Verteilung der Sekundärelektronenkontamination im Photonenfeld eines Linearbeschleunigers bei 6 MV und 15 MV

Abstract

The combination of 6–MV and 15–MV photon beams by secondary electrons was investigated by means of a difference method based on two scans of each beam profile in air, with and without a brass cap over the ionization chamber. The analysis of the electron profiles obtained at different focal distances and with different field sizes showed that the exit plane of the flattening filter is the main source of secondary electrons, and that the electron field is modified by scattering in air and lateral limitation by the jaws. With PMMA and metal filters placed in the satellite plane of the accessory holder, the number of secondary electrons generated in these filters strongly depended on material and energy. The emission of secondary electrons increased with decreasing atomic number of the filter material and with increasing photon energy. Both effects resulted from the interplay of the initial emission of electrons by Compton effect and pair production, with their subsequent multiple scattering in the filter material.