Radiation-induced conductivity of Teflon in electret ionization chambers

Studies of radiation-induced conductivity (RIC) of 254 μm thick Teflon polyfluoroethylene propylene (FEP) film used in an electret ionization chamber (EIC) for charging and discharging of the electret are discussed. The EIC can be used to measure air-kerma from the amount of electret surface charge that has been neutralized by ions created in the chamber's sensitive volume. RIC permits the 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. A semi-empirical equation was developed to calculate time-dependent RIC as a function of air-kerma rate and electric field in the Teflon (FEP) film for steady-state and non-steady-state conditions. It was found that the air-kerma rate (dose rate) exponent is constant while the electric-field exponent is time dependent. It is found that RIC in Teflon (FEP) of this thickness causes air-kerma as measured with EICS to be very air-kerma rate dependent at low air-kerma rates. A discussion on the limitation of EICS due to this problem is given. (author)