Ion chamber response and A_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. In particular, the response divided by the wall correction factor A_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 which predicts 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+or-0.2%. Calculated values of A_wall are in good agreement with most experimental results and with those given in the AAPM protocol but with a considerably lower uncertainty of +or-0.2%. When using these values with the AAPM protocol, the beta _cep factor should not be used since it is included in these calculated values.