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An investigation into the presence of secondary electrons in megavoltage photon beams (radiotherapy application)

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.