Development of high-sensitive and real-time detector for radon dissolved in water

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The goal of this research is to develop a high sensitive radon detector for water, and to estimate radon background in the Super-Kamiokande solar neutrino data. There are two main items to be developed; one is a high sensitive radon detector for air and the other is a stable extraction system of radon dissolved in water.In order to measure radon in air at high sensitivity, a 950L volume of radon detector with electrostatic collection method was made. A new electronics device which enables us to supply up to -6000V, previous value was up to -1500V, was also developed The inside of the detector is finished with the finest electro-polish in order to reduce radon background from the detector itself. A preliminary result of 10 count/day was obtained as a background count at the 950L radon detector. It is similar level as the previous 70L radon detectors. At higher humidity, we have achieved 5-6 times larger count rate at the 950L radon detector against the current 70L radon detectors.To extract radon in water stably, it was turned out that a hollow membrane had a good property for it. We have measured radon emanation form several membrane modules, and chosen the most clean one. The radon emanation from the selected membrane module was less than 0.02mBq/mィイD13ィエD1/module, and it can be neglected comparing with the purified water of Super-Kamiokande detector. We have attached this membrane module and the 950L radon detector, then made a high sensitive radon detector for water. The preliminary calibration factor for degasified water of the new detector was obtained as 2 - 4 (count/day)/(mBq/mィイD13ィエD1). The same factor for the current 70L radon detector for water is about 0.3 (count/day)/(mBq/mィイD13ィエD1) In future, we are planning to carry out more precise calibration of the new detector.