Alpha-event characterization for germanium detectors Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 05/05

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.