Pulse Shape Discrimination Studies with A Broad-Energy Germanium Detector for Signal Identification and Background Suppression in the Gerda Double Beta Decay Experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0 nu beta beta) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q(beta beta)(Ge-76) = 2039 keV are (0.93 +/- 0.08)% for events from Co-60, (21 +/- 3)% from Ra-226 and (40 +/- 2)% from Th-228. This background suppression is achieved with (89 +/- 1)% acceptance of Th-228 double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0 nu beta beta-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Q(beta beta). The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.