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

TL;DR

This study sets new experimental limits on various double-beta decay processes in selenium-74 using a high-purity germanium detector, exploring rare nuclear transitions and their potential enhancements.

Contribution

First experimental limits on $eta^+$EC and ECEC processes in $^{74}$Se using natural selenium and HPGe detector, including discussion of near-degenerate state enhancements.

Findings

Limits on decay half-lives ranging from 1.9×10^{18} to 1.1×10^{19} years.

Discussion of potential decay rate enhancement due to near-degenerate excited states.

Prospects for future experiments to improve sensitivity and explore these processes.

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 $\beta^+$EC$(0\nu + 2\nu)$ transition to the ground state, $7.7\times 10^{18}$ y for the ECEC($2\nu$) transition to the $2^+_1$ excited state in $^{74}$Ge (595.8 keV), $1.1\times 10^{19}$ y for the ECEC($0\nu$) transition to the $2^+_1$ excited state in $^{74}$Ge (595.8 keV) and $5.5\times 10^{18}$ y for the ECEC($2\nu$) and ECEC($0\nu$) 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$(0\nu)$ process is nearly degenerate with an excited state in the daughter nuclide. Prospects for investigating such processes in future experiments are discussed.