Physical model for the calibration of activated charcoal radon detectors

Full Text:Radon (222Rn) is one of the ubiquitous pollutants of the indoor air. It penetrates dwellings from soil or by emanation from the walls. Prolonged exposure to the alpha radiation of its Progeny may result in lung cancer. In Israel, the Ministry of the Environment limits its concentration to less than 200 Bq m3 and imposes regulations on the testing procedures. Radon detection by passive adsorption into activated charcoal (AC) has evolved from the earliest improvised WWII gas mask filter to the present EPA standard diffusion barrier canisters and liquid scintillation vials. Radon atoms from the air are trapped by Van der Waals forces onto the huge pore surface of the charcoal. After the exposure, the AC is closed and the radon activity is measured by gamma or beta spectrometry. The radon concentration is measured as the ratio between the detector activity and a calibration constant, known as the effective volume, determined in a radon calibration chamber. Due to its simplicity, the AC detector has survived the competition of the more accurate electrostatic (electret), ionization chamber or solid-state detectors, and is still a widely used method for short-range radon tests. However, the physical processes involved in the radon detection are not trivial. Due to radon decay the detector is not a true integrator. Its response to a variable radon concentration is different from the mean value. In addition, the competition between the adsorption and desorption of radon atoms and water molecules lead to a non-linear response. The calibration constant is a function of exposure time, temperature and humidity. Due to statistical fluctuations of the radon collection, a precise calibration requires averaging between 5-10 detectors. A systematic calibration over