In this work, an experimental setup has been constructed to study the effect of aerosol concentration on the size distribution and unattached fraction of radon decay products. The experiments are conducted with 2 m~3 standard radon chamber under controlled conditions (aerosol concentration, temperature and humidity). Number concentration and size distribution of aerosols inside radon chamber are measured with a Diffusion Aerosol Spectrometer (DAS 2702M, range from 5 nm to 10 μm). The activity size distribution of radioactive aerosols is estimated with diffusion battery. Number distribution diameter at low aerosol concentration has interval 50-100 nm. When additional aerosols injected to the radon chamber, distribution peak shifted to large diameters 70-150 nm at high aerosol particle concentration. At high aerosol concentration the unattached fraction fell sharply to 10-15% of the total activity compared to 50-70% when the aerosol concentration is low. The ratio between the fractions of the unattached and attached is in the range from 0.9 to 1.2 at low aerosol concentration. At high aerosol concentration, this ratio becomes from 0.09 to 0.14.
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