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$(0\hat{I}½+2\hat{I}½)$ transition to the ground state, $7.7\times 10^{18}$ y for the ECEC($2\hat{I}½$) transition to the $2_1^{+}$ excited state in ${}^{74}$Ge (595.8 keV), $1.1\times 10^{19}$ y for the ECEC($0\hat{I}½$) transition to the $2_1^{+}$ excited state in ${}^{74}$Ge (595.8 keV) and $5.5\times 10^{18}$ y for the ECEC($2\hat{I}½$) and ECEC($0\hat{I}½$) 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$(0\hat{I}½)$ process is nearly degenerate with an excited state in the daughter nuclide. Prospects for investigating such processes in future experiments are discussed.

13 pages, 2 figures; presented at the 2-nd Symposium on "Neutrino and Dark Matter in Nuclear Physics" (Paris, September 3-9, 2006); v3: minor changes