First search for $2\varepsilon$ and $\varepsilon\beta^+$ decay of $^{162}$Er and new limit on $2\beta^{-}$ decay of $^{170}$Er to the first excited level of $^{170}$Yb

TL;DR

This study conducted the first search for rare double beta decay processes in erbium isotopes, setting new lower limits on their half-lives, with no observed decay events, thus constraining theoretical models of these nuclear processes.

Contribution

It provides the first experimental limits on $2 ext{e}$ and $ ext{e}eta^+$ decay modes of $^{162}$Er and improves the half-life limit for $2eta^-$ decay of $^{170}$Er to an excited state.

Findings

No decay signals observed within experimental sensitivity.

Half-life limits for $^{162}$Er decay modes range from $10^{15}$ to $10^{18}$ years.

New lower limit of $4.1 imes 10^{17}$ years for $^{170}$Er $2eta^-$ decay to excited state.

Abstract

The first search for double electron capture ($2\varepsilon$) and electron capture with positron emission ($\varepsilon\beta^+$) of $^{162}$Er to the ground state and to several excited levels of $^{162}$Dy was realized with 326 g of highly purified erbium oxide. The sample was measured over 1934 h by the ultra-low background HP Ge $\gamma$ spectrometer GeCris (465 cm$^3$) at the Gran Sasso underground laboratory. No effect was observed, the half-life limits were estimated at the level of $\lim T_{1/2}\sim 10^{15}-10^{18}$ yr. A possible resonant $0\nu KL_1$ capture in $^{162}$Er to the $2^+$ 1782.7 keV excited state of $^{162}$Dy is restricted as $T_{1/2}\geq5.0\times 10^{17}$ yr at 90% C.L. A new improved half-life limit $T_{1/2}\geq4.1\times 10^{17}$ yr was set on the $2\beta^-$ decay of $^{170}$Er to the $2^+$ 84.3 keV first excited state of $^{170}$Yb.