Comparison of the perturbation correction in a parallel plate and a cylindrical ion chamber.

The perturbation correction factors for a 0.03 cc parallel plate chamber and a 0.6 cc cylindrical ionization chamber have been compared. The current from these ion chambers was measured in electron beams of energy in the range 7 to 12 MeV and in a cobalt-60 gamma ray beam. It is concluded that there is no difference between the perturbation correction factor for these two chambers. A significant polarity effect was observed for the parallel plate chamber, which was a function of energy and depth in the phantom. This work supports the ICRU recommendation for displacement correction.