ABSORBED DOSE BEAM QUALITY FACTORS FOR THE DOSIMETRY OF HIGH-ENERGY PHOTON BEAMS

The parallelism between existing air kerma formalisms based on the 'absorbed dose to air' chamber factor, N(D) (or N(gas)), and the more simple approach based on calibrations in terms of absorbed dose to water, N(w), is described. The importance of avoiding steps performed by the user that introduce avoidable uncertainties in the dosimetric procedure, is emphasized. Radiation beam quality factors normalized to Co-60 gamma-rays have been calculated from a comparison between the two formalisms, and sets of tables produced for the different ionization chambers included in the 1987 IAEA Code of Practice. The calculated set of data is compared with existing experimental determinations at Primary Standard Dosimetry Laboratories, showing agreement within estimated uncertainties. Problems associated with the calibration of ionization chambers in terms of absorbed dose to water in high-energy photon beams are discussed. Attention is given to a solution in terms of a normalization at Co-60 which would allow a determination of absorbed dose to water consistent with present dosimetry protocols and with an actual 1% agreement among different primary standards of absorbed dose to water at this energy.