Collimation of high energy electron beams.

The collimation of high energy electron beams for radiation therapy is treated with special attention on the contamination of the beam by electrons which ideally should be stopped, but instead are scattered back into the beam through the edge of the collimator. It is demonstrated that the mean energy of these electrons is only 40 per cent of the mean energy in the beam and that the smallest electron contamination is obtained if the material closest to the beam is of high density.