12/4/08
Working of a typical TEM
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A light source located at the top of the microscope emits electrons that travel through a vacuum in the column of the microscope. Electromagnetic lenses focus the electrons into a very thin beam which travels through the specimen. Depending on the density of the material present, some of the electrons are scattered and disappear from the beam. At the bottom of the microscope the unscattered electrons reach a fluorescent screen, which gives rise to a shadow image of the specimen with its different parts displayed in varied darkness according to its density. The image can be studied directly by the operator or photographed with a camera.
The detailed operation is as fallows:
• An electron gun located at the top produces a stream of monochromatic electrons.The detailed operation is as fallows:
• This stream is focused to a small, thin, coherent beam by the use of two condenser lenses. The first one determines the spot size; the general size range of the final spot that strikes the sample. The second lens changes the size of the spot on the sample; changing it from a wide dispersed spot to a pinpoint beam.
• The beam is restricted by the condenser aperture, knocking out high angle electrons.
• The beam strikes the specimen and parts of it are transmitted and focused by the objective lens into an image.
• Optional objective and selected area metal apertures can restrict the beam; the objective aperture enhancing contrast by blocking out high-angle diffracted electrons and the selected area aperture enabling the user to examine the periodic diffraction of electrons by ordered arrangements of atoms in the sample.
• The image is passed down the column through the intermediate and projector lenses and enlarged.
• The image strikes the phosphor image screen and light is generated, allowing the user to see the image.
• The darker areas of the image represent those areas of the sample that fewer electrons were transmitted through because they are thicker or denser.
• The lighter areas of the image represent those areas of the sample that more electrons were transmitted through because they are thinner or less dense.
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