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

在Superkamiokande探测器中，溶解在纯水中的氡形成了一个很强的本底成分。太阳中微子观测的低能背景主要是来自^&lt；214&gt；Bi的β衰变电子，它是^&lt；222&gt；Rn的子体。Superkamiokande的优点是可以用5 MeV的阈值测量反冲电子能谱。为了达到5 MeV的阈值，纯水中的氡浓度最终应降至0.005(Bq/m^2)。通过水净化系统去除细小颗粒物，并连续监测纯净水和无氡空气中氡浓度的变化是必要的。我们研制了一种采用PIN光电二极管的高灵敏度测氡仪的静电采集装置。新的测氡仪能够测量到0.001(Bq/m^3)的氡浓度，具有很高的稳定性。对西宫市一口井的地下水氡浓度的观测结果表明，我们的技术用于地震预报是有效的。观测井深17米，位于1995年1月17日神户7.2级地震震中东北约30公里处。在1月7日，氡浓度显著增加了约十倍。地震发生后，我们修复了氡监测系统，并于1月22日重新开始观测，当时氡浓度已恢复到1994年10月之前的水平。

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.