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A comparison of methods for calibrating parallel‐plate chambers
Author(s) -
Reft C. S.,
Kuchnir F. T.,
DeWerd L. A.,
Micka J.,
Attix F. H.
Publication year - 1994
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 - 10.1118/1.597180
Subject(s) - imaging phantom , ionization chamber , calibration , dosimetry , ionization , cathode ray , beam (structure) , materials science , electron , nuclear medicine , physics , optics , nuclear engineering , nuclear physics , medicine , engineering , ion , quantum mechanics
All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel‐plate ionization chambers, but the method of determining their value of N gas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at d max with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60 Co for megavoltage chamber calibrations, two alternate procedures based on exposures in‐air, or in‐phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured N gas using each of these techniques for six of the most commonly used and commercially‐available parallel‐plate ionization chambers. The paired cylindrical and parallel‐plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60 Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for N gas that are within 1% of each other.