Applications of gamma ray absorption and scattering

Compton backscatter, using a sealed radioactive source and a nuclear detector, is widely used for the purpose of measuring density. A general theory is proposed to explain Compton backscatter and the disparate results found in the literature. An integral count Compton backscatter instrument, using digital techniques, called a moving source gauge, was designed and constructed. In this the source-detector separation, d, can be either fixed or variable. In variable-d mode, graphs of Idsup(n) against d, where I is the count rate and n is a constant > 1, are involved. The basis of the general theory is an additional assumption to the principle of similitude and characteristic differential equations for the maximum values of the curves for the two modes are derived. It is applicable to integral count backscatter results independent of mode, design, gamma source, or source strength, and reconciles the previous disparate results. An equal count rate procedure improved the accuracy of the experimental results. A set of normalised bandwidth characteristics allowed a comparison of theoretical and experimental curve shape. Two new Compton backscatter instruments were designed. The equal count rate moving source gauge uses the shape, rather than the amplitude, parameters to measure density, and an extended version of the moving source gauge which measures low densities, can be used to examine hydrogen containing materials. (U.K.)