Observations of radon at the Kamioka Mine and earthquake prediction

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We newly developed an underwater radon detector for ^<222>Rn concentration determinations in water. A PIN photodiode, an alpha particle detector, installed inside a container attracts radon families when charged electrostatically. Alpha particles emitted from ^<218>Po(RaA), ^<214>Po(RaC'), ^<210>Po(RaF) and ^<212>Po(ThC') are completely isolated from each other. This system cam be applied to a half year period observation for wide variation of radon concentrations from 0.5(Bq/m^3) in pure water to 10,000(Bq/m^3) in underground water. The observed results of pure water radon concentration in the KAMIOKANDE Tank showed that this technique is valid for the monitor of the degasfication system.Radon emanation rate were measured from various detector materials in KAMIOKANDE.The expected radon concentration from the exhalation in detector materials was estimated to be about 0.03(Bq/m_3). This value is 1/20 of the observed radon concentration 0.5(Bq/m^3) in KAMIOKANDE.This system can be applied to three years period observation for wide variation of radon concentrations in the fault underground water. The observed results of underground water radon concentration at a depth of 1,000(m) in the Kamioka Mine showed that this technique is valid for earthquake prediction. Significant decrease in the radon concentration were observed about two and a half days before a magnitude 3.9 earthquake occurred on Atotsugawa fault, and about ten days before a magnitude 4.7 earthquake occurred on the center of Gifu Prefecture.