Nuclear structure beyond the neutron drip line: the lowest energy states in ${}^9$He via their T=5/2 isobaric analogs in ${}^9$Li

Abstract

The level structure of the very neutron rich and unbound ${}^9$He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy ${}^9$He states exist with spins $J^{\ddot{I}}€=1/2^{+}$ and $J^{\ddot{I}}€=1/2^{-}$ and widths on the order of hundreds of keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the ${}^8$He$+n$ system, performed via a search for the T=5/2 isobaric analog states in ${}^9$Li populated through ${}^8$He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad $J^{\ddot{I}}€=1/2^{+}$ state in ${}^9$He located approximately 3 MeV above the neutron decay threshold.