Structure and decay of a three-quasiparticle isomer in 153 Eu

An isomer has been observed in the stable nuclide ${}^{153}\mathrm{Eu},$ which primarily decays via a 919-keV gamma ray to the ${17/2}^{+}$ level of the ground-state band. The structure of the isomer has been investigated using an array of eight germanium detectors and an electron spectrometer. The K-shell internal-conversion coefficient of the 919-keV transition has been measured to be $9.8(2.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ establishing that this transition has electric-dipole character. This measurement, and the observed decays of the isomer, indicate that the isomer has spin and parity ${19/2}^{\ensuremath{-}}.$ Using timing measurements against a pulsed beam, the half-life of the isomer has been measured to be 475(10) ns: this suggests high purity for the K quantum number and associated highly K-forbidden decays. The gamma-gamma coincidence data have revealed a rotational band built upon the isomer. The properties of the rotational band are consistent with the configurations $\ensuremath{\nu}[651]3/2\ensuremath{\bigotimes}\ensuremath{\nu}[505]11/2\ensuremath{\bigotimes}\ensuremath{\pi}[413]5/2,$ and $\ensuremath{\nu}[402]3/2\ensuremath{\bigotimes}\ensuremath{\nu}[505]11/2\ensuremath{\bigotimes}\ensuremath{\pi}[413]5/2,$ or a mixture of both.