An isomeric one-quasineutron state, likely based on the $[725]11/2^{-}$ Nilsson level, was identified in ${}^{261}\mathrm{Sg}$ by its decay via internal conversion electrons. The state has an excitation energy of $\approx 200$ keV and a half-life of $9.0_{-1.5}^{+2.0}$ $\mu$s. ${}^{261}\mathrm{Sg}$ has the highest $Z$ and $A$ of any nucleus in which the electromagnetic decay of an isomeric state was observed to date. A separate experiment was performed on the $\alpha$ daughter nucleus of ${}^{261}\mathrm{Sg}$, namely ${}^{257}\mathrm{Rf}$. Spectroscopy of delayed $\gamma$ rays and converted electrons from ${}^{257}\mathrm{Rf}$ resulted in the identification of a $K$ isomer at an excitation energy of $\approx 1125$ keV with a half-life of 134.9 $\pm$ 7.7 $\mu$s. The spin of the isomeric state is tentatively assigned $I=21/2,23/2$ and the state likely decays to a rotational band built on the $[725]11/2^{-}$ Nilsson level via a $\Delta K=5$ or 6 transition. The present results provide new information on excited states in the transactinide region, which is important for testing models of the heaviest elements.

• Received 31 March 2010
• Corrected 16 August 2010

DOI:https://doi.org/10.1103/PhysRevC.81.064325