Measurement of Atmospheric Tritium Activity Using Liquid Scintillation Counting Method

Abstract

  Tritium originating from seneral different sources is present in the atmosphere in various chemical forms, such as tritiated water vapor (HTO), tritiated hydrogen (HT) and tritiated methane (CH₃T). It is important to know the present background levels in each chemical form of tritium in the atmosphere before large-scale tritium handling is performed. Therefore, an analytical technique for atmospheric tritium with a low background liquid scintillation counter was developed and applied to actual air samples collected in Fukuoka, Japan from 1984 to the present. HTO concentrations ex pressed in Bq/L-H₂O vary within a range of 1.01 to 2.45, giving an overall average value of 1.86 ± 0.077. HTO concentrations expressed in mBq/m³-air vary within a range of 6.1 to 46.1 and have a strong correlation with the atmospheric humidity, being high in summer and low in winter. In the case of HT and CH₃T, no seasonal variations were observed with average monthly values of 20.7 to 61.0 mBq/m³-air and 8.2 to 23.9 mBq/m³-air, respectively. The present HTO concentrations are already close to the tritium level before nuclear testings. However, the present HT and CH₃T concentrations are still higher by a factor of about 125 and 30, respectively, than those before the testings.