Experimental derivation of beta for high-energy photons

The absorbed dose in a medium for a given beam of megavoltage photons is related to the collision kerma by the energy dependent parameter beta. Some theoretical methods of estimating and calculating beta have been proposed in the past. The majority of the methods take into account only Compton interactions, with just one method taking into account the beam spectrum, coherent and incoherent scattering and pair production effects. Experimentally measured data, on the other hand, implicitly include appropriate contributions of all these processes. Experimentally derived beta values are tabulated and the rationale for their measurement is discussed. The beta value for a low-Z ion chamber calibration in free space with a 60Co gamma-ray beam is 1.002. The dependence of beta on several factors such as energy, field size, phantom material and depth has been studied.