First search for double-β decay of 184Os and 192Os

A search for double-β decay of osmium has been realized for the first time with the help of an ultra-low background HPGe γ detector at the underground Gran Sasso National Laboratories of the INFN (Italy). After 2741 h of data taking with a 173 g ultra-pure osmium sample limits on double-β processes in 184Os have been established at the level of T 1/2 ∼ 1014-1014 y. Possible resonant double-electron captures in 184Os were searched for with a sensitivity of T 1/2 ∼ 1016 y. A half-life limit T 1/2≥5.3×1019 y was set for the double-β decay of 192Os to the first excited level of 192Pt . The radiopurity of the osmium sample has been investigated and radionuclides 137Cs , 185Os and 207Bi were detected in the sample, while activities of 40K , 60Co , 226Ra and 232Th were limited at the ≈ mBq/kg level.