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

溶解在纯水中的氡在Superkamiokande探测器中形成强背景成分。太阳中微子观测的低能背景主要是来自^ Bi的β衰变电子<214>，它是^ Rn的子体<222>。Superkamiokande的优点是反冲电子能谱的测量阈值为5 MeV。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）。水净化系统必须去除水中的小颗粒物，并连续监测纯水和无氡空气中氡浓度的变化。研制了一种静电收集PIN光电二极管高灵敏度氡探测器。这种新型氡探测器具有测量氡浓度低至0.001（Bq/m^3）、稳定性好等特点，通过对西之宫市某井地下水氡浓度的观测，表明了该技术在地震预报中的有效性。该观测井深17米，位于1995年1月17日发生的科比7.2级地震震源东北约30公里处。1月7日，氡气浓度显着增加，达到约十倍。地震后，我们修复了氡监测系统，并于1月22日再次开始观测，当时氡浓度已恢复到1994年10月前的水平。

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.