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Sci—Fri AM(2): Brachy—06: The NRC Electron Beam Primary Standard Water Calorimeter
Author(s) -
McEwen M,
Ross C
Publication year - 2009
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 - 10.1118/1.3244198
Subject(s) - calorimeter (particle physics) , primary standard , dosimetry , calibration , linear particle accelerator , ionization chamber , beam (structure) , cathode ray , nuclear physics , nuclear engineering , ion beam , absorbed dose , physics , materials science , ion , electron , medical physics , nuclear medicine , optics , medicine , detector , engineering , quantum mechanics , ionization
Electron dosimetry, for the majority of clinical dose measurements worldwide, is currently based on an ion chamber calibrated at the standards laboratory in a60 Co beam. Conversion factors obtained from a protocol such as AAPM TG‐51 are then required to derive the dose in a linac electron beam. Electron beam water calorimetry offers a direct method to calibrate ion chambers in the clinical beams that they will be used in. This presentation details the development of a primary standard water calorimeter at the National Research Council in Ottawa and the method to calibrate ion chambers. Materials and Method The calorimeter is based on the NRC60 Co absorbed dose primary standard but the design is optimized for use in electron beams down to 8 MeV. After validation of the design the calorimeter was used to calibrate a set of cylindrical and parallel‐plate chambers (NE2571, NACP‐02, PTW Roos) in 12, 18 and 22 MeV electron beams from an Elekta Precise linac. Results The N D , wvalues obtained were compared to those obtained using TG‐51 and found to agree at the 1% level. The measured energy dependence of ion chamber calibrations was compared with data from three other laboratories that have developed electron beam standards and agreement generally better than 1% was obtained there. The standard uncertainty in the calibration of an ion chamber is estimated to be 0.35%, which will give significant improvement in the measured dose uncertainty compared to using protocol‐based values.