Abstract
Yrast and near-yrast levels up to an spin value and a 6-MeV excitation energy have been delineated in the “two-neutron” nucleus following deep-inelastic reactions involving targets and a number of heavy-ion beams at energies above the Coulomb barrier. The level scheme was established on the basis of multifold prompt and delayed coincidence relationships measured with the Gammasphere array. In addition to the previously known states, many new levels were identified. For most of the strongly populated states, spin-parity assignments are proposed on the basis of angular distributions. The reinvestigation of the , isomeric decay results in the firm identification of the low-energy transitions involved in the cascade, and in a revised level half-life of 92(10) ns, nearly a factor of 2 longer than previously measured. Among the newly identified states figure spin levels associated with the multiplet, as well as yrast states involving , , and neutron couplings. The highest-spin excitations are understood as core excitations and the yrast population is found to be fragmented to the extent that levels of spin higher than could not be reached. Four transitions are present in the yrast decay; three of these involve the octupole component, as reflected in the 21(2) and Weisskopf unit enhancements of the rates of the first two. The fourth, transition corresponds to the core octupole excitation built on the state, in analogy to a similar coupling to the level in . Shell-model calculations performed for two-neutron states and core excitations are in good agreement with the data. Evidence was found for the existence of a hitherto unknown high-spin -decaying isomer in . Shell-model calculations of the levels suggest the possibility of a long-lived, -decaying state, and the delayed yields observed in various reactions fit rather well with a assignment.
2 More- Received 11 May 2018
DOI:https://doi.org/10.1103/PhysRevC.98.024324
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