Perturbation correction factors in ionization chamber dosimetry

The perturbation of the electron fluence in parallel-plate ionization chambers has been studied using an extrapolation chamber in which it is possible to change the material of the front wall, side wall and back wall separately. With this chamber it is possible to investigate the effect of both the wall material and the chamber geometry. A graphite chamber has a very small fluence perturbation. Using this chamber as a reference, measurements were made with either the chamber front wall, side wall or back wall replaced by aluminium or A-150 (tissue-equivalent plastic). Measurements were made in beams of 60Co γ-rays, 6 and 21 MV X-rays and 9 and 22 MeV electrons. Furthermore, some preliminary comparable calculations with the EGS4 Monte Carlo code have been performed. The experimental results for photons show that with a homogeneous chamber of graphite a specific ionization nearly independent of plate separation is obtained. This is what is expected from Fano's theorem. When changing the different walls from graphite to, for example, aluminium, large differences in the specific ionization are obtained. An aluminium front wall causes a decrease as large as 10 per cent in the specific ionization and an aluminium side wall or back wall gives an increase of up to 20 per cent in the specific ionization. In electron beams also large perturbation effects are obtained that vary with chamber geometry and wall material. This supports earlier results obtained by others with commercial parallel-plate chambers that indicate large perturbation effects, especially for low electron energies. The present results show that in order to reduce the perturbation it is important to have all chamber walls made of the same material and to use a large guard in contrast to most commercial parallel-plate and extrapolation chambers used today. (author). 24 refs, 6 figs