Development of a compact and sensitive electrostatic radon-222 measuring system for use in atmospheric observation

摘要

A new automated ~(222)Rn measuring system suitable for a wide range of atmospheric observations has been developed. This system comprises a ~(222)Rn analyzer and an air-sampling unit, along with a pump, drying equipment,and a filter. The operation of this ~(222)Rn analyzer is based on an electrostatic collection method involving the use of a PIN photodiode to separately detect a particles emitted from ~(218)Po and ~(214)Po released from the decay of ~(222)Rn in a hemispheric air-sample chamber. The response of the ~(222)Rn analyzer with respect to the applied voltage, chamber volume size, and sample flow rate is investigated to determine the appropriate measuring conditions. With this instrument, the detection limits have improved to 0.16-0.20 Bq m_3 for 1 h. The new ~(222)Rn analyzer shows several technical advancements over the previous electrostatic collection methods, such as an improved detection limit, higher sensitivity, higher time-resolution analysis, and longer life. During the preliminary observational test, the ~(222)Rn measuring system was able to detect the diurnal cycle in ~(222)Rn resulting from the mixing of rapid vertical air in the surface boundary layer. At the Minamitorishima station located far from the continent, very small ~(222)Rn peaks were clearly detected when long-range transport of polluted air masses from the Asian continent was observed at the station. We demonstrate that our compact 222Rn measuring system has the potential to be widely used for high time-resolution measurements of low-level ~(222)Rn.