Ion chamber response and Asub(wall) correction factors in a 60Co beam by Monte Carlo simulation

The responses and wall correction factors for ion chambers in broad parallel 60Co beams have been calculated using Monte Carlo techniques. The calculated responses are in good agreement with Bragg-Gray cavity theory. The response divided by the wall correction factor Asub(wall) is found to be independent of the detector's shape but dependent on the material used for the chamber wall in a manner predicted by Bragg-Gray cavity theory. A simple theory is given predicting the increase in response of a Farmer ion chamber due to an electrode of an arbitrary material and radius. The change in chamber response as a function of build-up cap composition is in good agreement with analytic expressions. The effect of guard regions in pancake chambers is found to be negligible. Embedding a pancake chamber in a flat phantom during calibration is shown to increase the response by 1.0+-0.2%. Calculated values of Asub(wall) are in good agreement with most experimental results and with those given in the AAPM protocol but with a considerably lower uncertainty of +-0.2%. When using these values with the AAPM protocol, the βsub(cep) factor should not be used since it is included in these calculated values. (author)