Observation of Radon Mitigation in MicroBooNE by a Liquid Argon Filtration System

TL;DR

This study demonstrates that a copper-based liquid argon filtration system effectively reduces radon levels by over 99.999%, enhancing purity for large-scale LArTPC detectors used in neutrino experiments.

Contribution

First experimental evidence of radon mitigation in liquid argon using a large-scale copper-based filter in a LArTPC.

Findings

Radon reduction factor exceeds 99.999% or 97% depending on the model.

Copper-based filter material is primarily responsible for radon removal.

Supports models of radon mitigation via slowing and trapping mechanisms.

Abstract

The MicroBooNE liquid argon time projection chamber (LArTPC) maintains a high level of liquid argon purity through the use of a filtration system that removes electronegative contaminants in continuously-circulated liquid, recondensed boil off, and externally supplied argon gas. We use the MicroBooNE LArTPC to reconstruct MeV-scale radiological decays. Using this technique we measure the liquid argon filtration system's efficacy at removing radon. This is studied by placing a 500 kBq $^{222}$Rn source upstream of the filters and searching for a time-dependent increase in the number of radiological decays in the LArTPC. In the context of two models for radon mitigation via a liquid argon filtration system, a slowing mechanism and a trapping mechanism, MicroBooNE data supports a radon reduction factor of greater than 99.999% or 97%, respectively. Furthermore, a radiological survey of the filters found that the copper-based filter material was the primary medium that removed the $^{222}$Rn. 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 LArTPCs.