Rain rate and radon daughters' activity

During a precipitation a transient increase of gamma activity is generated by Pb-214 and Bi-214, daughters of at-mospheric Rn-222, which are collected by rain droplets and brought to the ground. A continuous monitoring of this gamma radiation can be an efficient alternative to a Rn-222 direct measurement in clouds and allows for estimating rain-induced variations in environmental gamma radiation. This work presents the results of a seven months proximal gamma-ray spectroscopy experiment, specifically tailored for gathering reliable and unbiased estimates of atmospheric Pb-214 gamma activity related to rainfalls. We developed a reproducible model for reconstructing the temporal evolution of the Pb-214 net count rate during rain episodes as function of the rain rate. The effectiveness of the model is proved by an excellent linear correlation (r(2) = 0.91) between measured and estimated Pb-214 count rates. We observed that the sudden increase of Pb-214 count rates (Delta C) is clearly related to the rain rate (R) by a power law dependence Delta C proportional to R-0.50 +/- 0.03. We assessed that the radon daughter Pb-214 content (G) of the rain water depends on the rain rate with G proportional to 1/R-0.48 +/- 0.03 and on the rain median volume diameter (lambda(m)) with G proportional to 1/lambda(2.2)(m). We proved that, for a fixed rainfall amount, lower is the rainfall intensity (i.e. the longer is the rain duration), higher is the radon daughters' content of the rain water.