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Higher-seniority excitations in even neutron-rich Sn isotopes

2014; American Institute of Physics; Volume: 89; Issue: 4 Linguagem: Inglês

10.1103/physrevc.89.044324

1538-4497

Ł. W. Iskra, R. Broda, R. V. F. Janssens, J. Wrzesiński, B. Szpak, C. J. Chiara, M. P. Carpenter, B. Fornal, N. Hoteling, F. G. Kondev, W. Królas, T. Lauritsen, T. Pawłat, D. Seweryniak, I. Ştefânescu, W. B. Walters, S. Zhu,

Nuclear Physics and Applications

Excited states above the seniority $\ensuremath{\nu}=2$ 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 $\ensuremath{\gamma}$-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 $\ensuremath{\nu}=4$, 15${}^{\ensuremath{-}}$, and 13${}^{\ensuremath{-}}$ 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${}^{\ensuremath{-}}$ isomers, firm spin-parity assignments could be proposed for most of the seniority $\ensuremath{\nu}=4$ states. Higher-lying, seniority $\ensuremath{\nu}=6$ 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 $\ensuremath{\nu}=4$ isomers and is compared to that determined in earlier work for the $E2$ transition rates from the $\ensuremath{\nu}=2,3$ isomers.