Rain rate and radon daughters’ activity

During a precipitation a transient increase of gamma activity is generated by 214Pb and 214Bi, daughters of atmospheric 222Rn, 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 222Rn 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 214Pb gamma activity related to rainfalls. We developed a reproducible model for reconstructing the temporal evolution of the 214Pb net count rate during rain episodes as function of the rain rate. The effectiveness of the model is proved by an excellent linear correlation (r2 = 0.91) between measured and estimated 214Pb count rates. We observed that the sudden increase of 214Pb count rates (ΔC) is clearly related to the rain rate (R) by a power law dependence ΔC∝R0.50±0.03. We assessed that the radon daughter 214Pb content (G) of the rain water depends on the rain rate with G∝1/R0.48±0.03 and on the rain median volume diameter (λm) with G∝1/λm2.2. 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.