Development of the high sensitive radon detector and removement of radon from the pure water at the Kamiokande Experiment

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Radon dissolved in the pure water form a strong background component in the Superkamiokande detector. Low energy background for solar neutrino observation is dominated by beta-decay electrons from ^<214>Bi, being a daughter from ^<222>Rn. Advantage of the Superkamiokande is that the recoil electron energy spectrum can be measured with a threshold of 5 MeV.In order to attain a 5 MeV threshold, the radon concentration in the pure water should be finally decreased to the value of 0.005(Bq/m^2). It is essential to remove the small particles by the water purificatin system, and to monitor continuously the changes of the radon concentration in the pure water and the radon free air. We developed an electrostatic collection of the high sensitive radon detector with PIN photodiode. The new radon detector is capable of measuring radon concentration down to 0.001(Bq/m^3) with high stabitity.The observed results of groundwater radon concentration at a well located in Nishi-nomiya city showed that our technique is valid for earthquake prediction. The observation well is 17m deep and is located about 30km northeast of the hypocenter of the magnitude (M) 7.2 the Kobe earthquake that occurred at 17 January 1995. A significant increase in the radon concentration was reacheded to be about ten times on 7 January. After the earthquake, we repaired the radon monitoring system and started the observation again on 22 January, by which time the radon concentration had returned to the pre-October 1994 level.