An Online α-Impactor for Short-Lived Radon Daughters

A low-pressure cascade impactor was developed for continuous measurements of the size distributions of aerosol-attached radon and thoron decay products under environmental conditions. The emitted α-particles of the size fractionated activities are registered during air sampling with surface barrier detectors (active area = 2000 mm2) with an energy resolution < 6% (FWHM). The impactor consists of nine stages and back-up filters and operates at a flow rate of 5.1 m3 /h. The 50% cutoff diameters range between 65 and 15,530 nm. The detection limit for the short-lived radon daughters is ∼ 2 Bq/m3 for a sampling time of 1 h and a statistical error of 10%. To minimize energy losses of the ∼-particles and blow-off effects the sampling units are rotatable.