Continuous Registration of 222Rn Concentration in Air Varying with Time

As 222Rn may not be measured separately from its short-lived daughters, the activity of one of its daughter products has to be chosen as indicator for short-term variations in radon concentrations. 218 Po is suitable for the following advantageous reasons: (a) 218Po is the first daughter of 222Rn and a simple mathematical correlation exists between both activitities; (b) 218Po may be enriched on a detector surface by electrostatic separation (''stripping effect''); and (c) 218Po is an alpha-emitter and may be distinguished easily from other alpha-emitting radon daughters by energy discrimination. The differential equation giving the disintegration of radionuclides is the basis for calculation of radon activity and its variation with time from the measured 218Po activity. Disregarding a yield factor, the 218Po activity is equal to the count rate which may be read from the ratemeter. To separate polonium ions electrostatically from a definite volume, a cylindrical chamber was chosen; one of the plane faces of the chamber and the detector opposite to it, serve as the electrodes. The total volume of the chamber is 2.3 litres. The detector is an ORTEC silicon-surface barrier detector with a sensitive area of 9.5 cm2, corresponding to 3.5 cm diam. The surface of the detector is coated with a thin gold layer, 150 Å thick. Electrical pulses from the detector, proportional to the energy of incident alpha particles, are fed into a single-channel pulse-height analyser where the pulses, the height of which corresponds to alpha-particle energies of between 5.5 and 6.2 MeV, are selected and passed to a ratemeter. The energy resolution of the equipment proved to be about 2%, or 120 keV for an alpha- particle energy of 6 MeV. This means that 218Po alpha particles can be separated easily from those of 214Po (7.7 MeV) and 210Po (5.5 MeV). For calibration purposes, the chamber was connected to a closed circuit consisting of a radon source, a diaphragm pump and a membrane filter for the removal of radioactive aerosols from the air before entering the measuring volume. Calibration of the chamber for radon concentrations between 10-8 and 10-11 Ci/litre demonstrated proportionality between count rate and radon concentration in air. The yield of the equipment proved to be 0.051 counts/dis. The lowest level of detection is subject to the following parameters: separation volume; yield; and required measuring accuracy. For a given concentration measuring accuracy is, however, dependent on the value of the damping constant of the ratemeter, and therefore on the required time resolution of the equipment. For a measuring accuracy of ±20%, which is sufficient for health physics purposes, and a time resolution of 10 min, the limit of detection of the equipment described is, in radon concentrations, of the order of 10-11 Ci/litre. This value corresponds to the maximum permissible concentration (mpc) of 222Rn in air as proposed by the International Commission on Radiological Protection (ICRP) for 168 hours' occupational exposure per week. (author)