Proton Shell Structure in Neutron-rich ²³F

The proton shell structure in neutron-rich fluorine ²³F was investigated using the in-beam γ-ray spectroscopy technique via the proton transfer reaction onto the ²²O nucleus. We also measured the α inelastic scattering on ²³F, the neutron knockout from ²⁴F, and the two-nucleon knockout from ²⁵Ne using a cocktail secondary beam. The excited states were identified by de-excitation γ-rays coincident with outgoing ²³F, and we found the proton single-particle states at 2268 and 4059 keV by comparison of the population strengths among these reactions. These states were respectively assigned to have ℓ = 0 and ℓ = 2 by the angular distributions obtained in the transfer reaction, and therefore they were considered to have the configurations of π s_{1 / 2} and π d_{3 / 2}. The excitation energies of these states are located at higher excitation energies than the shell model predictions for the USD and SDPF-M interactions, which may indicate that the centroid energy gaps between π d_{5 / 2} and π s_{1 / 2} are wider than predictions.