A search for double beta decay in 76Ge

Abstract

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

The Gerda experiment for the search of 0νββ decay in 76Ge

“…Indeed, the first searches for 0νββ decay used natural Ge detectors [18][19][20][21][22][23][24][25][26][27][28]. Enriched detectors allow for a better signalto-background ratio and also yield reduced costs for a fixed mass of 76 Ge in the experiment.…”

“…There is a considerable history to the use of 76 Ge for the search for 0νββ decay. After initial experiments [18][19][20][21][22][23][24][25][26][27][28], the Heidelberg-Moscow (HDM) collaboration [29] and IGEX [30][31][32][33] were the driving forces in this field setting the most stringent limits. In 2004 a subgroup of the HDM collaboration [34] claimed a 4σ significance for the observation of 0νββ decay with a best value of T 1/2 = 1.19 · 10 25 yr; the quoted 3σ range is (0.…”

Final report on the search for neutrinoless double-βdecay ofGe76from the Gotthard underground experiment

“…T h e experimental setup and some earlier results have been described in Refs. [2,3]. T h e detector, shown schematically in Fig.…”

“…The electrical contact t o the ground consists of a thin gold layer (p+-type contact) evaporated a t the surface of a center cavity. This cavity is a cylinder coaxial t o the detector, about 9 m m in diameter by 38 m m in 'Present address: Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MA 2 , are housed in a single cryostat made from high purity copper. All the components located inside the cryostat [field-effect transistor (FET), solder, wires, resistors, capacitors] or used for the cryostat itself were tested for radioactive contamination with an actively and passively shielded, low background, 90 cm3 germanium detector.…”

“…The most relevant ones are 1 3 4~s and 13'Cs with respective half-lives of 2.06 and 30.2 yr. (iii) Cosmogenic activation by fast neutrons: this activation occurred when the detector was above ground and not protected against cosmic neutrons. This leads to nuclear reactions in copper (present in the shielding and in the cryostat) yielding a series of y emitting isotopes comprising 5 6~o , 5 7~~, 5 8~~, 6 0~~, and 5 4~n [2]. Similar reactions in the germanium of the detector itself give rise to 6 5~n and especially 6 8~e , produced by 7 0~e (n,3n) 6 8~e (T,,,=288 d).…”