Observation of Radon Mitigation by MicroBooNE's Cryogenic System

MicroBooNE is an 85-tonne active mass liquid argon time projection chamber at Fermilab which uses a liquid argon filtration system to remove electronegative impurities. To study the efficiency of the filtration system, we placed a 500 kBq radon source in the cryogenic system, upstream of the filters, and use MeV-scale reconstruction to search for activity in the MicroBooNE TPC. We find that the liquid argon filtration system was able to remove greater than 99.99% of the radon injected into the system. This is the first observation of radon mitigation in liquid argon with a large-scale copper-based filter and could offer a radon mitigation solution for future large liquid argon time projection chambers and could help support a low-energy program at DUNE.