Development of high sensitivity radon detectors

High sensitivity detectors for radon in air and in water have been developed. We use electrostatic collection and a PIN photodiode for these detectors. Calibration systems have been also constructed to obtain collection factors. As a result of the calibration study, the absolute humidity dependence of the radon detector for air is clearly observed in the region less than about 1.6 g/m3. The calibration factors of the radon detector for air are 2.2±0.2 (counts/day)/(mBq/m3) at 0.08 g/m3 and 0.86±0.06 (counts/day)/(mBq/m3) at 11 g/m3. The calibration factor of the radon detector for water is 3.6±0.5 (counts/day)/(mBq/m3). The background level of the radon detector for air is 2.4±1.3 counts/day. As a result, one standard deviation excess of the signal above the background of the radon detector for air should be possible for 1.4 mBq/m3 in a one-day measurement at 0.08 g/m3.