EFFECT OF RADIATION RATE ON ELECTRET IONIZATION CHAMBERS

The dependence of radiation-induced conductivity (RIC) on radiation rate in 254 mu m thick Teflon polyfluoroethylene propylene (FEP) film used in a electret ionization chamber (EIC) is discussed and measured with regard to radiation dosimetry. Ideally, air-kerma, a quantity linearly related to dose, can be determined through measurement of the surface-charge density on the electret surface before and after irradiation and observing the difference. However, RIC permits trapped surface charge to migrate through the polymer and recombine at the collecting electrode which results in a reduced EIC charging efficiency and an overestimation of air-kerma in radiation dosimetry. The response of the EIC to a given air-kerma is recorded as a function of air-kerma rate. It is predicted and observed that surface-charge loss does depend on air-kerma rate. With proper adjustment of physical parameters however, a suitably wide range of air-kerma rates can be found over which response is flat.