Spectroscopy of ²⁸Na: shell evolution toward the drip line

الأصل · EN

Excited states in ²⁸Na have been studied using the β-decay of implanted ²⁸Ne ions at GANIL/LISE as well as the in-beam γ-ray spectroscopy at the NSCL/S800 facility. New states of positive (Jπ=3,4+) and negative (Jπ=1-5-) parity are proposed. The former arise from the coupling between 0d_5/2 protons and a 0d_3/2 neutron, while the latter are due to couplings with 1p_3/2 or 0f_7/2 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 ²⁶F and ³⁰Al. This points to a possible change in the proton-neutron 0d_5/2-0d_3/2 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 0f_7/2 and 1p_3/2 levels below Z=12. These features are discussed in the framework of Shell Model and EDF calculations, leading to predicted negative parity states in the low energy spectra of the ²⁶F and ²⁵O nuclei.