Conversion electron studies on the new high-spin isomer in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}$

High-resolution conversion electron spectra between 6.05 and 122.68 keV have been measured using thin ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}_{}^m{}_{}{}^{}$ sources produced from the ($p,n$) reaction on ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$. In addition to the expected and observable conversion lines from ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}_{}^m{}_{}{}^{}$, four other conversion lines were observed, and they could be attributed to the $L_2$-, $L_3$-, $M_2$-, and $M_3$-conversion lines of an $E2\mathrm{}$, 32.61-keV transition. This energy matches the difference of the 100.80- and 68.14-keV transitions, and an $E2\mathrm{}$ assignment is appropriate for the transition connecting these two levels. No evidence of conversion lines of the isomeric transition was observed in this energy range and the upper limit of the isomeric transition energy is set at 13 keV. The multipolarities of all six transitions were determined from the electron measurements. The relative $\gamma$-ray intensities were found to be significantly different from earlier results. A decay scheme is proposed which can account for nearly all the observations. The half-life of ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}_{}^m{}_{}{}^{}$ was determined to be 45.8 ± 0.3 min.

RADIOACTIVITY ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}_{}^m{}_{}{}^{}$ measured $T_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$, $I_{\mathrm{ce}}$; deduced $\gamma$ multipolarities, ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}$ levels, $J$, $\pi$. Ge(Li) detectors, double-focusing $\beta$-ray spectrometer.