Higher-seniority excitations in even neutron-rich Sn isotopes

Excited states above the seniority nu = 2 isomers have been investigated in even neutron-rich Sn118-128 isotopes produced by fusion-fission of 6.9MeV/A Ca-48 beamswith Pb-208 and (238)Utargets and by fission of 6.7MeV/A Ni-64 beams on a U-238 target. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold gamma-ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques, including prompt and delayed cross-coincidence methods. As a result, seniority nu = 4, 15(-), and 13-isomers were observed and their half-lives determined. These long-lived states in turn served as steppingstones to delineate the isomeric decays and to locate higher-lying states with good sensitivity. As the observed isomeric decays feed down to 10(+) and 7(-)isomers, firm spin-parity assignments could be proposed for most of the seniority nu = 4 states. Higher-lying, seniority nu = 6 levels were assigned tentatively on the basis of the observed deexcitation paths as well as of general yrast population arguments. Shell-model calculations were carried out down to Sn-122 in the g(7/2), d(5/2), d(3/2), s(1/2), and h(11/2) model space of neutron holes with respect to a Sn-132 core. Effective two-body interactions were adjusted such that satisfactory agreement with data was achieved for Sn-130. The results reproduce the experimental level energies and spin-parity assignments rather well. The intrinsic structure of the states is discussed on the basis of the calculated wave functions which, in many instances, point to complex configurations. In a few cases, the proposed assignments lead to unresolved issues. The smooth, systematic decrease of the level energies with mass A is accompanied by the similarly regular behavior with A of the reduced transition probabilities extracted from the isomeric half-lives. This A dependence is discussed for the E1 and E2 transitions in the decay of the seniority nu = 4 isomers and is compared to that determined in earlier work for the E2 transition rates from the nu = 2,3 isomers.