Spectroscopy of Na 28: Shell evolution toward the drip line

A. Lepailleur, K. Wimmer, A. Mutschler, O. Sorlin, J. C. Thomas, V. Bader, C. Bancroft, D. Barofsky, B. Bastin, T. Baugher, D. Bazin, V. Bildstein, C. Borcea, R. Borcea, B. A. Brown, L. Caceres, A. Gade, L. Gaudefroy, S. Grévy, G. F. GrinyerH. Iwasaki, E. Khan, T. Kröll, C. Langer, A. Lemasson, O. Llidoo, J. Lloyd, E. Lunderberg, F. Negoita, F. De Oliveira Santos, G. Perdikakis, F. Recchia, T. Redpath, T. Roger, F. Rotaru, S. Saenz, M. G. Saint-Laurent, D. Smalley, D. Sohler, M. Stanoiu, S. R. Stroberg, M. Vandebrouck, D. Weisshaar, A. Westerberg

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Abstract

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.

Original languageEnglish
Article number054309
JournalPhysical Review C - Nuclear Physics
Volume92
Issue number5
DOIs
StatePublished - Nov 16 2015

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