Abstract
Recent developements in Computerised Axial Tomography, non-destructive testing of materials, and elemental analysis have encouraged a renewed theoretical and experimental interest in the determination of attenuation coefficients. In particular, the low energy region, < 100 keV, is important because of the rapid changes of mu with absorber atomic number and photon energy. Also, this region is important for medical applications and there is increasing importance being placed upon the limited validity of the mixture rule near absorption edges. This thesis reviews the development of the experimental measurements of mu showing a significant lack of measured data in this energy region. The present state of theoretical knowledge is reviewed in chapter 3 for the most significant modes of interaction, being photoelectric effect and incoherent and Rayleigh scattering. Measurements of mu and mu/p have been made with a Si(Li) detector and a commercial Am-241 radioisotope source. The isotope is distributed in a ceramic matrix and placed in a steel casing with a thin beryllium window permiting low energy emission over 2?. Fifeteen peaks were used in the range 14 to 60 keV. The absorbers used were compounds or elements in either liquid or solid form, ranging from 2 cm of water and various organic solutions to much thinner specimens of compressed powdered solids, and foils of aluminium, silicon and graphite. The transmitted energy spectra were recorded on paper tape and analysed for peak location and integrated peak count on the CDC7600 at the University of London Computer Centre. Subsequent ananlysis and computations were carried out with the Surrey Univerity PRIME computer. Detailed considerations has been given to corrections required to the measured value of mu and mu/p arising from scatter contributions from incoherent and Rayleigh interactions.