Search for $\alpha$ decay of ${}^{151}$Eu to the first excited level of ${}^{147}$Pm using underground $\gamma$-ray spectrometry

The alpha decay of ${}^{151}$Eu to the first excited level of ${}^{147}$Pm ($J^{\pi }=5\mathrm{/}{2}^{+}$, $E_{exc}=91.1$ keV) was searched for at the HADES underground laboratory ($\approx 500$ m w.e.). A sample of high purity europium oxide with mass of 303 g and a natural isotopic composition has been measured over 2232.8 h with a high energy resolution ultra-low background n-type semi-planar HPGe detector (40 cm${}^3$) with sub-micron deadlayer. The new improved half-life limit has been set as $T_{1\mathrm{/}2}\ge 3.7\times {10}^{18}$ yr at 68% C.L. Possibilities to improve the sensitivity of the experiment, which is already near the theoretical predictions, are discussed. New half-life limit for $\alpha$ decay of ${}^{153}$Eu is also set as $T_{1\mathrm{/}2}\ge 5.5\times {10}^{17}$ yr.