passive radon dosimeter alpha track radon dosimeter alpha track detector Alpha track radon dosimeters are used domestically for radon monitoring over long periods of time. A passive radon dosimeter in which a small plastic film or polycarbonate chip is exposed to radon gas. When radon decays, it emits alpha particles that leave microscopic tracks on the film. After exposure, the film is chemically etched to reveal the tracks, which are counted under a microscope to determine radon levels.