An analytic theory of the point-source nonuniformity correction factor for thick-walled ionisation chambers in photon beams

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Abstract
In exposure and air kerma measurements, axial and radial nonuniformity perturbation correction factors are used to account for the nonuniformity of the incident photon field in the vicinity of the ionisation chamber cavity. An analytic theory for calculating the point source nonuniformity correction factor for thick-walled ionisation chambers is developed. The theory developed herein is compared with Monte Carlo calculations for chambers with pancake, cylindrical and spherical geometries similar in size to the instruments employed by Standards laboratories. The perturbations, which differ from those calculated or measured by some Standards laboratories, suggest that corrections of the order of 0.3% should be applied to typical pancake geometries, smaller corrections of the order of 0.05% or less for typical Farmer-type chambers, and no correction for spherical chambers.