Monitoring precipitation rates using γ rays from adsorbed radon progeny as tracers

Abstract

A model correctly predicting the observed atmospheric γ radiation from radon progeny adsorbed onto the surface of rain and snow enables monitoring of precipitation rates to better than 0.3 mm/h. This model predicts that the surface (volume) deposition of radon progeny yields γ-ray flux rates proportional to the 2/5 (3/5) power of rain rates, respectively. The γ-ray rates observed were proportional to the 0.45 power for rain, but for snow which does not form spherical droplets a linear relationship was observed with proportionality constant dependent upon snow type. Atmospheric γ radiation was measured at one site in Mitaka, Japan since 1999 and more recently at another site a distance 220 m from the first using 12.9 cm3 NaI detectors. Correlation coefficients ranging from 0.85 to 0.95 and improving with elevation were obtained when data were fit with predominantly surface adsorption model and an age correction factor for the 30 min half-life of dominant radon progeny. Measurement of atmospheric γ radiation from radon progeny naturally adsorbed by rain at 20 m above ground (moderate building height) may serve as a reliable, live monitor of precipitation rates limited only by counting statistics. The regional applicability of this technique has been confirmed and will most likely have global and remote applicability.