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

In precision ion chamber dosimetry in air, the wall attenuation and scatter are corrected for by A wall . Using the EGS4 system the authors show that Monte Carlo calculated A wall 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 A wall . However, the AAPM final values of A wall agree within 0.2% with the more accurate values calculated.