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 (π0d5/2, π1s1/2 and π0d3/2) 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 π0d5/2−π1s1/2 and π0d5/2−π0d3/2 at 23,25F has been examined. Results show the evolution of quasi-shell gap at 23F 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 121+ and 321+ states of 23F, and 121+ state of 25F are in good agreement with their respective orbital energy gaps. The origin of 321+ state of 25F and its excitation energy has been delineated as an effect of the breakdown of semi-magic core 24O.