Physical parameters and correction factors for ionization chambers for absolute measurement of air kerma in γ-ray fields

Values of physical parameters and correction factors essential for the absolute measurement of air kerma in Cs-137 and Co-60 gamma-ray fields were obtained using an EGS5 program for spherical, cylindrical and pancake ionization chambers. The mean mass collision stopping power ratio for graphite and air, was found to vary depending on the cutoff energy of electrons employed in calculation. The ratio between the energies deposited in cavity air due to Compton electrons emitted from the air and those from the graphite wall increases as the chamber size is increased. It also increases as the gamma-ray energy is reduced and is equal to 0.09 for Cs-137 gamma-rays in a spherical ionization chamber of cavity diameter 12 cm. Correction factors for gamma-ray attenuation in chamber walls and those for the contribution of scattered gamma-rays to chamber responses were obtained separately. The wall correction factor, which is equal to the product of these two factors, is close to unity for pancake chambers. (c) 2007 Elsevier B.V. All rights reserved.