Electron contamination of a high-energy X-ray beam.

The Lorentz force on an electron is utilised to separate out the electron component from the open beam of a Clinac-18, operating in the X-ray mode. The experimental arrangement is such that the mixed beam (comprising the primary photons, the scattered photons and the contamination electrons) enters an external magnetic field, which is perpendicular to the central axis of the beam. The electron contribution is analysed as a function of collimator setting, depth and distance. Based on the data, one is led to believe that the shift in dmax with field size is due to a relative enhancement in the soft X-ray component as the collimator jaws are opened.