Radon (or Rn-222) is a radioactive noble gas with 3.8 days half-life. Rn-222 even exists in normal outdoor air, and its typical concentration is 1-10 Bq/m3 (Becquerel per cubic meter). The radon concentration may become higher in the underground caverns. For example, in Kamioka underground observatory, the radon concentration reaches at about 1000Bq/m3. There are many daughter nucleus of Rn-222. For example, beta decay of Bi-214 has 3.27 MeV maximum energy . So, if radon contaminate in a detector, it will become a background source in MeV energy region in the detector. In the underground site, significant amount of the cosmic ray could be reduced. But, since radon concentration may become high, the monitoring and protection against radon will be necessary. Such technique is a common important topic in low-energy physics experiments in the world. In recent years, the exchange of information and technology are actively done among researchers in the world.
Our group has developed a real-time Rn-222 detector which has a sensitivity of mBq/m3 level in pure water and in the air. We have been applied the detector to search for the possible radon source in the Super-Kamiokande experiment. This radon detector and/or similar detector based on the detector have been used by several domestic and international research groups in the low-energy physics experimental projects.
Currently, we are developing a detection technique to measure residual radon in purified noble gases. So far, we have developed the technique to measure micro Bq/m3 level radon in purified argon gas. We are going to develop a precise radon monitoring and reduction techniques in purified xenon gas.