Double beta decays of /sup 76/Ge and /sup 100/Mo by β-γ coincidence measurements

Double beta decays of /sup 76/Ge have been measured by means of a 171 cc Ge detector surrounded by a 4π-geometry NaI detector for 8621 hours at the Kamioka underground laboratory. Lower limits on the neutrino-less decays (0νββ) are obtained as T/sub 1/2//sup 0ν/(0/sup +/ → 0/sup +/) > 0.73x10/sup 23/ y and T/sub 1/2//sup 0ν/(0/sup +/ → 2/sup +/) > 0.57x10/sup 23/ y, respectively, with a 68% CL. The corresponding upper limits on the Majorana neutrino mass is 2x10/sup 20/ y and T/sub 1/2//sup 2ν/ > 1x10/sup 20/ y, respectively. A detector system which consists of 11 layers of Si(Li) detectors with 10 thin /sup 100/Mo foils (10 grams) interleaved between the detectors have been developed to study ββ decays of /sup 100/Mo. The preliminary 1866 hour run yields lower limits on the ββ half-lives of T/sub 1/2//sup 2ν/ > 2.5x10/sup 17/ y, T/sub 1/2//sup 0ν/ > 1.9x10/sup 20/ y, and T/sub 1/2//sup 0νB/ > 1.1x10/sup 19/ y, all with a 68% CL. Uranium and thorium chain isotopes contaminated mainly on surfaces of the molybdenum foils. The detector system itself has a sensitivity to the half-life of 2x10/sup 18/ -- 2x10/sup 19/ y for the 2ν and 2x10/sup 21/ y for the 0ν ββ decays