Electromagnetic decays of excited states inSg261(Z=106) andRf257(Z=104)

Abstract

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.