Developments in Purity Monitoring at CSU

Purity monitors (PrMs) measure liquid argon (LAr) purity in Liquid Argon Time-Projection Chambers (LArTPCs). LArTPCs drift ionization electrons from charged particle tracks toward an anode to form a 2D image. Full 3D track reconstruction is made using the drift time coordinate determined by prompt scintillation light. LAr impurities (e.g., oxygen, water) reduce signal-to-noise ratios by attenuating electrons and scintillation light before detection. PrMs can monitor purity during a LAr fill, when a detector is not fully operational, or when cosmic rays for calibration are sparse (deep underground). A PrM drifts electrons from a cathode to an anode in LAr. Our PrM uses a xenon flash lamp and optical fibers to transmit UV light to a photo-cathode. Electrons ejected from a photo-cathode (via the photo-electric effect) drift in an electric field to the anode. Charge amplifiers integrate currents from the cathode and anode, producing a voltage nominally proportional to the input charge. The charge difference due to attachment by impurities is used to calculate the electron lifetime. This talk will describe our PrM system, including the PrM built at CSU, the electronics, methods to increase signal size, and tests in argon.