The forward production of high-energy electrons from megavoltage photon beams

The forward production of high-energy electrons from materials with various atomic numbers from carbon to lead has been measured for megavoltage photon beams from 4- to 25-MV peak bremsstrahlung energy by placing a thin-window parallel-plate ionization chamber directly behind foils of the various materials. The relative forward production of electrons decreases with atomic number for energies less than or equal to 10 MV until about Z = 50, after which it rises. For photon energies greater than or equal to 15 MV, forward production increases with atomic number with a break point at Z approximately 50, beyond which the curve becomes steeper.