The ${}^{257}$Rf isotope has been populated via the ${}^{208}$Pb(${}^{50}$Ti, $n$) fusion-evaporation reaction and delayed $\gamma$-ray and electron decay spectroscopy has been performed. The existence of a high-$K$ isomeric state in ${}^{257}$Rf has been confirmed. The isomeric state decays into a rotational band based on the 11/2${}^{-}$[725] excitation, which was observed up to spin of (23/2${}^{-}$). Three multipolarity-$E1$ $\gamma$ transitions depopulating the isomeric state have been observed, which fixes the spin for that state to (21/2${}^{+}$). This assignment agrees with theoretical predictions calculated with the microscopic-macroscopic approach, which suggest the isomeric state to be formed by coupling an unpaired $11/2^{-}$[725] quasineutron to the $(1/2^{-}$[521]$\otimes 9/2^{+}$[624])${}_{5^{-}}$ two-quasiproton state. The same two-quasiproton excitation is possible for the lowest isomer in ${}^{256}$Rf.

• Received 15 August 2013

DOI:https://doi.org/10.1103/PhysRevC.88.044313