QUANTITATIVE-ANALYSIS OF KNOCK-ON AND THERMAL-DAMAGE OF BIOLOGICAL SPECIMENS BY ELECTRON-IRRADIATION IN TRANSMISSION ELECTRON-MICROSCOPY

High-energy electrons are able to transfer momentum to nuclei, which results in displacement on to the interstitial lattice sites with a maximum transferred energy of 4 . 10(4) eV for carbon at 100 keV. Moreover, most of the energy dissipated in energy losses is converted into heat, which results in melting and evaporation. The specimen temperature rise was calculated by the heat conduction theory and confirmed by the specimen drift due to the thermal damage. The damage can be reduced by a small area of illumination, the use of a metal-coated microgrid and small area scanning. A further displacement due to the knock-on collision and the resulting etching rate of biological specimens was measured. The damage is proportional to the current density in c cm-2 at the specimen. The allowable maximum dose was obtained from the measurement of an etching rate with the weight loss and dry density of the specimens. It was found that the images affected by the electron irradiation, in which -H, C-H, C-N bonds break molecules in proteinaceous biological specimens are removed, and the remaining molecules are changed to stable carbon-rich molecules by deposition, polymerization and contamination. In addition, defect images were observed in high contrast, when compared with unaffected images taken with a small area scanning method.