Optimized Procedure for the Continuous Measurement of ~(222)Rn in the Atmosphere

摘要

In conjunction with other natural and artificial radionuclide measurements, such as ~(210)Pb, ~7Be, ~3H,~(14)C, ~(85)Kr and ~(133)Xe ~(222)Rn data provide a useful constraint in evaluating air transport models and in identifyingglobal atmospheric conditions. Long range transport studies are best made from coastal or high-altitude sitesfree of local ~(222)Rn sources. In the frame of the Global Atmospheric Watch Programme of the WorldMeteorological Organization measurements are also performed at the environmental research platformSchneefernerhaus (2600 m a.s.l.) below the top of Germany`s highest mountain Zugspitze by the GermanMeteorological Service.With the aim to measure concentrations less than 30 mBq/m3 a measuring device was constructed based on theprinciple of electrostatic deposition of ~(222)Rn daughters: the air is drawn through a filter that removes all theambient progeny but allows the inert gas ~(222)Rn to pass. The ~(222)Rn then flows continuously in a decay chamberwith a silicon detector. The dimensions and geometry of the chamber, the high voltage and the pressure insidewere optimized to deposit the decay product ~(222)Po on the silicon detector with a sufficient efficiency. Taking aradioactive equilibrium into account the concentration of ~(222)Rn was calculated. It was demanded for a systemwith minimized manual handling and an easy way of continuous calibration. The sensitivity of the instrument isdependant on the volume of the decay chamber, the flow rate, the chamber geometry, progeny captureefficiency, humidity, sampling interval and counter efficiency. The calibration procedure was performed using aRn calibration source traceable to a primary source. The sensitivity accuracy and the instrumental backgroundwere evaluated.