Double-electron capture of 74Se and the search for neutrinoless decay

Experimental results on the measurement of the double-electron capture (εε) decay process of 74Se are described. This decay is particularly interesting, because 74Se is almost degenerate to the second excited state at 1204 keV in the daughter nucleus 74Ge. Such a degeneracy favors a neutrinoless decay because of the large phase-space suppression for a decay with two neutrinos. Subsequent de-excitation would generate a 2γ-ray cascade of 608 and 596 keV, whose coincident detection provides a unique signature for this particular εε process. The 2γ-ray cascade has been searched for by a HPGe γ-ray detector in coincidence with a NaI detector using 3 kg of natural selenium.