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Collection efficiency of an ionisation chamber in a pulsed swept beam: collimator scatter effects

The expressions for calculating the collection efficiency of an ionisation chamber in a pulsed swept beam, as derived by Boag (see ibid., vol 27, p.201-10, 1982), explicitly assume that the stationary beam has a Gaussian radial intensity distribution and implicitly assumes that this distribution, upon being swept, is unperturbed by extra-phantom scatter. Consequently a hyperbolic pulse-size distribution is expected on the central axis. Measurements of pulse-size distributions at the isocentre of a Sagittaire accelerator for small collimator settings yield hyperbolic distributions in accord with this model. However, measurements of large-field pulse-size distributions yield markedly non-hyperbolic distributions for electron energies from 7 to 32 MeV. It is shown that application of the model in such cases might result in significant errors depending on the inherent collection efficiency of the chamber.