Nuclear structure far off stability --Implications for nuclear astrophysics

Abstract

The single-particle structure and shell gap of 100Sn as inferred from previous in-beam γ-ray spectroscopy has been confirmed in recent studies of seniority and spin-gap isomers by γγ, βγ, βpγ, pγ and 2pγ spectroscopy. The results for 94,95Ag, 98Cd and its N=50 isotones 96Pd and 94Ru stress the importance of large-scale shell model calculations employing realistic interactions for the isomerism, np-nh excitations, seniority mixing and E2 polarisation of the 100Sn core. The strong monopole interaction of the Δl=0, 1 spin/isospin-flip partners πg 9/2-νg 7/2 along the N=50 isotones and the πf 5/2-νg 9/2 pair of nucleons along the Z=28 Ni isotopes are decisive for the evolution of the shell structure towards 100Sn and 78Ni. It can be traced back to the tensor force in the effective nucleon-nucleon interaction and provides a straightforward explanation for new shells in neutron-rich light nuclei, implying qualitative predictions for new N=32, 34 subshells in Ca isotopes, persistence of the 78Ni proton and neutron shell gaps and non-equivalence of the g 9/2 valence mirror Ni isotopes and N=50 isotones. This is corroborated by recent experimental data on 56,58Cr and 70–76Ni. The implication of monopole driven shell evolution for apparent spin-orbit splitting towards N ≫ Z and structure along the astrophysical r-path between N=50 and N=82 is discussed