Abstract
The presence of secondary electrons in photon beams of 60Co and 4, 8, 10 15 and 25 MV X-rays has been studied by measuring surface charge using thin window ionisation chambers. Measurements have been made for square fields from 4*4 cm2 to 35*35 cm2 for locations from the collimator head to a distance of 4 m from the target. In addition, measurements have been made from rectangular fields at 10 MV and 25 MV for fields of equivalent area from 5*5 cm2 to 25*25 cm2. By eliminating the inverse square effect, the presence of contaminants from the head and the effect of build-up in air are clearly seen and well separated. Comparison between the curves at different energies indicates an increasing effect of contamination from the head with energy and a decreasing effect of electron production in air with increasing energy.