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.

Variations in BEI contrast make it possible to identify and target individual particle components of specific interest. For example, lead-rich components in paint grains cannot be readily identified in the SEI.

However, because lead has a high atomic number, its presence can be readily isolated from the backscattered electron signal. In the accompanying BEI, where the contrast has been adjusted so that increasing brightness is a response to increasing mean atomic number composition, the lead-rich phases (arrowed) appear as bright regions in the image.

Similarly, in multi-component samples, like the non-combustible waste produced by coal burning power plants, the SEI may reveal only limited information.

Additional details which are not apparent in the SEI may be revealed in the BEI. The BEI can often resolve inter- and intra-particle variations in composition, differences in crystalline form, and variations in surface topography.