Characterizing Bulk Signals in an Inverted Coaxial Point-Contact Detector to Inform Rare-Event Searches

A novel HPGe Compton scanner was constructed at the Max Planck Institute for Physics in Munich. In this apparatus, highly penetrating 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 reasonable 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 Ge-based neutrinoless double-beta decay experiments, where at the tonne-scale even small underperforming volumes could populate the signal window with energy degraded signals. Using the Compton scanner, the first experimental images of the depletion surface of a large-volume, non-segmented HPGe detector were produced. It was shown that a modified impurity model of the detector, determined using the evolution of the depletion surface at different biases, outperforms the conventional model of the impurity profile. Pulse-shape simulation is heavily reliant on the impurity model. Thus, a precise understanding of impurities is critical for the development of background rejection techniques used in Ge-based rare-event searches.Doctor of Philosoph