Air equivalence of some solid fluoropolymers in photon beams

Abstract

Two fluoropolymers polyvinylidene fluoride (PVDF) and ethylene tetrafluoroethylene (ETFE), which differ only in chemical structure and have the same atomic composition, were evaluated for use in air simulation for phantom measurements in photon beams. Photon interaction was modelled using US-NIST datasets. In order to mimic the radiation properties of air with another material, values of the following quantities were chosen for comparison: total mass attenuation coefficients, mass-energy absorption coefficients, total electron mass stopping powers, mass collision stopping powers, and mass scattering powers. The calculated ratios of the corresponding coefficients or stopping powers between the material and air were the values of interest. The results for fluoropolymers were compared with other polymers commonly used in the manufacture of phantoms. The results show that in the photon energy range from 150 keV to 1 GeV both fluoropolymers exhibit excellent dosimetric properties slightly better than air equivalent plastic C552. The effect of a carbon-based filler added to the fluoropolymer to ensure the electrical conductivity of the resulting composite was also investigated. It has been found that the increased carbon-based filler content (up to 20 wt% of carbon) added to the fluoropolymer has a very small effect on the radiation properties of the composite.