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Microionization chamber air‐kerma calibration coefficients as a function of photon energy for x‐ray spectra in the range of 20–250 kVp relative to 60 Co
Author(s) -
Snow J. R.,
Micka J. A.,
DeWerd L. A.
Publication year - 2013
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 - 10.1118/1.4794491
Subject(s) - kerma , ionization chamber , calibration , dosimetry , optics , physics , range (aeronautics) , beam (structure) , materials science , laser beam quality , photon energy , photon , nuclear medicine , ionization , medicine , ion , laser , quantum mechanics , laser beams , composite material
Purpose: To investigate the applicability of a wide range of microionization chambers for reference dosimetry measurements in low‐ and medium‐energy x‐ray beams.Methods: Measurements were performed with six cylindrical microchamber models, as well as one scanning chamber and two Farmer‐type chambers for comparison purposes. Air‐kerma calibration coefficients were determined at the University of Wisconsin Accredited Dosimetry Calibration Laboratory for each chamber for a range of low‐ and medium‐energy x‐ray beams (20–250 kVp), with effective energies ranging from 11.5 keV to 145 keV, and a 60 Co beam. A low‐ Z proof‐of‐concept microchamber was developed and calibrated with and without a high‐ Z silver epoxy on the collecting electrode.Results: All chambers composed of low‐ Z materials ( Z ≤ 13), including the Farmer‐type chambers, the scanning chamber, and the PTW TN31014 and the proof‐of‐concept microchambers, exhibited air‐kerma calibration coefficients with little dependence on the quality of the beam. These chambers typically exhibited variations in calibration coefficients of less than 3% with the beam quality, for medium energy beams. However, variations in air‐kerma calibration coefficients of greater than 50% were measured over the range of medium‐energy x‐ray beams for each of the microchambers containing high‐ Z collecting electrodes ( Z > 13). For these high‐ Z chambers, which include the Exradin A14SL and A16 chambers, the PTW TN31006 chamber, the IBA CC01 chamber, and the proof‐of‐concept chamber containing silver, the average variation in air‐kerma calibration coefficients between any two calibration beams was nearly 25% over the entire range of beam qualities investigated.Conclusions: Due to the strong energy dependence observed with microchambers containing high‐ Z components, these chambers may not be suitable dosimeters for kilovoltage x‐ray applications, as they do not meet the TG‐61 requirements. It is recommended that only microchambers containing low‐ Z materials ( Z ≤ 13) be considered for air‐kerma calibrations for reference dosimetry in low‐ and medium‐energy x‐ray beams.