Effect of size and composition of the central electrode on the response of cylindrical ionization chambers in high-energy photon and electron beams

The authors present the Monte Carlo calculated central electrode correction factors for cylindrical ionization chambers used in high-energy photon and electron beams. The EGS4 code system with the application of a correlated sampling variance reduction technique has been employed in the calculation of the response of ionization chambers of different size and material. The results show that for an NE2571 chamber with a 1 mm diameter aluminium electrode the chamber response is increased by 0.6% in a ⁶⁰Co beam and 0.4% in a 24 MV beam compared to a graphite electrode. In a ⁶⁰Co beam the effect of an aluminium electrode compared with a graphic electrode increases with diameter from about 0.6% for 1 mm diameter to 3.4% for 5 mm diameter. The hollow aluminium electrode in an NE2561 chamber increases the chamber response by about 0.6% in a ⁶⁰Co beam compared to an air electrode. For electron beams the electrode effect varies with incident electron energy and phantom depth.