Electromagnetic decays of excited states in Sg261 (Z=106) and Rf257 (Z=104)

An isomeric one-quasineutron state, likely based on the $[725]11/{2}^{\\ensuremath{-}}$ Nilsson level, was identified in $^{261}\\mathrm{Sg}$ by its decay via internal conversion electrons. The state has an excitation energy of $\\ensuremath{\\approx}200$ keV and a half-life of $9.{0}_{\\ensuremath{-}1.5}^{+2.0}$ $\\ensuremath{\\mu}$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 $\\ensuremath{\\alpha}$ daughter nucleus of $^{261}\\mathrm{Sg}$, namely $^{257}\\mathrm{Rf}$. Spectroscopy of delayed $\\ensuremath{\\gamma}$ rays and converted electrons from $^{257}\\mathrm{Rf}$ resulted in the identification of a $K$ isomer at an excitation energy of $\\ensuremath{\\approx}1125$ keV with a half-life of 134.9 $\\ifmmode\\pm\\else\\textpm\\fi{}$ 7.7 $\\ensuremath{\\mu}$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 $[725]11/{2}^{\\ensuremath{-}}$ Nilsson level via a $\\ensuremath{\\Delta}K=5$ 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.