Angular and Spectral Distributions of Backscatter Radiation from Slabs of Water, Brass, and Lead Irradiated by Photons between 50 and 250 keV

Photons within a medium are exposed to absorption and scattering processes. It is mainly the photoelectric absorption and Gompton scattering that in­ fluence photons below a few hundred keV. The coherent scattering may be neglected without introducing an error of more than a few per cent of the total absorption coefficient. After photoelectric absorption the absorbing atom may emit characteristic fluorescent radiation whose energy depends on the nuclear charge of the absorber. For heavier atoms the energy and sometimes also the intensity of the characteristic radiation are so high that the radiation must be taken into consideration in the design of shields. In a Compton process a photon collides with an electron of the absorber; this process can be repeated and on each collision the photon is degraded in energy and its direction is changed. If the process could continue uninterruptedly, it would result in photons of ever decreasing energies. The multiple scattering is about the same in all absorbers if the influence of the photoelectric absorption be neglected. However, the photoelectric absorption becomes more important at lower energies. This results in a cut-off of the energy degradation originating from multiple Compton scattering. At a certain energy, photoelectric absorption is more probable than scattering of the degraded photon, with further energy loss. This energy is

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