Abstract: A radon progeny detector for measuring the concentration of attached and unattached factions in an atmosphere. A laminar flow of air is drawn sequentially through a diffusion battery and a particle trap. Unattached progeny having a particle size of 10 nm or less are captured upon the plates of the diffusion battery. Attached progeny is passed into the trap where the attached progeny is captured by a filter. Film strips for recording tracks produced by alpha particles emitted by the progeny are mounted in both the diffusion battery and the particle trap to record the concentrations of both fractions.