Development of a Liquid Argon Purity Monitoring System

Abstract

Liquid argon time-projection chambers (LArTPCs) are used to detect charged particles and measure their properties. Charged particles that pass through the liquid argon (LAr) in a LArTPC ionize and excite argon atoms, producing ionization electrons and prompt scintillation light. The ionization electrons drift through the LAr volume in a uniform electric field and produce a signal at the anode. The scintillation light is used to determine the drift coordinate of an event, which allows for 3D reconstruction of tracks and interactions. Electro-negative impurities lead to the reduction of the ionization electrons and scintillation light. They worsen a detector’s ability to perform event reconstruction by reducing the signal-to-noise ratios. A purity monitor is a device that is often used alongside LArTPCs to monitor the LAr purity. It extracts electrons from a photo-cathode via the photoelectric effect and drifts them through LAr to an anode using an electric field. When traversing the purity monitor, some of the electrons will be lost due to impurities along the way. As a result, the drift-electron lifetime, which is related to the LAr impurity concentration, can be determined by measuring the difference in charge between the cathode and anode. This method allows for continuous purity monitoring of the LAr used in a LArTPC. This thesis describes the development and testing of a purity monitoring system that is used in conjunction with a LArTPC at Colorado State University.