Radon-Concentration-in-Cloud and Rainfall-Rate Dependency of Short-Lived Radon Daughters in Rainwater

Abstract

Data obtained from continuous on-line monitoring system using NaI (Tl) scintillation counter for a period of 7 years at a number of sites in Fukui Prefecture in Japan have been used to study the seasonal variations of the specific activity of the short-lived radon daughters (RaB and RaC) in rainwater. The levels of the specific activity of rainwater tend to show the seasonal variations, in which the levels in winter are higher than in summer. The specific activity in each rainwater is a decreasing function of the rainfall rate, and in the case of the same rate of rainfall the specific activity of rainwater in winter is higher than in summer. From these observations, the seasonal variations of the specific activity of Ra(B+C) in rainwater have been treated as the phenomena which depends upon the seasonal variations of both rainfall rate and the concentration of radon (²²²Rn)in cloud. Namely, we have assumed the two factors on which the specific activity depends. One of them is the rainfall rate, the other is the concentration of radon in cloud.

It is concluded that the seasonal variations of the specific activity of rainwater depends almost upon the variations of the concentration of radon in cloud, and the rainfall rate dependency is 25∼40% of the total variations.

KEYWORDS:

• on-line monitoring system
• NaI(Tl)
• external radiation
• specific activity
• short-lived radon daughters
• radon in cloud
• seasonal variation
• additional doses with precipitation
• rainfall rates