Theory of Measurement of Collision Kerma in Photon Beams with an Ionization Chamber

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Despite the accepted limitation to the determination of exposure for photons of energies higher than a few MeV, it is shown that the collision kerma for a suitable material or the exposure is a measurable quantity which is useful to describe the high-energy photon field. These quantities are measured at a point in free space or inside a material different from air by using ionization chambers, which have small gas cavities, calibrated in terms of exposure, air kerma or absorbed dose in the cavity gas for photons of a specified quality. As for measurements of collision kerma in soft tissue, the correction factor for attenuation and scatter in the wall and buildup cap of the chamber when exposed in air to the calibration photons or user's beams may be experimentally determined if the chamber wall and buildup cap are of tissue-equivalent materials.

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