Wall effects in plane-parallel ionization chambers.

All dosimetry protocols recommend the use of plane-parallel chambers for dose determination in electron beams with energies below 10-15 MeV. The protocols have assumed chamber perturbation effects to be negligible. The new AAPM Protocol (TG39) includes a cavity replacement factor prepl that differs from unity for some chambers, but assumes that the wall perturbation factor, pwall, may be taken as unity. In this paper the perturbation of the wall has been determined, using a large plane-parallel ionization chamber with exchangeable front and back walls. The results show that in many commercial chambers there is an energy dependent pwall factor, mainly due to differences in backscatter from the often thick chamber body as compared to the phantom material. Backscatter in common phantom and chamber materials may differ by as much as 2% at low electron energies (Plastic Water as compared to polystyrene). The front walls are often thin, resulting in negligible perturbation, but the 0.5 mm front wall of graphite in the NACP chamber was found to increase the response by 0.7% in a PMMA phantom. The experimental results have been compared with EGS4 Monte Carlo calculations using the DOSRZ code. There is an agreement within statistical and experimental uncertainties (0.5%) indicating that it is possible to use Monte Carlo calculations to calculate perturbation factors with good accuracy.