Badanie wysokospinowych stanów z seniority ν > 2,3 w neutrononadmiarowych izotopach Sn produkowanych w reakcjach fuzja‒rozszczepienie

Abstract

Excited states above the seniority ν = 2 isomers have been investigated in even neutron‒rich 118‒128Sn isotopes produced by fusion‒fission of 6.9 MeV/A 48Ca beams with 208Pb and 238U targets and by fission of a 238U target initiated by 6.7 MeV/A 64Ni beam. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold γ‒ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques. In the data analysis, the delayed gamma coincidence technique was used to establish high‒spin state structures in the Sn isotopes with isomeric half‒lives below few sec. For cases with long‒lived isomeric states, the gamma cross‒coincidence method was employed to identify such structures. This required a more detailed knowledge of the fusion–fission process in order to establish which of the complementary fragments accompanying a specific Sn isotope should be expected in the reaction exit channel. As a result of employed analysis methods the seniority ν = 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 ν = 4 states. Higher‒lying, seniority ν = 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 122Sn in the g7/2, d5/2, d3/2, s1/2, and h11/2 model space of neutron holes with respect to a 132Sn core. Effective two‒body interactions were adjusted such that satisfactory agreement with data was achieved for 130Sn. 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 ν = 4 isomers and is compared to that determined in earlier work for the E2 transition rates from the ν = 2,3 isomers.