Detection of the flowing sections of groundwater measuring dissolved radon by an aquatic alpha track method

Dissolved radon is a good tracer to track the movement of groundwater in a stratum. Radon logging is a very useful technique for detecting groundwater flow in a boring well, but radon requires prompt measurement, because all isotopes of radon have short half-lives. However, prompt measurement of radon concentration is very difficult in the field, because it takes several hours or days for the extraction from groundwater and counting radioactivity in a laboratory after sampling. In this study, a method for in-situ measurement of radon was developed without sampling groundwater. The in-situ method uses the alpha track method, which is commonly used to measure radon concentrations in air, to measure radon dissolved in groundwater. Since alpha particles have a very short range of a few tens of micrometers in water, the alpha track method is not usually used to measure dissolved radon concentration. However, to date, no practical solid detector has been produced to measure radon dissolved into water. If an alpha track detector that measures radon dissolved in water can be produced, the short range of alpha particles emitted from radon and its daughters in water will conversely greatly help to trace groundwater movement because the dissolved radon and its daughters can not make tracks on the surface of a solid detector without radon directly touching the surface. Furthermore, this new aquatic alpha track method does not require prompt measurement, because radon concentration is left on the surface of detectors as the integrated numbers of tracks in proportion to the length of time the solid detector has been exposed in the water. Consequently, this method will be useful for tracking the in-situ groundwater movement in groundwater surveys or earthquake prediction. (author)