Investigation on the Effect of Chamber Wall Aluminum Alloy on the Response of Free-Air Well-Type Ionization Chamber for 137CS, 57Co and 241Am Gamma Rays

Free-air well-type ionization chambers are used in brachytherapy medical centers for measurement of brachytherapy source strengths. In this investigation, the HDR-33004 free-air well-type ionization chamber has been simulated using MCNP-4C computer code for several aluminum alloys as chamber wall material. The relative response (electric current) behavior of the simulated chamber for various source positions along the chamber central axis has been calculated and shows good agreement with measured values. We have also observed that the calculated and measured reference points are situated at the same positions. The results showed that the calculated electric current for unit source strength of 137Cs does not depend on the chamber wall aluminum alloy type, but for 57Co and 241Am sources, a strong dependency on the type of aluminum alloy as chamber wall material is observed. The presence of high atomic number elements such as zinc in some aluminum alloys resulted in higher response of the chamber. It is also found that for similar Al-7091 alloys the calculated and measured responses for 137Cs, 57Co and 241Am sources are in good agreement, showing that the alloy type of the chamber wall material would likely be an alloy similar to Al-7091. The results of this investigation can be applied for design of free-air well-type ionization chambers for low energy gamma sources.