UC Davis

Liquid-argon time-projection chambers (LArTPCs) are central to rare-event searches in neutrino and dark-matter physics. This dissertation presents two pieces of work that push those programs forward: a neutron–argon cross section measurement relevant to event reconstruction in liquid-argon (LAr), and engineering needed for stable high-voltage operation in multi-tonne LArTPCs.For DUNE, percent-level neutrino energy reconstruction is required. A major, hard-to-model piece is the “missing energy” carried by neutrons. In LAr, neutrons can travel meters before interacting and depositing energy. If the transport and capture distances are mis-modeled, reconstructed energies and the measurement of δCP and other oscillation parameters will be biased. This motivates new precision cross section measurements of neutrons on argon in the tens-of-keV to MeV range.Through my participation in the design, construction, and running of the ARTIE experiment at the Los Alamos Neutron Science Center, I measured the neutron total cross section of natural argon near 57 keV, as detailed in Chapter 3. A time-of-flight analysis with careful control of systematic uncertainties yields enhanced inputs for neutron transport and capture modeling in argon. In addition to examining systematics for the official ARTIE analysis, this thesis presents an independent analysis employing a global energy calibration method.DarkSide-20k is a dual-phase LArTPC designed for background-free, multi-tens-of-ton-year exposure and leading WIMP sensitivity. Achieving this sensitivity requires uniform and robust extraction/collection fields generated by high-voltage electrodes. Sustaining high-voltage in cryogenic noble liquids is nontrivial; large TPCs have been limited by breakdowns from microscopic defects, bubble nucleation, dielectric charging, particulates, and poorly optimized terminations.I carried out part of the engineering needed for stable operation in DarkSide-20k, as described in Chapter 6. This includes the design, construction, and testing of novel high-voltage cables and cathode terminations; field-gradings to suppress field emission; cryogenic cameras for in-situ diagnostics; and a LAr purity monitor. Together these developments support safe delivery of O(105)Volts in LAr and the monitoring required for long-term stability.By tightening neutron-transport inputs and delivering high-voltage-critical hardware, this work advances the sensitivity of LArTPC experiments searching for neutrinos and dark matter.