Spectroscopic study of neutron shell closures via nucleon transfer in the near-dripline nucleus 23O

Neutron single particle energies have been measured in O-23 using the O-22(d,p)O-23(*)-> O-22+n process. The energies of the resonant states have been deduced to be 4.00(2) MeV and 5.30(4) MeV. The first excited state can be assigned to the nu d(3/2) single particle state from a comparison with shell model calculations. The measured 4.0 MeV energy difference between the nu s(1/2) and nu d(3/2) states gives the size of the N=16 shell gap which is in agreement with the recent USD05 (universal sd from 2005) shell model calculation, and is large enough to explain the unbound nature of the oxygen isotopes heavier than A=24. The resonance detected at 5.3 MeV can be assigned to a state out of the sd shell model space. Its energy corresponds to a similar to 1.3 MeV sized N=20 shell gap, therefore, the N=20 shell closure disappears at Z=8 in agreement with Monte Carlo shell model calculations using SDPF-M interaction.