Recoil-isomer tagging with the ${}^{54}\mathrm{Fe}$$+$${}^{92}\mathrm{Mo}$ reaction was used to establish a 10(2)-$\mu$s isomeric state in ${}^{139}\mathrm{Eu}$. Prompt versus delayed $\gamma$-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 $g_{7/2}$ configuration. The decay of the isomer takes place through a single 26-keV $E1$ transition. The $\gamma$-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 ${}^{139}\mathrm{Eu}$ is reasoned to be because of difference in shape of the proton $g_{7/2}$ state and the proton $h_{11/2}$ ground state to which it decays.

• Received 22 September 2010

DOI:https://doi.org/10.1103/PhysRevC.83.014316