A pulse-counting ionization chamber for measuring the radon concentration in air

Abstract A pulse-counting ionization chamber with atmospheric air as filling gas has been constructed for the main purpose of measuring the radon concentration in air through its alpha activity. The problem with slow pulses due to the long collection times of the ions has been solved with a combination of a specially designed electrode structure and pulse-shaping electronics optimized for highest energy resolution and count rate. An energy resolution of 0.25 MeV FWHM has been achieved for the peaks associated with the alpha decay of radon and its daughters. This makes it possible e.g. to observe thoron ( 220 Rn) in the presence of radon ( 222 Rn). The instrument is portable and has a sensitivity of 1 cpm for a concentration of about 50 Bq/m 3 (1.4 pCi/l).