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We developed a new radon observation system for 222Rn concentration determinations. A PIN photodiode (PD), an alpha particle detector, installed inside a container attracts radon families when charged electrostatically. Alpha particles emitted from 218Po(RaA), 214Po(RaC'), 210Po(RaF) and 212Po(ThC') are completely isolated from each other. This system can be applied to a long time period observation for wide variation of radon concentrations from 10Bq/m3 in indoor air to 10, 000Bq/m3 in underground water. The observed results of underground water radon concentration at a depth of 1, 000m in the Kamioka Mine showed that this technique is valid for earthquake prediction. A significant decrease in the radon concentration was observed about two and a half days before a magnitude 3.9 earthquake occurred on Atotsugawa fault about 49km from the observation station.
Tasaka et al. (Wed,) studied this question.