Abstract: A detector for radon concentrations is provided having a housing, a cover member and a track registration device. The housing has a groove defined by a base, an inner wall and an outer wall with the inner wall and the outer wall extending from the base. The cover member has a side wall and is supported on the housing. The side wall intersects the groove thereby forming a U-shaped channel. The U-shaped channel creates a path for entry of radon concentrations from an exterior of the housing to an interior of the housing. The track registration mechanism is supported entirely within the housing and is capable of developing tracks thereon caused by radon-generated alpha particles.