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 ${}^{256}\mathrm{Rf}$, 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 ${}^{256}\mathrm{Rf}$ 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, $~$5(2)$\%$ that of the ground state instead of the expected $~$30$\%$, 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.

• Received 25 February 2011

DOI:https://doi.org/10.1103/PhysRevC.83.064311