How to scanning electron microscopes and transmission electron microscope work?
- Friday Nov 6,2009 08:34 PM
- By diddy
- In Others
i’m studying them in my science class, however the textbook is out of date and all the sites i’ve looked up give me various, different answers.
does anyone know a not too complicated way to explain how they work?
Microscope Work, Scanning Electron Microscopes, Science Class, Textbook, Transmission Electron Microscope
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The old SEM i used in the 80’s, a bulb which high currents passes through, providing a high energy x-ray source. the chamber is vacuum of all air, and supercooled with liquid nitrogen. the x-rays pass through a series of magnetic rings to narrow the beam, which is rasped, (moved back and forth like the old TV’s CRT), of a specimen. the specimen must be made non-conductive, by sputter coating it with graphite or other materials. A SEM, gives us a good visual of the surface of a specimen, and when the specimen is hit with narrow focused electron beam between 100 and 300 thousand volts, it gives off secondary x rays, which are picked up by crystal detectors (EDXRA). these detectors can give counts of secondary x-ray wavelengths specific to elements. different type detectors can pick up different elements, because some wavelengths overlap. advantage, SEM can take a large specimen, sample may only may need little preparation,
without the vacuum, air molecules bounce the beam, and if too much air, it can have a arch of electrons like a bolt of lighting.
http://en.wikipedia.org/wiki/Scanning_electron_microscope
a TEM has much higher magnification, is designed to look through very thin specimens, to get not surface as a SEM does, but to look inside a cell for example, , and different type of analysis can be done, we used crystal diffraction, see crystal structure at atomic level on asbestos fibers. the advantage, very high magnification, can see inside specimen in 3-D, the drawback, it takes long time to get a sample to the level, thin enough, it can be looked at in a TEM
http://en.wikipedia.org/wiki/Transmission_electron_microscopy
Hope this helps.
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