Changes in the U.S. Primary Standards for the Air Kerma From Gamma-Ray Beams

Abstract

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 60Co, 137Cs, and 192Ir 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 60Co beams, +0.64% to +1.07% (depending on the chamber) for 137Cs beams, and -0.06% for the single chamber used in the measurement of the standardized 192Ir source. The primary standards for air kerma will be adjusted in the near future to reflect the changes in factors described in this work.