Subject:
Quantitative Clarification on Wet Deposition Processes by Using Radon and Its Decay Products
Airborne substances are transported by “wind”, and then efficiently deposited on the ground by “rain”, where most of people live. Rain
also flush and remove substances on the ground surface elsewhere. The decay products of radon and thoron, in contrast to their parents, are removed from the atmosphere due to rain. This removal process has similarities with precipitation-related removal processes of other contaminants in air. Analysis of radon decay products in air and rain results in further understanding of “wet deposition” processes.
1. Observation of Variation in Concentrations of Radon Decay Products
Raindrops incorporate radon decay products in air and fall to the ground (Fig. 1). The short-lived radon decay products incorporated emit radiation to change to the other products. By measurement of their variations, physical processes from generation to precipitation of rainwater can be quantitatively understood.
Fig. 1 Temporal variations in rainfalls and depositions of 214Pb and 214Bi for 15 min (11 Mar. 2005 at Nagoya Univ.)
2. Quantitative Understanding of Mechanism of Increase in Gamma-ray due to Rainfall
During and just after rainfall, gamma ray radiation dose rate at the ground surface level becomes higher. This is due to gamma ray emission by natural radioactivity that is transported from the atmosphere to the ground by rain. Increase (amplitude) in the gamma ray dose rate highly varies rainfall-to-rainfall, and even during a rainfall event. This “unpredictability” sometimes brings noises to safety monitoring around nuclear facilities to interfere it. What determines the increase, and how? We study this problem by observation of radon decay products concentration in rainwater, analysis of gamma ray transport and numerical atmospheric transport and deposition models.
This study also can be applied to development of rapid estimation methods of the amount and distribution of artificial radioactivity on the ground and in air at the ground level, especially under a nuclear emergency condition.
Fig. 2 Movement of deposited radioactivity around a monitoring station and transport of gamma rays. Lead-214 and bismuth-214 precipitate with rainwater, and some part of them run off or permeate into soil.
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