Analysis of 4πγ ionization chamber response using EGS4 Monte Carlo code

The response of three types of 4πγ re-entrant well pressure ionization chambers filled with argon or nitrogen were calculated by means of the EGS4 Monte Carlo code as a function of photon energy and compared with experimental results. Excellent agreement was found between calculations and experimental results. This code was also used for the evaluation of corrections required in the change of source conditions such as the displacement of source position. In the medium and high energy region, the chamber response increased when the source was moved to off-center position from center whereas the response decreased in low energy region. This phenomena can be explained by a change ofgeometry and the absorption of photons in an inner cylindrical absorber tube of the chamber.