Breit-Wigner lineshape representing a resonance in the continuum. Accompanying shell-model calculations using the FSU interaction within NuShellX, combined with cross-section calculations using the eikonal reaction theory, indicate that these peaks in the decay energy spectrum are caused by multiple resonant states in ${}^{31}\mathrm{Ne}$.

Conclusions: Excited states in ${}^{31}\mathrm{Ne}$ were observed for the first time. Transitions from calculated shell-model final states in ${}^{31}\mathrm{Ne}$ to bound states in ${}^{30}\mathrm{Ne}$ are in good agreement with the measured decay energy spectrum. Cross-section calculations for the two-proton knockout populating ${}^{31}\mathrm{Ne}$ states as well as spectroscopic factors pertaining to the decay of ${}^{31}\mathrm{Ne}$ into ${}^{30}\mathrm{Ne}$ are used to examine the results within the context of the shell-model expectations.

• Received 16 July 2021
• Accepted 30 August 2021

DOI:https://doi.org/10.1103/PhysRevC.104.034313