Characterization of Point Contact Germanium Detectors and Development of Signal Readout Electronics for LEGEND

The Large Enriched Germanium Experiment for Neutrinoless ββ Decay, LEGEND, is a tonne-scale experimental program to search for neutrinoless double beta (0νββ) decay in the isotope 76Ge with an unprecedented sensitivity. Building on the success of the low-background 76Ge-based GERDA and Majorana Demonstrator experiments, the LEGEND collaboration is targeting a signal discovery sensitivity beyond 1028 yr on the decay half-life with approximately 10t ·yr of exposure. An essential prerequisite to achieve this ambitious goal is the reduction of backgrounds. In the first phase of the experiment, LEGEND-200, alpha- and beta-induced surface backgrounds are anticipated to be among the dominant contributions. The investigation of these backgrounds is one of the main objectives of this work. To this end, the response of a p-type point contact germanium detector to alpha and beta particles is studied in detail. It is shown that the passivated detector surface is prone to effects such as charge build-up leading to a radial-dependent behavior of important pulse shape parameters. The validity of the surface charge model is verified in dedicated pulse shape simulations. Another important role to enhance the experiment’s sensitivity plays signal readout electron- ics. The readout system needs to be placed as close as possible to the detectors to reduce the electronic noise and to enable the application of background rejection analysis techniques. However, the proximity also poses unique challenges for the radiopurity of the electronics. In LEGEND-200, an improved charge sensitive amplifier based on implementations by the predecessor experiments, GERDA and Majorana Demonstrator, that fulfills the requirements will be used. Within this work, data of a first full chain integration test are analyzed. It is shown that the novel readout system features an excellent performance. For the final stage of the experiment, LEGEND-1000, backgrounds must be decreased even further. To reduce the component originating from the signal readout electronics, the use of an application-specific integrated circuit (ASIC) amplifier located very close to the detector is foreseen. ASIC technology allows the implementation of the entire amplifier into a single low-mass, low- background chip while maintaining the spectral and noise performance achieved with conventional solutions. In this work, the feasibility of operating a large-scale germanium detector with a commer- cially available readout ASIC is demonstrated. It is shown that an excellent performance exceeding the requirements can be obtained. Finally, a first prototype of an ASIC specifically developed to meet the demands of LEGEND-1000 is characterized