A measurement of absorbed dose to water per unit incident 7 MeV photon fluence

After isolating the effects due to 7 MeV photons, measurements were made of the ratio of the 60 Co exposure equivalent charge from a Baldwin-Farmer ion chamber in a water phantom, divided by the on-axis fluence of incident 7 MeV photons. This quantity is the ratio between the conversion factor from fluence to absorbed dose to water and the chamber calibration factor specified by various dosimetry protocols. The measured value is a test of the Monte Carlo codes and dosimetric theory used to derive these two factors. A value of 4.13*10 -13 C kg -1 cm 2 +or-1.6% was measured. In order to assess the ion chamber charge due to scattered 7 MeV photons, a subsidiary experiment measured the dose due to scattered photons originating in a series of iron plates. Agreement was obtained between the measured dose as a function of iron plate thickness and that calculated using the EGS Monte Carlo simulation systems and also between the measured and calculated absorbed dose-depth curves. The results may be viewed as an experimental measurement, with 2-3% uncertainty, of the absorbed dose to water per unit incident 7 MeV photon fluence, or of the ion chamber calibration factor specified by dosimetry protocols.