New limits on the β+EC and ECEC processes in 74Se and 120Te

Abstract

Limits on double-beta processes in 74Se and 120Te have been obtained using a 400 cm3 HPGe detector and an external source consisting of natural Se or TeO2 powder. In particular, limit with a 90% C.L. on the 74Se half-life of 5.5 × 1018 y for ECEC(0ν) transitions to the 2+2 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. For other β+ EC and ECEC transitions (to the ground and excited states) obtained limits are on the level ∼ 1018-1019 y.