An annular diffusion channel equipped with a track detector film for long-term measurements of activity concentration and size distribution of nanometer 218Po particles

A new method is presented to estimate mean activity concentration and mean size distribution of nanometer 218Po over long time, i.e. several weeks in indoor atmospheres. It uses an annular channel, equipped with an LR 115 solid-state alpha-track detector, as diffusion sampler. Design, experimental characterisation and analytical methods are described. The calibration was carried out in a 0.35 m3 test radon chamber without aerosol particles by comparing the number of tracks registered on the film detector with the nanometer 218Po particle concentrations measured in parallel with a classical single screen-filter method. A calibration factor of 0.107 tracks cm-2 h-1 was established. The size distribution of these nanometer radioactive particles in the range of 0.5–5 nm was reconstructed from the track density recorded on the detector film with non-linear inversion techniques.