Effect of rain scavenging on the concentration of airborne radionuclides

Concentration of radon, which is the main component of the airborne radionuclides, is largely controlled by meteorological conditions including states of rain, wind and temperature profile. Among all, rain scavenging plays the leading role in changing the radon concentration in a short period. It also causes enhancement of external radiation level through wet deposition of radon daughters onto the ground. In this study, the time variation of external radiation level was used to monitor the change of the concentration. In this relation, simultaneous measurements of the radiation level and rain had been conducted for several years by the use of highly sensitive rainmeter (>0.004 mm) and Nal(Tl) scintillation monitor (2'φx2'). Aanayses of those data showed that (1) the external radiation level enhances at the onset of a rain, (2) the enhancement is large when the rain is discrete while it is vague during a continuous rain, (3) it takes about two days in winter and one day in summer for atmospheric radon concentration to recover after it suffered a rain scavenging, (4) the shortest recovery time is about three hours, and (5) the longest time is at most one month. The relative increment of the external radiation level (y:% of the base line level) at the onset of a rain can be formulated experimentally as a function of preceding dry period (t hour;t > 1.5hour) as y = log28.5 (1-exp (-2.21log t). Those findings can be used to develop a transport model of the atmospheric radon. (author)