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^- Nilsson level, was identified in Sg261 by its decay via internal conversion electrons. The state has an excitation energy of ≈200 keV and a half-life of 9.0_-1.5^+2.0 μs. Sg261 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 α daughter nucleus of Sg261, namely Rf257. Spectroscopy of delayed γ rays and converted electrons from Rf257 resulted in the identification of a K isomer at an excitation energy of ≈1125 keV with a half-life of 134.9 ± 7.7 μs. 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 ΔK=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.