Discovery of a 10 μs isomeric state in 63139Eu

Recoil-isomer tagging with the Fe54+Mo92 reaction was used to establish a 10(2)-μs isomeric state in Eu139. Prompt versus delayed γ-ray coincidence data have revealed the presence of a prompt rotational band built upon the isomer. The alignment properties of the states in this band show that the isomer is based upon a proton g7/2 configuration. The decay of the isomer takes place through a single 26-keV E1 transition. The γ-ray transition strength for this decay is consistent with those established in the neighboring isomeric gamma-soft nuclei. In these nuclei, isomers are expected to form as a consequence of differences in nuclear shapes or configurations, and the natural hindrance associated with configuration-changing E1 transitions. The isomeric nature of the state in Eu139 is reasoned to be because of difference in shape of the proton g7/2 state and the proton h11/2 ground state to which it decays. © 2011 The American Physical Society.