Standards of air kerma at the BEV for Cs-137 gamma rays

At the BEV two cylindrical cavity ionisation chambers have been established as primary standards to measure air kerma for Cs-137 gamma rays. The design of both chambers is the same, the chambers only differ by the specifications of the graphite they are made from. The main volume of the cavity was determined by two independent methods, i.e. length measurement of the diameter and of the depth of the cavity and also by filling the cavity with water. The applied correction factors for air density, recombination losses, humidity, stem scattering, wall attenuation, wall scattering, mean ori in of electrons, axial non-uniformity and radial non-uniformity are described. The air kerma rates measured by the two standards agree within 0.33%.