A continuous measuring system has been developed, which uses a plane multiwire-electrode ionization chamber and obtains radon concentrations by measuring in situ alpha energy spectra due to alpha-active nuclei in the air. The chamber has three rigid plane multiwire electrodes in a box of aluminum plates which have holes for the air to flow freely through them. The concentration measurements do not disturb the distribution of radon in the air. Values of concentration are obtained with good statistical errors because of the use of a large volume for the ionization chamber. The slow rise-time pulses of alpha particles from the chamber are analyzed with a specially designed analog-to-digital converter which is controlled with a microcomputer. The energy spectra have been obtained with a FWHM of about 600 keV. Time variations of radon concentrations have been measured in a room in a ferroconcrete building.
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