Scanning Electron Microscopy


Structure investigations of organic, in particular biological, material are frequently performed with a strong electron beam. If the dose is higher than le/Å2, as required e.g. for high resolution electron microscopy, the results are strongly influenced by radiation damage. There are no means for preventing breaking of chemical bonds and ionizing of atoms and fractures of molecules due to the electron impact. The secondary processes, however, such as diffusion or evaporation of the fragments, can be strongly reduced by cooling the specimen to 4 K (cryoprotection). A suitable instrument for experimenting with cryoprotection is a microscope equipped with a superconducting lens system.

Topics relevant for cryomicroscopy are: instrumentation; determination of cryoprotection factors of various materials by electron diffraction; direct imaging in particular for information on the steric structure of the material; preparation conditions for an effective cryoprotection.

Though the knowledge of the physics and chemistry causing radiation damage at 4 K is still limited, a useful application of cryoprotection is already possible.

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