Nuclear Shell Evolution and In-Medium NN Interaction
Résumé
We investigate the mechanisms responsible for changes of the nuclear shell structure depending on N/Z ratio. We perform the spin-tensor decomposition of an effective two-body shell-model interaction and then study single-particle (s.p.) energy variations in a series of isotopes or isotones. The technique allows to separate unambiguously contributions of the central, vector and tensor components of the realistic effective interaction. We show that while the global trend of the nuclear binding energy is mainly due to the central term of the effective interaction, local variations of s.p. energies are governed by various components of the NN force. Two most important contributions to the s.p. energy evolution are confirmed to be the central spin-isospin-exchange term and the tensor term, with the role of the latter being almost exclusive when considering behavior of spin-orbit partners. From the analysis of a well-fitted realistic interaction in sdpf shell model space, we reveal the role played by the different terms in the variation of the N=16, N=20 and N=28 shell gaps in neutron rich nuclei. Spin-tensor contents of a microscopic interaction and a fitted one are compared.