This invention relates to devices for detection of radon gas employing a detector material which records the impact of alpha particles emitted by the radon gas and its radioactive derivatives. More particularly this invention relates to radon detection devices which are dimensioned for convenient personal use as well as in permanent installations.
As described, for example, in U.S. Pat. No. 3,665,194 (Alter et al., 1972) devices for detection of radon gas have been developed in which the radon gas is detected by exposure of a film of detector material which is coated on one face of a conventional glass microscope slide or in the form of a separate sheet of material which is cemented or secured to a baseplate. The detector materials is exposed to alpha particles emitted by the radon gas and the detector material has the property of forming damage tracks along paths traversed by the alpha particles. After exposure, the material is etched with a reagent to enlarge the tracks, making them visible so they can be counted. The number of tracks formed during the exposure is a measure of the amount of radon in the monitored area. As more fully described in the aforementioned patent, cellulosic plastics, particularly cellulose nitrate have been employed as the detector materials because radioactive decay products or daughters or radon gas which have plated-out or deposited on the surface of these materials will emit alpha particles which are too energetic to form detectable damage tracks on these materials. Consequently, these materials will only register and form damage tracks by alpha particles originating at least a few centimeters away from the surface of the material and degraded in energy by passing through the few centimeters of air so that their energy is less than or equal to the track formation threshold. Cellulose nitrate, for example, has a track formation threshold of approximate 3.5 MeV. As a result these materials can register alpha particles emitted by radon gas migrating within the vicinity of their surface but at the same time exclude registration of alpha particles emitted by radon decay products which accumulate by direct deposition on the surface. The single registration surface of the detector member or film is exposed to the radon gas and to impingement of the emitted alpha particles, with the subsequent etching treatment to develop the detectable tracks.