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

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.