Rainout-Washout Model for Variation of Environmental Gamma-Ray Intensity by Precipitation

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A rainout-washout model is proposed in order to forecast the variation of the intensity of environmental γ-rays and study the behavior of radioactive nuclei in the atmosphere on rainy days. This model divides the atmosphere into two parts of in cloud and under cloud. Simultaneous differential equations are formulated to give the time variations of the concen-trations of daughter nuclei of 222Rn during precipitation in each state of free atoms or ions, aerosol particles, cloud droplets and rain drops. Coefficients used in the equations for removal of daughter nuclei from one state to other one and for washout by rain drops are estimated from published data. Variations of various concentrations are obtained. Counting rates of a Ge (Li) detector are calculated for 352 keV (214Pb) and 609 keV (214Bi) γ-rays. They are mainly due to γ-rays from daughter nuclei accumulated on the earths surface, most part of which is brought by rainout in cloud. Experimental variations of counting rates are well reproduced on rainy days. For the forecast of the variations it is necessary to measure rainfall rates, concentrations of 222Rn and cloud base heights at short interval.