Quasi shell gap at 23F

Abstract The experimental states of nuclei, close to semi-magic nuclei, having one extra proton on top of closed proton core, in general, connote the arrangement of proton orbitals within a shell. Herein, we have studied the arrangement of proton orbitals ( π 0 d 5 / 2 , π 1 s 1 / 2 and π 0 d 3 / 2 ) in sd-shell for Fluorine isotopes within the nuclear shell model framework. The contribution of different components of effective nucleon-nucleon interaction in the evolution of the energies of proton orbitals, and the orbital energy gaps π 0 d 5 / 2 − π 1 s 1 / 2 and π 0 d 5 / 2 − π 0 d 3 / 2 at 23,25 F has been examined. Results show the evolution of quasi-shell gap at 23 F due to key contribution of central force, in particular, of its even-channel, and destructive interference in the contribution of spin–orbit and tensor force. The excitation energies of 1 2 1 + and 3 2 1 + states of 23 F, and 1 2 1 + state of 25 F are in good agreement with their respective orbital energy gaps. The origin of 3 2 1 + state of 25 F and its excitation energy has been delineated as an effect of the breakdown of semi-magic core 24 O.