Abstract: The device is a collapsible alpha track registration detector which will cost less to use and which can be used for measuring radon concentration over short and long term exposure times. The device includes a collapsible housing, defining an interior volume, in which is a strip of track registration material which forms damage tracks along paths traversed by alpha particles emitted during radioactive decay. The strip is retained within the housing on a cardboard holder to locate the strip at a predetermined location within the housing. The housing is a combination of a gusseted plastic bag and cardboard such that when collapsed it is a flat container but when in use it appears as a prism. The selected cardboard and plastic bag block particles and radon daughter products while admitting air and radon to the interior of the housing. The plastic bag has a hole to allow air to enter when being placed in use but the hole is sealed to one of the cardboard faces which prevents unwanted leakage.