Sci-Fri PM: Planning-09: An EGSnrc investigation of ion chamber response to Co-60 beams

The EGSnrc Monte Carlo code was evaluated for its ability to calculate the relative response of a variety of ion chambers to Co-60 beams as a means of justifying the use of this code in future investigations of cavity theory. EGSnrc calculations were compared to measurements with four separate ion chambers, which were each configured with several wall materials (ranging from plastic to lead) and cavity sizes (or cavity air pressures). The experimental results included measurements by Nilsson et al. in 1992, and experiments by Whyte, Attix et al. and Cormack and Johns in the mid-to-late 1950's designed to evaluate Spencer-Attix cavity theory. Experiments by Whyte involved measurements of the response per unit mass as a function of cavity air pressure for a large cylindrical chamber, whereas the other experiments consisted of measurements of the response per unit mass (or ionization current) as a function of the distance between the front and back wall (cavity height) of a plane-parallel chamber. EGSnrc calculations, which could account for the change in response associated with changes in wall material in most cases, were generally within 1-3% of experimental values, even for experimental data that required calculations of unreported wall corrections determined using experimental techniques. The ability of EGSnrc to accurately model these experiments, which showed variations up to 300%, confirms its suitability for detailed Monte Carlo studies of cavity theory.