Ultralow-background study of neutrinoless double. beta. decay of /sup 76/Ge; new limit on the Majorana mass of. nu. /sub e/

A lower limit of 1.16 x 10/sup 23/ yr (1sigma) is reported for the half-life of no-neutrino ..beta../sup -/..beta../sup -/ decay of /sup 76/Ge which results from 3763 h of counting with an ultralow-background, 135-cm/sup 3/ prototype detector located 1438 m underground. A limit of 1.7 x 10/sup 23/ yr (1sigma) results from the best combination of our data with that from other experiments. Straightforward application of shell-model matrix elements to this limit implies that <3.2 eV (1sigma).