Characterization of Three Ion Chambers for High Energy Photons Reference Dosimetry

Aim: to investigate different parameters for ion chambers for the proper evaluation of absorbed radiation doses

Introduction: Many standards, codes of practice and protocols were issued internationally in order to standardize the methodologies and formalism of the use of ionization chambers in purposes of the evaluation of absorbed radiation doses in high-energy photons and electrons beams from medical linear accelerator. Methods: Three ion chambers were selected for this study: Semiflex, Farmer type (PTW), and Pinpoint ion chambers. Many correction factors and parameters controlling the behavior of ionization chambers were included in the study such as ion recombination, polarity, and response to high energy photons for each ion chambers. Results and discussion: The collection efficiencies of each ion chamber was calculated and evaluated numerically and compared to experimental data. Additionally, the beam quality correction factors were studied for each chamber, including several beam quality indices experimentally and numerically for two high energy photons: 6 MV, and 10 MV high energy photons. Two experimentally determined beam quality indices were studied for each ion chamber with respect to the reference beam quality (assumed to be the Cobalt-60 photons energy): tissue-phantom ratio TPR, and percentage depth dose PDD. All studied parameters were applied for the evaluation of the absorbed dose to water according to IAEA TRS-398 and obtained values were compared to reference values. Uncertainties associated to the evaluated values of absorbed radiation doses were discussed and presented. Conclusion: All the studied parameters are of much importance and should be considered for purposes of Radiation Metrology.