Secondary electron detector (SED) – Everhart-Thornley Detector
Due to the low energies of secondary electrons (SE) (~2 to 50 eV) they are ejected only from near-surface layers. Therefore, secondary electron imaging (SEI) is ideal for recording topographical information. To attract (collect) these low-energy electrons, a small bias (often +/- ve selectable but usually around +200 to 300V) is applied to the cage at the front end of the detector to attract the negative electrons towards the detector. [If the cage is negatively biased it functions as a BS detector]. A higher kV (e.g. 7 to 12kV) is applied inside the cage i.e. to the scintillator, to accelerate the electrons into the scintillator screen.
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Backscattered electron detector (BSD) – solid state diode detector
The BSD is mounted below the objective lens pole piece and centered around the optic axis. As the specimen surface is scanned by the incident electron beam, backscattered electrons (BSE) are generated, the yield of which is controlled by the topographical, physical and chemical characteristics of the sample. Both compositional or topographical backscattered electron images (BEI) can be recorded depending on the window of electron energies selected for image formation.
See Principles of operation – Electromagnetic lenses, apertures and beam size for more on SEM theory.