Abstract: An improved radon detector has an enclosed housing with a removeable cap to provide a chamber within which a detector member is mounted for detecting the impact of alpha particle emissions from radon gas entering the chamber through apertures formed in the cap. The detector member has at least one planar surface for detecting the emissions and is supported within the chamber to orient the planar detection surface in a generally vertical plane parallel to the central axis of the upstanding enclosed wall which positions the detection surface in parallel opposition to preferably the major portion of the enclosing wall on which radioactive decay products from radon are deposited.