Changes in the US primary standards for the air kerma from gamma-ray beams

Monte Carlo photon-electron transport calculations have been done to derive new wall corrections for the six NBS-NIST standard graphite-wall, air-ionization cavity chambers that serve as the U.S. national primary standard for air kerma (and exposure) for gamma rays from Co-60, Cs-137, and Ir-192 sources. The data developed for and from these calculations have also been used to refine a number of other factors affecting the standards. The largest changes are due to the new wall corrections, and the total changes are +0.87% to +1.11% (depending on the chamber) for Co-60 beams, +0.64% to +1.07% (depending on the chamber) for Cs-137 beams, and -0.06% for the single chamber used in the measurement of the standardized Ir-192 source. The primary standards for air kerma will be adjusted in the near future to reflect the changes in factors described in this work.