Development of a Sensitive Noble Liquid Purity Monitor

Purity monitoring is critical for the operation of large-scale noble liquid time projection chambers used in rare-event searches such as neutrino and dark matter experiments. We present the design, construction, and characterization of a high-sensitivity 50 cm long purity monitor for the nEXO experiment, capable of measuring electron lifetimes up to 10 ms in noble liquid. The monitor is based on a double-gridded ion chamber configuration, optimized for low noise and high stability at cryogenic temperatures. Design elements such as electric field configuration, photocathode, high voltage filtering and charge readout are discussed. In this work, we aim to achieve an oxygen equivalent sub-ppb level of electron impurities within the detector, which corresponds to electron attenuation length of a few meters.