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I owe a great debt to my advisor, Prof. Felix Boehm, not only for guiding me through this project and my studies at Caltech, but also for giviug me the opportunity to experience life in Switzerland while doing so. Invaluable guidance was also provided by Jean-Luc Vuilleumier, Kurt Gabathuler, Petr Vogel and Herb Henrikson, all of whom played irreplacable roles in completing this project. Else Garcia deserves special credit for always being there to handle the last minute difficulties and as well as the everyday ones. The other collaborators on this experiment played invaluable parts and they were Eric Bovet, Jean-Pierre Egger and Daniel Reusser .Thanks go to the institutions involved, Caltech, the Swiss Institute for Nuclear Research and the Institute of Physics at the University of Neuchatel, for providing excellent support. Special thanks to my parents for patience and understanding and to friends on both sides of the Atlantic for making both work time and free time enjoyable. Also special thanks to Howard Stone and Louis Fluri for many interesting discussions and David Mellon and Frank Booth for providing many interesting diversions.Finally, sentiments I cannot express to Jane Ann Kincade who has been with me throughout and without whose patience and understanding I could not have completed this project. This thesis is dedicated to her. 1

BRIEF REPORTS Brief Reports are accounts of completed research which do not warrant regular articles or the priority handling given to Rapid Communications; however, the same standards of scientific quality apply. (Addenda are included in Brief Reports.) A Brief Report may be no longer than fourprintedpages and must be accompanied by an abstract.

An array of eight high purity germanium crystals is operated in the Gotthard road tunnel. Based on a data sample of 6.2 kg years, limits on different modes of double beta decay in 76Ge are obtained. In particular, the half-life for the neutrinoless double beta decay to the ground state of 76Se is greater than 2.0*1023 (3.4*1023) years at the 90% (68%) confidence level. A low energy sample of 51.6 kg days was used to set limits on cold dark matter. In particular, Dirac neutrinos with masses between 10 and 2400 GeV are ruled out. A new lower limit on the half-life for the decay of the electron into weakly interacting particles of 1.9*1023 years at 68% confidence level was also derived.