Spectroscopy of Na-28: Shell evolution toward the drip line

Excited states have been studied in Na28 using the β-decay of implanted Ne28 ions at the Grand Accélérateur National d'Ions Lourds/LISE as well as the in-beam γ-ray spectroscopy at the National Superconducting Cyclotron Laboratory/S800 facility. New states of positive (Jπ=3+,4+) and negative (Jπ=1 - 5-) parity are proposed. The former arise from the coupling between 0d5/2 protons and 0d3/2 neutrons, while the latter are attributable to couplings of 0d5/2 protons with 1p3/2 or 0f7/2 neutrons. While the relative energies between the Jπ=1+-4+ states are well reproduced with the USDA interaction in the N=17 isotones, a progressive shift in the ground-state binding energy (by about 500 keV) is observed between F26 and Al30. This points to a possible change in the proton-neutron 0d5/2-0d3/2 effective interaction when moving from stability to the drip line. The presence of Jπ=1 - 4- negative-parity states around 1.5 MeV as well as of a candidate for a Jπ=5- state around 2.5 MeV give further support to the collapse of the N=20 gap and to the inversion between the neutron 0f7/2 and 1p3/2 levels below Z=12. These features are discussed in the framework of shell-model and energy-density-functional calculations, leading to predicted negative-parity states in the low-energy spectra of the F26 and O25 nuclei.