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

TL;DR

This study investigates the unbound neutron-rich nucleus $^9$He through isobaric analog states in $^9$Li, finding no narrow states and identifying a broad $1/2^+$ state around 3 MeV above the neutron threshold, challenging previous claims.

Contribution

First high-resolution measurement of $^9$He excitation energies via $^8$He+p scattering, providing new insights into its level structure and refuting earlier narrow state observations.

Findings

No narrow structures observed in $^9$He.

Evidence for a broad $J^{}1/2^+$ state at ~3 MeV.

Results challenge previous interpretations of $^9$He states.

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 $J^\pi=1/2^+$ and $J^\pi=1/2^-$ 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 $J^{\pi}=1/2^+$ state in $^9$He located approximately 3 MeV above the neutron decay threshold.