Development of highly sensitive monitoring techniques for environmental radon and thoron

The threshold level of radon detection is 10 Bq/m3 of air and 0.1 Bq /l of water at present and it is ten to a hundred times higher for the respective methods for continuous monitoring. To analyze behaviors of environmental radon in detail, it is needed to develop a more sensitive monitoring method for radon and thoron. A short-life polonium, Po-214 ( half life : 164 μsec) was selected from its daughter nuclides of the decay series and only two continuous emissions in a short time were counted by liquid scintillation counter. A new method to discriminate the time interval was established by the first two studies of this project. The detection and amplification systems for the conventional liquid scintillation analyzer were utilized and a time analyzer which allows time-course isolation of a nuclide. And the setting conditions of the system for time-interval discrimination were determined and the errors in quantification and detection threshold were estimated based on the results of experiment and theoretical calculation. Furthermore, a cell for liquid scintillation counter able to make continuous measurement of radon and thoron through inflow of test sample onto the detector was designed and the structure of the best functioning detector was determined. The efficacy of this monitor was examined in a field study. An area mainly composed of graphite was chosen as the test field to detect an opening cleavage of bedrock based on radon distribution. The concentration of radon was markedly higher at the point of fault intersection than the surrounding area, although there remained some doubt about the absolute values of measurement. These results indicate that this monitor is available for field geographic study. (M.N.)