New electret charger and charge reader systems

Electrets are candidate materials for radiation dosimetry with applications in medical physics. Dosimeters produced with electret materials are able to detect γ - and X-rays, α, β, electrons and other charged particles and, with appropriate converters, fast and slow neutrons. To produce and use an electret dosimeter one must have a system to inject an initial charge into the electret material, and another system to read and compare the initial charge with that remaining after the radiation exposure. We have developed a charging device to produce large area thin film electrets using corona discharge with an array of 130 electrodes. Also we have developed one reader and improved another that accurately measure the electret charge density. The first is a plane plate reader connected to an electrometer that measures an induced charge from which the average charge density on the electret can be inferred. The second is a coaxial probe that, together with a scanning system can record 2D images of non uniform exposures on an electret dosimeter film with a spatial resolution determined by the dimensions of the probe and its distance from the film. We report results from both of these charge readers and compare electrets produced from 25 μm thick ETFE (Ethylenetetrafluoroethylene),PFA (Tetrafluoropropylene-per-fluoromethoxyethylene) and FEP (Tetrafluoroethylene-hexa-fluoropropylene) polymer films. We demonstrate that the corona charging technique and both reading systems are adequate for electret film dosimeter measurements. We present the relationship between the responses from the two reading systems and we show that the planar probe is more sensitive and rapidly measures average exposure.