Neutron-unbound excited states in ³¹Ne

The Island of Inversion near the N=20 shell gap is home to nuclei with interesting nuclear structure effects due to a shell inversion with respect to spherical shell model expectations. Studies of ³¹Ne have revealed that it exhibits a halo structure, characterized by a valence neutron orbiting a deformed ³⁰Ne core. This lightly bound nucleus is expected to have neutron-unbound excited states as it has has a very small separation energy of S_n = 170 keV. An experiment was performed at NSCL at MSU to study the low-lying excited states in ³¹Ne for the first time, utilizing the MoNA-LISA neutron array. A two-proton knockout reaction populated ³¹Ne states from an 89 MeV/u ³³Mg beam. The neutron-unbound states of ³¹Ne decay immediately into ³⁰Ne and a neutron. The two-body decay energy was reconstructed via the invariant mass method and shows two broad features. Accompanying shell model calculations using the FSU interaction within NuShellX combined with cross-section calculations using eikonal reaction theory were performed that indicate the spectrum is likely caused by several resonant states in ³¹Ne.