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| The elastic narrow-angle scattering on the atoms defines the scatter absorption contrast which is used, for example, to image cell structures in biological specimens In crystalline materials, the elastic Bragg scattering permits structures and defects to be analysed down to atomic level. With inelastic interaction, primary electrons change their energy. Due to chromatic aberration, the image contrast deteriorates. Element specific information can only be obtained if use is made of the inelastic scattering processes. |
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| | Electron scattering in specimen
During elastic scattering at the specimen atoms, primary electrons with E0 energy are scattered at large angles Q without losing any measurable energy.
Inelastically scattered electrons only scatter at small angles, but lose energy and thus change their wavelength.
Elastically scattered electrons remain monoenergetic and display an energy bandwidth dE which is largely defined by the cathode. The bandwidth of inelastically scattered electrons, on the other hand, increases dramatically with greater mass and specimen thickness.
EFTEM contrast generation method
In EFTEM, the transmitted electrons are subjected to an additional energy selection after the angle selection.
In the OMEGA filter the electrons are separated according to their energy (wavelength). The OMEGA filter acts as a spectrometer. The slit is used to select energy and energy bandwidth.
This additional electron selection results in contrast enhancement for all imaging modes and also provides the possibility of selecting electrons with specific scattering effects for imaging, thereby generating object or element specific contrasts.
Contrary to the CTEM method, the image is formed exclusively by a “monoenergetic” electron beam*.
CTEM contrast generation method
The specimen is exposed to monoenergetic electron radiation. As a result of the scattering in the specimen, a polyenergetic electron beam is produced.
In CTEM, electrons are selected from the electron beam via the lens aperture diaphragm according to their scatter angles. This angular selection results in scatter or diffraction contrast.
The image is formed by all electrons passing the lens aperture diaphragm, in other words with a polyenergetic electron beam. |