Radon radiation detector. This is designed to be used inside buildings to detect the radiation emitted by the radioactive gas radon. Radon occurs naturally, and is found in small quantities in the atmosphere. It is produced by the decay of radioactive isotopes in the Earth's crust. In some locations, radon gas rising from the ground can build up to harmful levels in houses and other buildings. This device, built by Radosys, uses a filter to let the air circulate past its detector, a plastic polymer called poly-allyl diglycol carbonate (PADC). Radiation emitted by the radon etches marks in the polymer. The amount of marks, and the exposure time, allow the amount of radon present to be calculated.