Certification of three new electrodes for the XENONnT upgrade in cryogenic liquid xenon

Liquid xenon based dual-phase time projection chambers (LXe TPCs) are a well established detector concept and lead the search for direct dark matter detection. Over the past decade, these detectors tremendously grew in size and continuously established lower backgrounds, thereby becoming increasingly sensitive. So far, the elusive dark matter candidate, the weakly interacting massive particle (WIMP), did not leave its signature in these detectors. Instead, liquid xenon based dual-phase TPCs start observing other rare events as 8B neutrinos through coherent elastic neutrino-nucleus scattering (CEνNS) [1, 2] or the second-order weak decay of 124Xe [3, 4], the longest half-life ever directly measured to date. While increasing in sensitivity, the technical realization and complexity of large-scale TPCs becomes more and more challenging. Among the most crucial of these challenges are those associated by scaling up the TPC electrodes. They need to provide both mechanical stability and optical transparency, while the amount of material used must be kept as small as possible to minimize their background contribution. Additionally, these components need to operate reliably at −100 °C and when biased at high-voltage, e.g., −30 kV. Hence, testing of large-scale components is necessary to ensure the successful operation of the experiment. One of the current-generation liquid xenon based dual-phase TPCs is the XENONnT experiment operated at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. Since its commissioning in 2020 the detector was operating with a significantly reduced drift field due to mechanical issues with one of its wire electrodes, this has limited the detector’s sensitivity. This thesis describes the PANCAKE platform, which allows for testing of large-scale components with diameters of up to 2.75 m and reports on the successful commissioning of this facility using 300 kg of xenon. With PANCAKE being operational, three electrodes–newly designed for the XENONnT experiment–underwent thorough testing in liquid xenon environment in PANCAKE. This thesis presents the preparations and the results of this certification campaign. These tests are the first demonstration of their kind for 1.4 m diameter electrodes in liquid xenon. The insights obtained are crucial for upcoming test campaigns in the context of the next-generation XLZD detector.