Rayleigh scattered photons for substance identification.

Rayleigh scattering, that is the elastic and coherent scattering of photons from bound atomic electrons, has been investigated for use as an active probe for substance identification. An angular dispersive scattering system has been developed for materials identification utilising the 22 keV Kalpha X-ray line from a low power (25 Watt) silver anode X-ray tube. Differential scattering cross-sections have been measured for a wide range of low- to high-Z materials and compared with free atom theoretical predictions. Experimental results compare favourably with theory and evidence has been found to suggest that inter-atomic and inter-molecular interference effects have little or no effect upon the total scattering cross-section. The potential of Rayleigh scattering for detection of inclusions of low-Z media within extended objects of similar atomic constitution has been investigated with particular emphasis on the detection of potential contaminants in food products. The contrasts obtained with scatter techniques are substantially in excess of those achievable with conventional transmission measurements. By exploiting the ratio of the scattering in two regions of momentum space as the scatter parameter the contrast between inclusion and matrix may be increased to over five times that of transmission methods. This technique may be used in many situations where alternative photon interrogation methods such as X-ray transmission or X-ray fluorescence are unsuitable and a wide range of potential applications including quality control, sorting of plastics, security screening and oil/water discrimination have been considered