First search for 2$\epsilon$ and $\varepsilonβ^+$ processes in ${}^{168}$Yb

Abstract

The double-electron capture and the electron capture with positron emission in ${}^{168}$Yb have been investigated for the first time at the STELLA facility of the Gran Sasso underground laboratory (Italy) measuring 371 g of highly purified ytterbium oxide placed on the end-cap of a 465 cm${}^3$ ultra-low-background high purity Germanium detector (HPGe). No gamma associated to double beta processes in ${}^{168}$Yb have been observed after 2074 h of data taking. This has allowed setting the half-life limits on the level of $\lim T_{1/2}\sim$ $10^{14}-10^{18}$ yr at 90\% C.L. Particularly, a lower half-life limit on a possible resonant neutrinoless double-electron capture in ${}^{168}$Yb to the $(2{)}^{-}$ 1403.7 keV excited state of ${}^{168}$Er is set as $T_{1/2}\ge 1.9\times 10^{18}$ yr at 90\% C.L. Half-life limits $T_{1/2}^{2\hat{I}½(0\hat{I}½)}\ge 4.5(4.3)\times 10^{16}$ yr were set on the $2ν(0ν)2β^-$ decay of ${}^{176}$Yb to the $2^{+}$ 84.3 keV first excited level of ${}^{176}$Hf.

23 pages, 6 figures, 4 tables