Excited states above the seniority $\nu =2$ isomers have been investigated in even neutron-rich ${}^{118–128}$Sn isotopes produced by fusion-fission of 6.9 MeV/$A$ ${}^{48}\mathrm{Ca}$ beams with ${}^{208}\mathrm{Pb}$ and ${}^{238}\mathrm{U}$ targets and by fission of 6.7 MeV/$A$ ${}^{64}\mathrm{Ni}$ beams on a ${}^{238}\mathrm{U}$ 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 ${}^{122}\mathrm{Sn}$ 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 ${}^{132}\mathrm{Sn}$ core. Effective two-body interactions were adjusted such that satisfactory agreement with data was achieved for ${}^{130}\mathrm{Sn}$. 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.

• Received 26 February 2014

DOI:https://doi.org/10.1103/PhysRevC.89.044324