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Dependence of signal development in germanium detectors on temperature and operating voltage


Hagemann,  Felix
Max Planck Institute for Physics, Max Planck Society and Cooperation Partners;

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Hagemann, F. (2020). Dependence of signal development in germanium detectors on temperature and operating voltage. Master Thesis, TU Berlin, Berlin.

Cite as: https://hdl.handle.net/21.11116/0000-0008-1B6F-F
The search for neutrinoless double-beta decay in 76Ge is inspired by the possibility that neutrinos are their own antiparticles. The observation of this process would also provide information on the absolute mass-scale for neutrinos and on a possible path to explain the matter dominance in the universe. The search is, however, very challenging as the next generation of experiments targets half-lives of more than 10²⁸ years. Such large-scale experiments require exposures of 10 ton years with a background-free environment to be sensitive enough to observe a signal consisting of only a few events. To identify background and to optimise experimental designs, a deep understanding of the pulses of the germanium detectors employed in the search is needed. This also includes a deep understanding of the influence of operational parameters like temperature and operating voltage. For this, a Compton Scanner was built, commissioned and operated at the Max-Planck-Institute for Physics in Munich. It was used to deposit energy spatially controlled in an n-type and a p-type segmented point-contact germanium detector. Pulse shape libraries were extracted for well-defined detector volumes. Simulated pulse shapes as predicted by the software package SolidStateDetectors.jl were compared to the data to test the theoretical models used to describe the charge carrier drift in germanium. Events with energy deposits close to the detector surface and with long horizontal inward drifts along the <100> and <110> axes of the germanium detector were selected. This inward drift was studied together with the fast upward drift of the charge carriers, which are collected on the point contact at the top of the detector. For temperatures between 76 and 99 K, only the inward drift along the axes was seen to depend on the temperature, while the upward drift caused by high electric fields was not. The temperature dependence of the pulse lengths can be well described by a Boltzmann-like function and is most pronounced for the electron drift along the <100> axis. For operating voltages between -1350 and -4500V, changing the voltage predominantly influences the transition between the inward and upward drift. The upward drift starts sooner at higher operating voltages, leading to shorter pulses for higher operating voltages. The inward drift is basically not influenced by the operating voltage. A comparison of simulated to measured pulses revealed that the observed electron drift is not well predicted by the commonly accepted electron drift model and that further studies are required to understand the impurity density profiles of germanium detectors. The results provide insights into the physics of germanium detectors which will help to choose optimal detector geometries and operational conditions in upcoming large-scale experiments searching for neutrinoless double-beta decay in 76Ge.