Correction factors in gamma-ray dosimetry

The effect of build-up factors on the measurements of dose have been investigated theoretically and experimentally. The K-fluorescent radiation emission is calculated for cerium and the lighter elements (Cu, Fe and S) to obtain the mass energy transfer coefficients for these elements, and hence the build-up factors of several dose meters - with fluorescent radiation assumed to be (1) all escaping from, and (2) all absorbed in the dose-meter volume. The fractions of the fluorescent radiation that would be absorbed in these dose meters are also estimated. In order to compare the theoretical prediction with experiment, theoretical expressions for the absorbed dose rate and the build-up factors due to an extended source are next derived, which, when expressed in terms of a form factor for the source, retain all the expressions for a point source. Experiments were performed at the Radiation Laboratory of the US Army Natick Research and Development Command in which four chemical dose meters were used: the ferrous-cupric, and the ceric-cerous systems at three different concentrations. The experiments were carried out with a 60Co cylindrical γ-ray source in air and at 64 cm distance from a large 60Co source in a water pool. The buil-up factors for these two sources correspond to μr=0.2 and 4.25, respectively. It is found experimentally that, for μr=4.25, the absorbed dose for a ceric-cerous system with 0.1M ceric-cerous concentration is higher than that with 0.01M ceric-cerous concentration by a factor of 1.25. The theoretical prediction for this factor is 1.21. For four different dose meters, the G-values as determined by these experiments are: 2.33 for 0.01M Ce4++0.01M Ce3+, 2.33 for 0.016M Ce4++0.009M Ce3+; 1.95 for 0.01M Ce4++0.09M Ce3+, all in 0.4M H2SO4; and 0.560 for 0.006M FeSO4+0.060M CuSO4 in 0.005M H2SO4. (author)