Wall attenuation and scatter corrections for ion chambers: measurements versus calculations

Publisher Website
Google Scholar
Abstract
In precision ion chamber dosimetry in air, the wall attenuation and scatter are corrected for by Awall. Using the EGS4 system the authors show that Monte Carlo calculated Awall factors predict relative variations in detector response with wall thickness which agree with all available experimental data within a statistical uncertainty of less than 0.1%. However, the calculated correction factors for use in exposure and air kerma standards are different by up to 1% from those obtained by extrapolating these same measurements. Using calculated correction factors would imply increase of 0.7-1.0% in the exposure and air kerma standards based on spherical and large diameter, large length cylindrical chambers and decreases of 0.2-0.5% for standards based on large diameter pancake chambers. Calculations are also shown to agree within 0.05% with the measurements of Rocha and co-workers for clinical chambers. These experimental data are not in exact agreement with the gamma values used in the AAPM protocol to obtain Awall. However, the AAPM final values of Awall agree within 0.2% with the more accurate values calculated.
Keywords