A consistent system for ionisation chamber calibration and measurements in air and in phantom

Measurements have been made to establish experimental support for the proposal that the dose to a point in an irradiated medium can be derived via in-air calibration of an ionisation chamber, with any material as a build-up cap. 60Co γ rays, 8 MV X rays and 18 MV X rays were used as the radiation sources, and build-up materials from Perspex to copper were studied. Experimental measurements have also shown that, for readings made in water, the presence or absence of a Perspex build-up cap does not alter the final measured dose. This finding is in agreement with theoretical predictions. (author)