Proton-neutron particle-hole symmetry in nuclei with valence nucleons in same shell

Abstract The variation of excitation energies E2+ of yrast 2+ states has been studied for the nuclei where the valence protons and neutrons occupy the same major shell for the mass range A = 20−136. It is observed that the local depression in the E2+ energy does not coincide with the maximum NpNn values predicted by the NpNn scheme, although it coincides with the condition Np^particlep ≈Nn^hole. In the shell-model description, nuclear collectivity is largely governed by the proton–neutron quadrupole interaction, whose strength is expected to reach maximum when Np^particle ≈ Nn^hole. The shell model framework reproduces these trends reasonably well, particularly in regions with a small number of valence nucleons.