Editing Scanning electron microscope (section)

==Detection of backscattered electrons==
[[File:SEM SE vs BE Zr Al.png|thumb|Comparison of SEM techniques: <br />Top: backscattered electron analysis{{snd}} composition <br /> Bottom: secondary electron analysis{{snd}} topography]]

Backscattered electrons (BSE) consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms. Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image, BSEs are used to detect contrast between areas with different chemical compositions.<ref name="Goldstein-1981">{{cite book |title=Scanning electron microscopy and x-ray microanalysis |last=Goldstein |first=G. I. |author2=Newbury, D. E. |author3=Echlin, P. |author4=Joy, D. C. |author5=Fiori, C. |author6= Lifshin, E. |year=1981 |publisher=Plenum Press |location=New York |isbn=978-0-306-40768-0}}</ref> The Everhart–Thornley detector, which is normally positioned to one side of the specimen, is inefficient for the detection of backscattered electrons because few such electrons are emitted in the solid angle subtended by the detector, and because the positively biased detection grid has little ability to attract the higher energy BSE. Dedicated backscattered electron detectors are positioned above the sample in a "doughnut" type arrangement, concentric with the electron beam, maximizing the solid angle of collection. BSE detectors are usually either of scintillator or of semiconductor types. When all parts of the detector are used to collect electrons symmetrically about the beam, atomic number contrast is produced. However, strong topographic contrast is produced by collecting back-scattered electrons from one side above the specimen using an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality.

Backscattered electrons can also be used to form an [[electron backscatter diffraction]] (EBSD) image that can be used to determine the crystallographic structure of the specimen.