Spectroscopy of even Sn nuclei and generalized-seniority breaking

Abstract

Properties of even Sn nuclei are described in a broken-pair or generalized-seniority ( ν g ) scheme. Special attention is paid to the degree of ν g mixing in various types of low-lying states. A finite-range interaction as well as a surface-delta interaction (SDI) are employed and single-particle energies are deduced from spectra of odd Sn isotopes. It appears that up to 3 MeV no experimental indications for states with ν g > 4 exist. The only low-lying states which are not included in the model are the members of the well-known two-proton-hole band. With the SDI the ν g mixing is in general a factor two less than with the finite-range force. For the latter an improvement of the description of energy spectra as well as electromagnetic decay is obtained due to about 20% ν g = 4 admixtures in predominantly ν g = 2 states. Only ground states, 2 1 + and 3 1 − states have less than 10% of ν g = 4 admixtures. We argue that the main origin of ν g mixing is a particle-phonon coupling mechanism. A strong fragmentation of two-phonon 0 + , 2 + , 4 + states by ν g mixing emerges from the calculations. For the 0 + states the total two-phonon E2 transition strength is much less than predicted by phenomenological phonon or boson models. Excitation strengths for unnatural-parity states are reduced by 30–40% by ν g mixing. For natural-parity states this reduction is less; for 2 1 + and 3 1 − enhancements by ground-state correlations overcompensate the reduction by fragmentation.