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

Abstract Experimental results on the measurement of the double-electron capture ( ee ) decay process of 74 Se are described. This decay is particularly interesting, because 74 Se is almost degenerate to the second excited state at 1204 keV in the daughter nucleus 74 Ge. 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 2 γ -ray cascade of 608 and 596 keV, whose coincident detection provides a unique signature for this particular ee process. The 2 γ -ray cascade has been searched for by a HPGe γ -ray detector in coincidence with a NaI detector using 3 kg of natural selenium.