Development of a real-time detector for a small amount of radon in degasified water

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In this research, the method to extract a small amount of radon from degasified and purified water was studied at first. We used a hollow membrane module to extract radon stably and efficiently. We made a special membrane module encapsulated in an acrylic vessel, and the acrylic vessel was attached to the bottom of the radon detector vessel. We studied some other methods, for example, mount the membrane module in different way, use an air pump, but, all of them were not appropriate.Next, we have performed various calibrations by using the 700L radon detector for degasified water. We have obtained 14.6 ± 2.1(count/day)/(mBq/m^3) as a calibration factor of the 700L detector against radon in degasified water. The calibration factor for the current 70L radon detector used at Super-Kamiokande was about 0.3(count/day)/(mBq/m^3). Therefore, the sensitivity increased by about factor 40. Considering the background level, the detection limit became 0.7mBq/m^3. These results were summarized in a paper, and submitted to NIM A. The paper was accepted by NIM A (as of Oct. 2002).After summarizing these results in a paper, we made a pre-production sample of low background PIN photodiode in which Uranium-238 was reduced by factor about 50. We also optimized the electric field in the 700L radon detector, then the collection efficiency against radon in air was increased by factor about 2. We are planning to test the new 700L radon detector with the optimized electric field and the low background photodiode in the future.