title: Radiation induced currents in parallel plate ionization chambers: measurement and Monte Carlo simulation for megavoltage photon and electron beams.
abstract: Polarity effects in ionization chambers are caused by a radiation induced current, also known as Compton current, which arises as a charge imbalance due to charge deposition in electrodes of ionization chambers. We used a phantom-embedded extrapolation chamber (PEEC) for measurements of Compton current in megavoltage photon and electron beams. Electron contamination of photon beams and photon contamination of electron beams have a negligible effect on the measured Compton current. To allow for a theoretical understanding of the Compton current produced in the PEEC effect we carried out Monte Carlo calculations with a modified user code, the COMPTON/ EGSnrc. The Monte Carlo calculated COMPTON currents agree well with measured data for both photon and electron beams; the calculated polarity correction factors, on the other hand, do not agree with measurement results. The conclusions reached for the PEEC can be extended to parallel-plate ionization chambers in general.
authors.affiliation: Department of Medical Physics, McGill University Health Centre, 1650 Avenue Cedar Montreal, Quebec H3G 1A4, Canada. wabdel@medphys.mcgill.ca
authors.forename: Wamied
authors.initials: W
authors.lastname: Abdel-Rahman
authors.forename: Jan P
authors.initials: JP
authors.lastname: Seuntjens
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
authors.forename: Ervin B
authors.initials: EB
authors.lastname: Podgorsak
mesh_terms: D016477:Artifacts; D003198:Computer Simulation; D017076:Computer-Aided Design; D004574:Electromagnetic Fields; D004583:Electrons; D004867:Equipment Design; D019544:Equipment Failure Analysis; D008962:Models, Theoretical; D009010:Monte Carlo Method; D017785:Photons; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D012542:Scattering, Radiation