Electret - A New Tool for Measuring Concentrations of Radon and Thoron in Air

Abstract

Electret is an electrical analogue of a permanent magnet and it carries permanent electrical charge. A negatively charged teflon electret is used for the collection of decay products of radon or thoron (positively charged) formed inside a 10-l. wire mesh chamber whose sides are covered with 25-mm thick polyurethene foam. Teflon electret provides an electric field inside the chamber equivalent to that provided by a battery of 2000–3000 V. In dry conditions, nearly 70% of all the decay products of radon and thoron formed in the chamber are collected on the surface of the electret. After a known sampling time the electret is subjected to programmed a-counting to compute concentrations of radon and thoron. With a 3-hr sampling and subsequent counting, minimum detectable limits work out to be 30 pCi/m3 for radon and 110 pCi/m3 for thoron. The method is ideally suited for measurement of levels normally encountered in the environment and in dwellings. The paper discusses the method of preparing and characterizing electrets; the dependence of the collection efficiency upon relative humidity, area of the electret and the charge on the electret. Field measurements made by this method and the standard double filter system are shown to agree well.