Measurement of ambient radon progeny decay rates and energy spectra in liquid argon using the MicroBooNE detector

TL;DR

This study measures radon progeny decay rates and energy spectra in liquid argon using the MicroBooNE detector, demonstrating detection techniques and setting new radiopurity limits relevant for future neutrino experiments.

Contribution

It introduces new charge-based reconstruction tools for detecting radon decay products in liquid argon and reports the first radiopurity limit for a liquid argon detector with liquid-phase purification.

Findings

No significant $^{214}$Bi activity detected, limit <0.35 mBq/kg

Demonstrated calorimetric detection of $eta$ and $eta$ particles in liquid argon

Set a new radiopurity limit relevant for DUNE detector design

Abstract

We report measurements of radon progeny in liquid argon within the MicroBooNE time projection chamber (LArTPC). The presence of specific radon daughters in MicroBooNE's 85 metric tons of active liquid argon bulk is probed with newly developed charge-based low-energy reconstruction tools and analysis techniques to detect correlated $^{214}$Bi-$^{214}$Po radioactive decays. Special datasets taken during periods of active radon doping enable new demonstrations of the calorimetric capabilities of single-phase neutrino LArTPCs for $\beta$ and $\alpha$ particles with electron-equivalent energies ranging from 0.1 to 3.0 MeV. By applying $^{214}$Bi-$^{214}$Po detection algorithms to data recorded over a 46-day period, no statistically significant presence of radioactive $^{214}$Bi is detected, and a limit on the activity is placed at $<0.35$ mBq/kg at the 95% confidence level. This bulk $^{214}$Bi radiopurity limit -- the first ever reported for a liquid argon detector incorporating liquid-phase purification -- is then further discussed in relation to the targeted upper limit of 1 mBq/kg on bulk $^{222}$Rn activity for the DUNE neutrino detector.