Surface event pulse shape studies of semi-coaxial Ge-detector

The GERmanium Detector Array experiment aims to search for the neutrinoless double beta decay (0νββ) of 76Ge by using isotopically enriched germanium crystals as source and detector simultaneously. The bare semiconductor diodes are operated in liquid argon at cryogenic temperatures in an ultra-low background environment. In addition, GERDA applies different active background reduction techniques, one of which is pulse shape discrimination of the closed-ended coaxial HPGe detectors of Phase I. The analysis of the signal time structure provides a powerful tool to discriminate single site events (SSE) of the ββ-decay from multi site events (MSE) of common gamma-ray background or surface events. To study the correlation between the signal shape and the interaction position on the surface of the inner detector contact, measurements with a 90Sr β-source have been performed. A summary of the used electronic/detector assembly is given and is followed by a discussion of the obtained data for a semi-coaxial detector like the ones used in GERDA. Finally, a comparison of the experimental results with the pulse shapes expected from Monte Carlo simulations is presented along with an evaluation of the relevance for the GERDA experiment.