Long term measurement of the $^{222}$Rn concentration in the Canfranc Underground Laboratory

TL;DR

This six-year study at the Canfranc Underground Laboratory analyzed $^{222}$Rn levels and their correlation with environmental factors, highlighting seasonal patterns and the effectiveness of radon mitigation strategies for underground experiments.

Contribution

It provides long-term measurements of radon levels, their correlations with environmental variables, and evaluates mitigation strategies in an underground laboratory setting.

Findings

Positive correlation between $^{222}$Rn and relative humidity.

Seasonal periodicity observed in radon and humidity levels.

Effective radon mitigation maintained low residual levels.

Abstract

We report the results of six years (2013-2018) of measurements of $^{222}$Rn air concentration, relative humidity, atmospheric pressure and temperature in the halls A, B and C of the Canfranc Underground Laboratory (LSC). We have calculated all the Pearson correlation coefficients among these parameters and we have found a positive correlation between the $^{222}$Rn concentration and the relative humidity. Both correlated variables show a seasonal periodicity. The joint analysis of laboratory data and four years (2015-2018) of the meteorological variables outside the laboratory shows the correlation between the $^{222}$Rn concentration and the outside temperature. The collected information stresses the relevance of designing good Rn-mitigation strategies in current and future experiments at LSC; in particular, we have checked for two years (2017-2018) the good performance of the mitigation procedure of the ANAIS--112 experiment. Finally, in another measurement (2019-2021) for two years of live time, we report an upper limit to the residual $^{222}$Rn content of the radon-free air provided by the radon abatement system installed in the laboratory.