Search for β+EC and ECEC processes in 74Se

Limits on double-beta processes in 74Se have been obtained using a 400 cm3 HPGe detector and an external source consisting of natural selenium powder. In particular, limit with a 90% C.L. on the 74Se half-life of 5.5×1018 yr for ECEC(0ν) transitions to the excited state in 74Ge (1204.2 keV) has been obtained. This 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. For other β+EC and ECEC transitions (to the ground and excited states) obtained limits are on the level ∼1018−1019 yr.