First direct search for $2\hat{I}\mu$ and $εβ^+$ decay of ${}^{144}$Sm and $2β^-$ decay of ${}^{154}$Sm

Abstract

The first direct search for the double electron capture ($2\hat{I}\mu$) and the electron capture with positron emission ($εβ^+$) in ${}^{144}$Sm to the ground state and to the excited levels of ${}^{144}$Nd was realized by measuring - over 1899 h - a 342 g sample of highly purified samarium oxide (Sm${}_2$O${}_3$) with the ultra-low background HP-Ge ${\hat{I}}^3$ spectrometer GeCris (465 cm${}^3$) at the STELLA facility of the Gran Sasso National Laboratory (LNGS). No effect was observed and half-life limits were estimated at the level of $T_{1/2}\sim (0.1-1.3)\times 10^{20}$ yr (90% C.L.). Moreover, for the first time half-life limits of the double beta ($2β^-$) decay of ${}^{154}$Sm to several excited levels of ${}^{154}$Gd have been set; they are at the level of $T_{1/2}\sim (0.06-8)\times 10^{20}$ yr (90% C.L.).

20 pages, 7 figures and 4 tables