Nuclear structure far off stability —Implications for nuclear astrophysics

Abstract

Abstract. The single-particle structure and shell gap of 100 Sn 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 94,95 Ag, 98 Cd and its N = 50 isotones 96 Pd and 94 Ru 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 100 Sn core. The strong monopole interaction of the ∆l = 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 100 Sn and 78 Ni. 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 78 Ni 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 56,58 Cr and 70-76 Ni. 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.