Double Beta Decay in GERMANIUM-76.

Double beta decay in Ge^{76 } was searched for using a multi-detector germanium spectrometer system. The modes of decay searched for were betabeta_{0nu} (0^+ to 0^+), betabeta _{0nu}(0^+ to 2^+), betabeta_{2nu} , and betabeta_{0 nu} with Majoron emission. Analysis of the neutrinoless modes allows stringent limits to be placed on physics beyond the standard electroweak model, including lepton number nonconservation, right-handed leptonic currents, and the coupling between the Majoron and the electron neutrino. The experiment, located in the powerhouse of the Oroville Dam in Northern California, employs the 660 foot overburden of the dam and 8" of lead as passive shielding and a 4pi anticoincidence shield of sodium iodide scintillators. The germanium detectors provide a relatively large number of nuclei of the betabeta decay candidate Ge ^{76} (7.8% of the detector volume), offer good resolution (3.4 keV FWHM at the beta beta_{0nu} decay energy of 2.041 MeV), and low intrinsic background radioactivity. A comprehensive analysis of the components of the background radioactivity allowed the most stringent limits to be set on the lifetimes of the decay modes. The lower half-life limit of 7 times 10^ {23} years for the beta beta_{0nu}(0^+ to 0^+) decay correspond to an upper limit of ~1 eV for a massive Majorana neutrino, lower than any other experimental limit. The lower limit on the half-life of 1.4 times 10^{21 } years for betabeta_ {0nu} decay with Majoron emission corresponds to an upper limit of the Majoron-electron neutrino coupling <10^{-3} . Lower half-life limits of 1.3 times 10^{23} years and 1.9 times 10^ {20} years were set for the beta beta_{0nu}(0^+ to 2^{+} ) decay and betabeta_ {2nu} decay respectively.