Electromagnetic decays of excited states in 261Sg (Z = 106) and 257Rf (Z = 104)

An isomeric one-quasineutron state, likely based on the [725]11/2- Nilsson level, was identified in 261 Sg by its decay via internal conversion electrons. The state has an excitation energy of ≈200 keV and a half-life of 9.0 +2.0 -1.5 μs. 261 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 α daughter nucleus of 261 Sg, namely 257 Rf Spectroscopy of delayed γ rays and converted electrons from 257 Rf 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.