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

Detailed γ-ray spectroscopy of the exotic neon isotope <sup loc="post">28</sup>Ne has been performed for the first time using the one-neutron removal reaction from <sup loc="post">29</sup>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 <sup loc="post">28</sup>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 <math altimg="si1.svg"><mi>N</mi><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mn>20</mn></math> and <math altimg="si2.svg"><mi>N</mi><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mn>28</mn></math> 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.