Correction factors for thick-walled ionisation chambers in point-source photon beams

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Abstract
In absolute exposure and air kerma measurements, such as those performed at Standards' laboratories, axial and radial nonuniformity correction factors are used to account for the nonuniformity of the incident photon field in the vicinity of the ionisation-chamber cavity. A theory for calculating the correction due to source nonuniformity is developed which applies to thick-walled ionisation chambers irradiated by point-source photon field with arbitrary incident energy distributions. The equations are derived within the framework of a fundamental theory of ionisation-chamber response and are suitable for Monte Carlo calculation. Monte Carlo calculations for estimating the correction in pancake, cylindrical and spherical geometries are described and comparisons with the experimental results of Kondo and Randolph indicate agreement to better than 0.5% demonstrating the viability of the theory under even the most extreme measurement conditions.

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