 Electret detectors can be used to measure radon over periods ranging from a few days to several months (depending on the element used.) The electret ion chamber contains an electostatically charged element, specially insulated, whose electrical charge is altered by ionising radiation. This charge is measured before and after exposure and the change in voltage can be read as a radon value. Allowance has to be made for gamma radiation from the ground and building materials. |
 Alpha track devices can be deployed for periods of one to several months. They have the advantage that they have a large dynamic range, i.e. they can be used over a large range of radon concentrations. The sensitive detector surface in this case is physically damaged by the alpha particles given off by radon as it changes into other elements. The amount of damage can be measured to produce a value for the radon concentration it was exposed to. Alpha track devices can only be used once. They can however be retained (usually by the laboratory) to provide a permanent record. |
 Charcoal detectors are only exposed for a few days and work by absorbing radon. The radon ‘decays’ in the charcoal and it is the subsequently released radiation which is measured by another radiation detector in the laboratory. Radon can sometimes de-absorb from the charcoal so if there is a large fluctuation over the test period or the canister is not sealed on completion of the test a lower reading may be obtained. Allowance for water absorption has also to be made and the canister would be weighed by the laboratory when it was manufactured and on its return. |
