The Soudan 100Mo double-beta decay experiment: A status report

Double-beta decay from the ground state of 100Mo to the 0+ excited state at 1130.29 keV in 100Ru has been observed. A 956 g sample of molybdenum metal powder isotopically enriched to 98.468% 100Mo was counted in a Marinelli geometry with a well-shielded, ultralow-background germanium detector. The cascade gamma-rays at 539.53 and 590.76 keV in 100Ru were observed at the 99.998% confidence limit. The systematic errors have not yet been evaluated, but the data support a half-life of ∼1021 y.