Alpha-event characterization for germanium detectors

A detailed study of alpha interactions on the passivated surface of a germanium detector is presented. Germanium detectors can be used to search for both neutrinoless double beta decay of 76Ge and direct interaction of dark matter. In order to increase the sensitivity to both neutrinoless double beta decay and dark matter beyond the current state of the art, the next generation of germanium-based experiments has to have a mass of about one ton and has to reduce the background by a factor of ten. The choices of detector technology facilitating both searches and the background reduction are one of the biggest challenges for such an experiment. Surface contaminations on the material close to the detectors or on the detectors themselves, can generate a background due to alpha particles, which was found to be limiting in some experiments. The characterization of events induced by alpha particles will help to identify such events and thus eliminate them as sources of background. An especially designed segmented true-coaxial detector was probed with alpha particles from an 241Am source inside the test-stand GALATEA, located at the MPI f¨ur Physik in Munich. Pulse shape analysis was performed to identify the characteristics of alpha events. The properties of the detector directly underneath the passivation layer on the end-plate were also studied. As part of the detector characterization, the thickness of the effective dead layer was determined. The studies presented here suggest improvements on detector design, which would allow an effective reduction of alpha background in next generation of germanium-based experiments.