Size distribution of naturally occuring radioactive dust measured by a cascade impactor and autoradiography

Abstract

The size distribution of dust particles suspended in the atmosphere near the ground which carry the naturally occuring radioactive substances was studied by the cascade impactor and autoradiography, i.e., the cascade impactor was used for classifying the dust particles into 4 groups by their particle sizes, and the autoradiography was used for counting the number of alpha-tracks on each stage of the impactor at 0 hrs and 18 hrs after collection. According to the results of measurements, a large part of naturally occuring radioactive dust was concentrated in the size range below 0.5 μ, and the radioactivity was radiated almost solely from radon and thoron daughter products of short half lives.