Polarity and Ion Recombination Correction Factors of a Thimble Type Ionization Chamber with Depth in Water in the Megavoltage Beams

When the PDD (percentage depth dose) in the megavoltage beams is measured in the water phantom using the dosimetry system, the polarity and ion recombination effects of ionization chambers with depth in water are not usually taken into consideration. The polarity and ion recombination effects may vary depending on the radiation type and spectrum, which vary with depth in water [1-4]. If those effects with depth could be ignored, the PDI (percentage depth ionization) curve could be a good approximation to the PDD curve, while those effects, if not, should be applied to every depth for the correction of the PDI curve. For most chamber types the polarity effect will be negligible in photon beams, while the effect may be significant in charged particles, particularly electrons. For plane-parallel chambers the polarity effect is usually more pronounced in low energy electron beams. However, for certain chamber types it has been shown that the polarity effect increases with energy [4]. For this reason the polarity effect should always be investigated at all electron energies. The polarity effects of ionization chambers are relatively more prominent in the electron beam than in the photon beam. Moreover, this effect increases with the lower energies of electron beams and therefore this effect should be determined at various depths in water especially in the electron beams. In electron beams, the PDD should be measured to determine R50, the half-value depth in water, used as the beam quality index for electron beams. Ion recombination and polarity corrections are required at all depth. Instead POLARITY AND ION RECOMBINATION CORRECTION FACTORS OF A THIMBLE TYPE IONIZATION CHAMBER WITH DEPTH IN WATER IN THE MEGAVOLTAGE BEAMS