Development of determination system for radon concentration using air luminescence

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Air luminescence method has been applied for the determination of ^<222>Rn and the daughter nuclides. In order to improve light collection, the two photo-multiplier tubes (PMTs) were enclosed in a light-tight chamber and covered with reflectors. The pulses from the PMTs were fed into an amplifier, a coincidence circuit, a 1024-channel pulse height analyzer. Discrimination between noise pulses and pulses from real luminescence events was satisfactory.The counting efficiency of this system using a standard alpha-emitting nuclide (^<241>Am) and known amounts of ^<222>Rn was found to be 2.4 and 7.38%, respectively. This difference in counting efficiencies may be attributed to the fact that ^<222>Rn is accompanied by two short-lived alpha-emitting daughter nuclides (^<218>Po, ^<214>Po).Luminescence observed in this study is attributed to the ionization and excitation of nitrogen and oxygen caused by the passage of alpha-particles in air. Therefore, the counting efficiency for nitrogen luminescence and oxygen luminescence was also measured using a liquid scintillation system and found to be 60.2 and 28.2%, respectively.Based on the relatively high counting efficiency of nitrogen luminescence, an effective method for sampling environmental ^<222>Rn in air was developed : After removal of CO_2 and O_2 from air, ^<222>Rn and nitrogen gas in air was cooled to ca.-195.82℃ using an apparatus for liquefaction of nitrogen gas. Thus, concentrated environmental ^<222>Rn was collected together with nitrogen and easily measured by this method.