Energy-dependent polarity correction factors for four commercial ionization chambers used in electron dosimetry.

The polarity effects of four commercially available ionization chambers were characterized and correction factors as a function of mean energy at depth were tabulated. These included a Farmer-type chamber, two parallel plate chambers, and one cylindrical chamber used in a scanning water phantom dosimetry system. Polarity effects were measured at representative depths along the depth dose curves of 6, 9, 12, 16, and 20 Me V electron beams. The term "polarity error" is introduced and is defined as the error which is introduced if polarity effects are ignored. Polarity errors for the four ionization chambers studied were shown to monotonically decrease with increasing mean energy at depth and were largely independent of the energy of the incident electron beam. Only at very low energies, that is, very near the end of the practical range, did the correction factors for beams of different incident energy diverge. Three of the four chambers studied had correction factors which were independent of field size, to within +/- 1/2%. One chamber showed an increase in correction factor with increasing field size which was shown to be mainly due to stem and cable irradiation.