The differential build-up factor
The build-factor is a magnitude which allows to correct the photon exponential attenuation model to obtain the real value of a certain dosimetric magnitude, like air exposure. Its main weaknesses are the dependences on the response function of such a magnitude, the geometry, and the source directionality. A formalism to deal with the first is presented in this work, which leads to the definition of the differential build-up factor. Analog Monte Carlo calculations were used to calculate this factor, as well as describing its uncertainty, in the 30 keV to 10 MeV energy range for the most typical materials up to 10 mean-free-paths. Coherent scattering and binding effects were considered in the simulations, which make the results suitable for calculations with the most complete descriptions of the attenuation coefficients. Differences due to geometry, source directionality, and detector response variability were typically found to be up to 30 % in the studied range.