Radon-gas Monitoring by Gamma-ray Measurements on the Ground for Detecting Crustal Activity Changes : Preliminary Study by Repeat Survey Method

Concentrated stresses due to deformation of the crust may generate highly compressed interstitial fluids at depths of more than several kilometers. Such fluids tend to migrate upwards through a crack system in the crust under high pressures. A mass of radon gas is considered to be discharged into the air from underground, because radon is generated from abundant radioactive uranium and radium in the crust. Its concentration in the air is increased by the compression of fluids within the crust. Therefore, the concentration of discharged radon gas in the air is one of the important indicators of crustal activity. As the emergence of radon may be random in space and time, we have to set up a dense network of observation stations. Before constructing such a net-work system, preliminary surveys were attempted. The presence of radon is confirmed by gamma rays released from 214Bi, an intermediate decay product of radon. Using a RE-100 scintillation counter, a product of Ohyo Koken Kogyo Co., Ltd., which enables data to be recorded continuously from two bands in the gamma-ray spectrum, we took measurements close to the ground surface to monitor underground radon emissions into the air. During the last 10 years, repeated continuous observations at a fixed station and on a moving automobile or train along fixed routes positively showed the validity of the method adopted in the study presented in this paper. Repeated ob-servations at a fixed point showed a long-term increasing trend of radon concentration in Inagawa Town, Hyogo Prefecture. Mobile measurements taken aboard a Shinkansen bullet train traveling from Kyoto to Tokyo disclosed burst-like emissions of radon at such sites as near Kyoto, as well as regional variations in the intensity of radon emissions