The effect of build-up cap materials on the response of an ionization chamber to 60Co gamma rays

Knowledge of the effect of wall and build-up cap materials on ionization chamber response is necessary to determine the absorbed dose in a medium using a calibration factor based on exposure or kerma in air. The attenuation and scattering effects of 60Co gamma rays in the ionization chamber wall and build-up cap, as well as their non-equivalence to air, were studied with an OFS ionization chamber (Delrin wall) and a set of build-up caps specially built for this purpose. The results for a specific material were plotted as functions of the wall and cap total thickness, extrapolated to zero wall thickness, and then corrected for the mean centre of electron production in the wall (=0.136 g cm-2). The correction factors for a specific thickness were analysed in relation to the cap material, as well as to the relative responses compared with the values calculated by using the AAPM, SEFM and IAEA formalisms for cap effects. A Monte Carlo calculation was performed in order to compare the experimental values with the theoretical ones. The calculations thus obtained show an agreement within 0.1% with the experimental values and a wall effect of approximately 1.6%.