Use of the BIPM calorimetric and ionometric standards in megavoltage photon beams to determine Wair and Ic.

The BIPM graphite calorimeter standard for absorbed dose to water has been used in conjunction with an ionization chamber of known volume and with Monte Carlo simulations of these arrangements to determine the value for Wair in (60)Co radiation and in accelerator photon beams up to 25 MV. The results show no evidence for a variation in Wair at the 0.2% level over this energy range. Taking the constancy of Wair as established, the best estimate is Wair = 34.03 eV with a standard uncertainty of 0.21%. Consistent with this analysis, and assuming the use of the grain density in evaluating the stopping power of graphite, is the value Ic = 81.1 eV for the mean excitation energy for graphite, with standard uncertainty 1.8 eV.