Secondary Emission Monitor as a Linear Accelerator Electron Beam Dose Monitor

Abstract

The secondary electron emission monitor (SEM) is useful for monitoring electron beams at high dose rates since it is a vacuum device and free of the saturation problem encountered with ionization chambers. In the SEM, the current associated with secondary electrons ejected from a thin foil is collected and used to monitor the electron beam. A description is given of a SEM unit which exhibits a desirable voltage saturation characteristic and good linearity with beam current. The SEM current is about 0.035 of the transmitted beam current for a single aluminum collecting foil located between adjacent polarizing foils. The SEM is basically a current monitor, but for a scattered electron beam of constant incident energy a unique relationship exists between SEM current and the absorbed dose rate at a specified point in the radiation field. However, if the energy varies by only a few percent, the relationship between SEM current and the dose rate is greatly altered. Errors of 20 to 50% were encountered with a particular linac before the beam energy stability was improved. These errors are primarily due to a variation in scattered electron intensity stemming from the energy dependent nature of the scattering process, but also to the dependence of secondary electron yield on beam energy. The change in dose rate arising from the energy dependent aspect of scattering is similar to that occurring in bremsstrahlung production under appropriate conditions. Over a beam energy range of about 1 to 600 MeV, the variation of yield with beam energy is similar to that for electron stopping power. By improving the energy stability of a particular linear accelerator the variation in dose monitor calibration was reduced to a few percent; an improvement by a factor of about 10. The effects of voltage changes, beam loading, and temperature are discussed. In the accelerator described, the microwave power was recirculated. By servo controlling the phase of the feedback power, an important improvement in energy stability was achieved.