Search for A 2-Quasiparticle High-K Isomer in (256)Rf

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in (256)Rf, which suggest that a long-lived, low-energy 8(-) isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a (256)Rf nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-mu s isomer was identified. However, its low population, similar to 5(2)% that of the ground state instead of the expected similar to 30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z = 104, which describes nuclei with Z = 92-103 very well.