The effect of wall thickness on the response of a spherical ionization chamber

Air-filled ionization chambers are used widely for radiation dosimetry. For some applications it is important to know the effect on the chamber response of photon attenuation and scattering in the chamber walls. Traditionally, the wall effect is determined by measuring the chamber response as a function of wall thickness and extrapolating linearly to zero thickness. We have constructed a spherical graphite chamber with variable wall thickness. The change in the chamber response with wall thickness has been measured in a 137Cs gamma-ray beam. Our data show that the change in response is not linear with wall thickness, in agreement with the theoretical prediction of Bielajew (1990 Med. Phys. 17 583-7). A linear versus non-linear extrapolation of the measured data to zero wall thickness leads to a difference of almost 1% in the estimate of the wall correction factor, Kw. The value of Kw obtained using the non-linear extrapolation is in good agreement with the result obtained using Monte Carlo techniques.