Calorimetric determination of the cavity-gas calibration factor Ngas

In the recently published AAPM protocol for the dosimetry of high-energy photons and electrons, the response of an ionization chamber is defined as the dose to the gas in the chamber per unit electrometer reading, Ngas. Using a graphite calorimeter, Ngas has been determined for a Farmer-type ionization chamber using 4- and 25-MV x rays. The procedure was to measure the dose to graphite using the calorimeter, and then obtain the response of the chamber at the same depth in a graphite phantom. Equation (9) of the AAPM protocol was then used to calculate Ngas. The values of Ngas determined with the calorimeter are within 1% of Ngas calculated according to the AAPM protocol, using the 60Co exposure-calibration factor.