A Novel HPGe Characterization Apparatus in the Search for Neutrinoless Double-Beta Decay

A novel HPGe Compton scanner was constructed at the Max Planck Institute for Physics in Munich. In this apparatus, gamma rays deposit energy in the Ge lattice, inducing a signal coincident with that of a position-and-energy sensitive camera. Position reconstruction in large-volume Ge detectors with a millimeter level resolution and practical detector scanning times was demonstrated. The scanning apparatus was employed to characterize the bulk of a 2 kg Inverted Coaxial Point-Contact detector. It was hypothesized that in such large volume detectors, deep hole trapping in the bulk could lead to the severe energy degradation of signals. At 95 K, no evidence of significant deep hole trapping was found. This is of importance for the next generation of germanium-based neutrinoless double-beta decay experiments, where at the ton-scale even small underperforming volumes could populate the signal window with energy degraded signals. Using the same apparatus, the first experimental images of the depletion surface of a large-volume, non-segmented germanium detector were produced. It was shown that a modified impurity model of the detector, which incorporates the evolution of the depletion surface at different biases, outperforms the conventional model of the impurity profile. Pulse-shape simulation is heavily reliant on such models. Thus, a precise understanding of impurities is critical to the development of background rejection techniques for germanium-based rare event searches.