Measurement of 220Rn (Thoron) Concentration in Air

Radon-220 concentrations in air were measured in two steps: the emanation of 228Th was adsorbed quantitatively by activated charcoal, and its daughter 212Pb was determined by y-ray spectrometry. Thus the activity of 21'2Pb is a measure of concentration of 220Rn in air which has passed through the charcoal trap. Calibration of the equipment was performed by the adsorption of 220Rn created in a 228Th standard solution. Three grams of activated charcoal proved to be sufficient for the complete adsorption of 220Rn from air at air-flow velocities of between 2 and 10 l/min, using a cylindrical charcoal trap of 2.8 cm diam. The temperature of the trap was kept constant at -80°C. The lowest concentration of 220Rn in air detected at air-flow velocities of 10 l/min was about 0.6 x 10-15 Ci/cm3, corresponding to 1% of the maximum permissible 220Rn concentration, as proposed by the International Commission on Radiological Protection (ICRP) for occupational exposure. Moreover, the equipment, if slightly modified, may be used for the detection of 220Rn in the expired air of individuals having incorporated 228Th, or its daughter 224Ra. Assuming an exhalation coefficient of 220Rn being of the order of 2%, a total-body 224Ra content of 3.4 x 10-7 Ci may be detected during a measurement of 10 min. Equipment which is being developed for health physics purposes allows the routine control of 220Rn concentrations in the air of working rooms, or in the exhaled air of workers in, e.g., the thorium industry. (author)