Fricke dosimetry: the difference between G(Fe³⁺) for ⁶⁰Co -rays and high-energy x-rays

A calibration of the Fricke dosimeter is a measurement of G(Fe³⁺). Although G(Fe³⁺) is expected to be approximately energy independent for all low-LET radiation, existing data are not adequate to rule out the possibility of changes of a few per cent with beam quality. When a high-precision Fricke dosimeter, which has been calibrated for one particular low-LET beam quality, is used to measure the absorbed dose for another low-LET beam quality, the accuracy of the absorbed dose measurement is limited by the uncertainty in the value of G(Fe³⁺). The ratio of G(Fe³⁺) for high-energy x-rays (20 and 30 MV) to G(Fe³⁺) for ⁶⁰Co -rays, G(Fe³⁺)^MV_Co, was measured to be 1.007(±0.003) (confidence level of 68%) using two different types of water calorimeter, a stirred-water calorimeter (20 MV) and a sealed-water calorimeter (20, 30 MV). This value is consistent with our calculations based on the LET dependence of G(primary products) and, as well, with published measurements and theoretical treatments of G(Fe³⁺).