The dose monitor and control system for electron beam and X ray on a 20 MeV medical linear accelerator is described. The system is composed of a transmission parallel-plate ionization chamber with appropriate electronic circuits. The thin foil electrodes (0.05mm) of the chamber are suitable for dose monitoring and measurement of high energy electron beam and X rays. The experimental results show that this system

with excellent linearity

is stable

reliable and fulfils design requirements.