Particle Composition By Backscattered Electron Imaging
Information on the composition of individual particles can be obtained from both the backscattered electron signal, and from the analysis of X-rays emitted by the particles. The backscattered electron signal rapidly supplies data on mean atomic number differences within and between particles. X-ray data provides details on the element composition of the particles. The information carried by each signal is complementary.
The backscattered electron image (BEI) is obtained from the probe electrons used to bombard the sample in the SEM. A proportion of these primary electrons will undergo a number of collisions within the sample to subsequently re-emerge from the sample surface. The number of backscattered electrons that re-emerge is controlled by the number of collisions which take place, which is in turn controlled by the (mean) atomic number of the element(s) composing the sample. The greater the atomic number of the sample contents, the greater the backscattered electron yield (heavier elements produce more backscattering events). Therefore, the image obtained from the collected backscattered electrons is an atomic number contrast image. Effectively, the BEI can be considered to reflect variations in density between the sample components.