Polarity effect of plane-parallel ionization chambers in electron radiation

Although plane-parallel ionization chambers have been in use for some time, there is still much to be learned about their performance characteristics. This work is concerned with the polarity effect in electron beams about which there is little published data. The investigations involved several popular ionization chambers; the PTW Markus chamber, the NACP chamber and its Calcam version, the Vinten-631 chamber, and the NE 2571 (Farmer-type) thimble chamber. The chambers were irradiated in electron beams of nominal energies between 4 MeV and 18 MeV. It was found in this study that the NACP, Markus and Vinten chambers require a correction of the order of 0.2% in the energy range between 4.5 MeV and 18 MeV. The overall behaviour of the Calcam chamber was similar with the exception of energies below 4 MeV. The depth dependence of the polarity effect seemed closely related to the mean beam energy at the depth of measurement. There is some evidence that the effect is also dependent on the angular spread of the electron beam and its spectrum. The authors considered how best to quantify the polarity effect practically, and propose that it should be expressed as a correction factor to be applied to readings with one particular chamber bias.