The effective atomic number and the calculation of the composition of phantom materials

Abstract

The formula for photoelectric absorption given by Walter (1926), used by Spiers (1946) for the calculation of an effective atomic number of a compound material, is adjusted to the values of the attenuation coefficients given by Grodstein (1957). The exponent of the atomic number has to be changed from 2·94 to 3·4 or higher. The coherent scattering is less dependent on the atomic number and is proportional to Z^1·7. A description is given of a method of calculating a phantom material which takes both effects into account. Some measurements of mass attenuation coefficients of water and of a water equivalent material composed of boron oxide, paraffin and magnesium carbonate, indicate the accuracy of the formulae, which is about 5 per cent.