Change in the number of ion pairs produced in free-air ionization chambers due to the applied electric field

Secondary electrons emitted by X-rays in free-air ionization chambers gain or lose energy depending on the electric fields applied to collect ion signal charges. The change in ionization due to the electric field was calculated by the EGS4 program for parallel-plate and cylindrical free-air ionization chambers. For cylindrical free-air ionization chambers, the electron range was assumed not to depend on electric field strength. Ionization enhancement and reduction peaked for X-rays at about 40 keV because the average energy of emitted electrons decreases transiently with the increase in X-ray energy as the number of Compton recoil electrons increases in relation to that of photoelectrons. Electrons emitted by photons at an energy within a certain range gain or lose over 0.3% of their energy. The net effect of the electric field on the signal current for typical parallel-plate and cylindrical free-air ionization chambers is usually small because energy gain and loss mutually compensate for each other.