Accuracy improvement on therapy level dosimeter calibration at Enea (Italy) by the National Absorbed-Dose-to-Water Standard

A standard of absorbed dose to water for Co-60 gamma-ray was established at ENEA (Italy) several years ago. This standard is based on a graphite calorimeter and a thick walled ionisation chamber used for absorbed dose conversion from graphite to water. To this end the ionisation chamber is irradiated at a point in graphite (where the absorbed dose to graphite is known) and then at a corresponding point in water where the energy spectrum of the radiation is the same as that at the reference point in graphite. The accuracy of this procedure was recently improved either by a more rigorous scaling of the water phantom dimensions or by a more accurate determination of the parameters involved in the conversion procedure. Moreover a more accurate gap correction for the graphite calorimeter was determined, accounting for field size dependence. The uncertainty on the absorbed dose to water is at present of about 0.45% (1σ). A comparison at BIPM of both the standards of absorbed dose to graphite and to water, respectively, was carried out and in both cases the deviations between the ENEA and the BIPM resulted about 0.3%. A comparison between the ionisation chamber calibration procedures based on the standards of absorbed dose to water and of air kerma was also made confirming the consistency of the two methods. Chamber calibration in terms of absorbed dose to water will be included as a recommended procedure in the AIFB dosimetry protocol in Italy