Cofrac interlaboratory comparison in air kerma and absorbed dose to water for 60Co photons

In 1996, the standard laboratories accredited in dosimetry by COFRAC have been for the first time made directly traceable to the national standard of absorbed dose to water for 60Co γ-rays. For this radiation, their traceability was formerly established only in air kerma. The aim of this interlaboratory comparison, organised in the frame of COFRAC, was to test the coherence of the calibrations carried out by these laboratories for both quantities. It consisted in comparing, for a radiotherapy type dosimeter, the calibration coefficients obtained by the participants in their own 60Co beams, to the values of BNM-LPRI. Results were in good agreement. Some problems arising from calibrations in absorbed dose to water, placing at the reference point the effective point of measurement of the ionization chamber instead of its centre, were pointed out. In that case, the differences in irradiation conditions during calibration and measurements (especially the source-to-detector distance) can result in errors of some tenths of percent. (authors)