Search for $β^+$EC and ECEC processes in $^{74}$Se

Abstract

For the first time, limits on double-beta processes in $^{74}$Se have been obtained using a 400 cm$^3$ HPGe detector and an external source consisting of natural selenium powder. At a confidence level of 90%, they are $1.9\times 10^{18}$ y for the $β^+$EC$(0ν+ 2ν)$ transition to the ground state, $7.7\times 10^{18}$ y for the ECEC($2ν$) transition to the $2^+_1$ excited state in $^{74}$Ge (595.8 keV), $1.1\times 10^{19}$ y for the ECEC($0ν$) transition to the $2^+_1$ excited state in $^{74}$Ge (595.8 keV) and $5.5\times 10^{18}$ y for the ECEC($2ν$) and ECEC($0ν$) transitions to the $2^+_2$ excited state in $^{74}$Ge (1204.2 keV). The last transition is discussed in association with a possible enhancement of the decay rate, in this case by several orders of magnitude, because the ECEC$(0ν)$ process is nearly degenerate with an excited state in the daughter nuclide. Prospects for investigating such processes in future experiments are discussed.