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 (0νββ) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Q_ββ( ⁷⁶Ge) = 2039 keV are (0.93 0.08)% for events from ⁶⁰Co, (21 ± 3)% from ²²⁶Ra and (40 ± 2)% from ²²⁸Th. This background suppression is achieved with (89 ± 1)% acceptance of ²²⁸Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0νββ-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_ββ. The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.