Relative efficiency of Gafchromic EBT3 and MD-V3 films exposed to low-energy photons and its influence on the energy dependence.

Energy-dependence of Gafchromic films exposed to low-energy photons has been reported to be a function of absorbed-dose. However, these studies are based on a relative-response, R, which considers the absorbed-dose in water and not within the film sensitive-volume. This work investigated the relative-efficiency, RE, (ratio of absorbed-dose required to produce the same net optical density (netOD) by Co gamma and by x-ray) of Gafchromic EBT3 and MD-V3 films exposed to five x-ray beams from 20 kV to 160 kV and Co gamma rays. A factor that accounts for the energy-dependence, f, based on RE, phantom-material and depth at which the films are placed during irradiation was used to remove the influence of absorbed dose. Values of RE indicated that the absorbed dose from Co gamma rays needs to be 4 and 3 times larger than those from 20 kV x-rays to produce the same netOD within the EBT3 and MD-V3 sensitive volumes, respectively. Thus, saturation could help explain why Gafchromic films show under-response to very low doses from low-energy photon beams, regardless of film model. Furthermore, RE, was found to be nearly independent of netOD and colour-channels. Consequently, f is independent of the absorbed dose and colour-channels. In contrast, besides the variation with the photon energy, f varied with film model, depth and phantom material used during the irradiation. Thus, the results suggest that f is a more reliable wide-ranging parameter for evaluating the degree of energy-dependence of the film rather than the relative-response method commonly considered.