A new method is presented to estimate mean activity concentration and mean size distribution of nanometer Po-218 over long lime, 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 m(3) test radon chamber without aerosol particles by comparing the number of tracks registered on the film detector with the nanometer Po-218 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.