Calibration of Liquid Argon Time Projection Chambers Using Ambient Radioactivity and Activity From 220Rn Injection

Abstract

Liquid argon time projection chambers (LArTPCs) are vital tools in modern particle physics,notably for neutrino and dark matter experiments. The Deep Underground Neutrino Experiment (DUNE) will leverage this technology to precisely measure neutrino oscillation parameters. It will feature an intense neutrino beam at Fermilab, a near detector to measure the neutrinos before oscillations, and a far detector (800 miles away) in Lead, South Dakota to measure the neutrinos after oscillations. Because neutrinos are neutral, their properties must be inferred by detecting the charged particles they produce upon interacting with matter. Accurate particle reconstruction requires precise detector calibration. However, because the DUNE Far Detector is located a mile underground, it is shielded from the cosmic-ray muons typically used for surface-level calibration, necessitating alternative methods. This thesis investigates using low-energy radioactivity, both naturally occurring and artificiallyintroduced, as an alternative calibration source for LArTPCs. While natural sources like 39Ar beta decays offer potential applications for calibrations, injecting artificial sources like 220Rn provides a broader range of low-energy signals. In particular, the 212Bi → 212Po → 208Pb (BiPo) decay produces a distinct, localized signature of both ionization charge and scintillation light from a delayed beta-alpha coincidence, making it an ideal candidate for uniform detector calibration. In this thesis, LArTPC calibrations are demonstrated using natural low-energy radioactivityin the 2x2 Demonstrator (a DUNE Near Detector prototype at Fermilab). Then, the performance of the 220Rn injection technique is assessed in a small-scale LArTPC test stand at Colorado State University. Finally, the injection technique is evaluated in the 2x2 Demonstrator, successfully performing multiple calibrations using the introduced radioactivity.