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Surface Characterization of Segmented Germanium Detectors with Alpha, Beta and Gamma Particles


Hauertmann,  Lukas Martin
Max Planck Institute for Physics, Max Planck Society and Cooperation Partners;

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Hauertmann, L. M. (2021). Surface Characterization of Segmented Germanium Detectors with Alpha, Beta and Gamma Particles. PhD Thesis, Technische Universität München, München.

Cite as: https://hdl.handle.net/21.11116/0000-000A-1B28-C
A detailed study of the response of two n-type true-coaxial germanium detectors to alpha, beta and gamma particles is presented. Alpha- and beta-induced events were a substantial part of the background events in 0νββ searches performed by the GERDA and MAJORANA collaborations. They are also expected to be important for the follow-up experiment LEGEND. Basically all alpha events induced on passivated surfaces are heavily affected by charge trapping, which can move their observed energy into the signal window. However, their identification is relatively easy, even if only the central contact of a detector is read out, because the trapping leaves a signature on the shape of the pulse of these events. Up to 50 % of beta-induced events on passivated floating surfaces are also affected by charge trapping, albeit a much smaller share of charge carriers are trapped. Thus, there is no signature left on the central pulse. These events can only be identified by analyzing data provided by the multi channel read-out as provided by the experimental detectors investigated for this thesis. The data from a single read-out channel as planned for LEGEND are not sufficient and the continuous spectra of beta sources can lead to unidentified events observed in the signal window. This is important for the selection of detector technologies for LEGEND. The amount of charge trapping in alpha events on the passivated surfaces was affected by the crystal axes for both detectors. The observed energy of alpha-induced events differs by up to 1 MeV, depending on whether the events are located close to a fast or close to a slow axis. In previous studies, the data were insufficient to detect such an effect. The effect of the metallization of a segment of a detector underneath a floating surface on the detector response was studied by comparing data taken before and after the segment was fully metallized. Before the metallization, extremely slow segment signals were observed as well as a strong dependence of the signal speed on the distance of the event to the read-out contact. The segment signals were significantly faster after the metallization. However, there was still a small but measurable effect of the distance to the contact. A new open-source pulse-shape simulation software, SolidStateDetectors.jl, for germanium detectors is presented in detail. It features fast 3D field calculations of arbitrary detector geometries including the environment. Its modular structure allows easy implementation of new and improved models describing the physics of a germanium detector. A first model of probabilistic trapping of the charge carriers during the drift through a “surface channel” was developed. The impurity distributions, needed as input to the field calculations, were tuned by fits to capacitance data. The resulting improved pulse shape simulation will improve the evaluation of the experimental backgrounds in experiments like LEGEND, which are based on existing and to be developed germanium detectors.