Neutron Shell Breaking in Neutron-Rich Neon Isotopes

The breaking of the N=20 shell gap in neutron-rich nuclei is investigated by studying the single-particle structure of 27Ne via neutron transfer onto a 26Ne beam. The observation of low-lying negative parity states in 27Ne above the 3/2+ ground state is further evidence of the raising of the νd3/2 orbital that is seen in other neutron-rich nuclei in the N=20 region. The previously unseen 7/2− state has been identified as unbound by 331 keV and lies above the already known 3/2− level. The measurements of the present work contradict SDPF-M Monte-Carlo Shell Model predictions, but are in good agreement with WBP shell model calculations in which the single-particle energies of the pf shell are artificially lowered by 1 MeV. This modification was made to mimic the closing of the N = 20 shell gap in neutron-rich nuclei. The predictions of the modifiedWBP calculations are also in agreement with experimental measurements in the adjacent nuclei of 25Ne and 29Mg. The calculations show that core-excited configurations play a significant role in both the 3/2+ ground state and the 7/2− intruder state in 27Ne