Search for the double-. beta. decay of /sup 76/Ge

A systematic study of the radioactive background in materials used in Ge-detector fabrication was conducted prior to the construction of an ultralow-background, 135 cm/sup 3/ prototype detector. The background from primordial radioactivities in the new system was lower by factors of between 1.5 x 10/sup 3/ and 2.5 x 10/sup 4/ when compared to commercial low-background detectors. Data were collected for 8089 h with the detector located 1438 m underground, resulting in improved lower limits for the half-lives for both two-neutrino and neutrinoless ..beta beta.. decay of /sup 76/Ge to the ground state of /sup 76/Se and for the neutrinoless ..beta beta.. decay to the first excited state of /sup 76/Se. The results of four recent theoretical calculations are compared in detail and used to extract limits on , the Majorana mass of ..nu../sub e-italic/, and on the amplitudes of the couplings of right-handed Majorana neutrinos. The best combination of the lowest background data from recent experiments results in a new limit T-italic/sub 1/2//sup 0//sup ..nu../approx. >3 x 10/sup 23/ yr, corresponding to <2.4 eV neglecting right-handed couplings or 2.8 eV including both right-handed neutrino couplings.