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Examination of the factors A c and A eq for cylindrical ion chambers used in cobalt‐60 beams
Author(s) -
Holt J. G.,
Fleischman R. C.,
Perry D. J.,
Buffa A.
Publication year - 1979
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.594582
Subject(s) - cobalt , ion , cobalt 60 , materials science , atomic physics , physics , nuclear physics , irradiation , metallurgy , quantum mechanics
The calibration of a cobalt‐60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor, A c , a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue–air ratios were measured with these chambers for cobalt‐60 gamma rays in a polystyrene phantom, and the ratio A c / A eq was experimentally determined. In order to calculate A c from this ratio, A eq was determined also. It was found that A c depended on chamber diameter only, and not on field size or depth. A value of 0.990 for A eq is recommended and a table of A c for chambers of different outer diameters is included.