Modeling of surface charge distributions in electret ionization chambers

Review of Scientific Instruments

1994

Additional considerations are discussed concerning the modeling of time-dependent surface charge distributions in electret ionization chambers during the charging and discharging of electrets with ionizing radiation. Estimates of position-dependent ion density in the charging chamber are made using (a) a single-scattering calculation of ionization due to recoil electrons and (b) an isotropic, inverse-square approximation of ionization density. These estimates are used in conjunction with a subroutine which solves Laplace’s equation for electric field and tracks charge carrier trajectories from the point of origin in the chamber to an electret surface where the charge becomes trapped. The results of improved measurements are also shown and compared with calculations.

Improved modeling of surface charge distributions in electret ionization chambers

Review of Scientific Instruments

1994

Radiation-induced conductivity of Teflon in electret ionization chambers

J. Phys. D: Appl. Phys.

Markovic

1993

Studies of radiation-induced conductivity (RIC) of 254 mu m 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 EC 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.

Effect of radiation rate on electret ionization chambers

1995

Phys. Med. Biol.

The dependence of radiation-induced conductivity (RIC) on radiation rate in 254 microns 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.