Intruder configurations in $^{29}$Ne at the transition into the island of inversion: Detailed structure study of $^{28}$Ne

TL;DR

This study provides detailed gamma-ray spectroscopy of $^{28}$Ne, revealing intruder states and shell gap breakdown, highlighting challenges in theoretical modeling of the island of inversion transition.

Contribution

First detailed level scheme and negative-parity state identification of $^{28}$Ne using one-neutron removal reactions, with experimental evidence of intruder p-wave strength.

Findings

Identification of negative-parity states in $^{28}$Ne.

Observation of significant intruder p-wave strength.

Discrepancy between experimental data and shell-model calculations.

Abstract

Detailed $\gamma$-ray spectroscopy of the exotic neon isotope $^{28}$Ne has been performed for the first time using the one-neutron removal reaction from $^{29}$Ne on a liquid hydrogen target at 240~MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for $^{28}$Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder $p$-wave strength providing evidence of the breakdown of the $N=20$ and $N=28$ shell gaps. Only a weak, possible $f$-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large $p$-wave and small $f$-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.