An energy monitor for electron accelerators

A monitor useful for checks of the energy selector scale of medical electron accelerators was developed and tested. It consists of a linear array of flat ionization chambers sandwiched between absorber plates of low-Z material. The first chamber at the electron beam entrance may be used to produce a reference signal Sr, if not another suitable reference signal is taken. The following chambers are electrically connected and deliver the measuring signal Sm. A clinical dosimeter can be used for recording current or charge. The energy-dependent electron range parameters Rp, R50 and R80 in water vary as linear functions of the ratio Sm/Sr. The best linear fit was obtained for the half value layer R50. Three types of the energy monitor are described, and experimental results obtained with a linear accelerator and a betatron between 5 and 25 MeV are reported. Uncertainties for checks of R50 with a calibrated energy monitor were not larger than 1 to 2 mm. Theoretical considerations by a computer model support these results.