The Role of Temperature in Limiting Radiation Damage to Organic Materials in Electron Microscopes

Abstract

The intensity of the electron beam in an electron microscope is at once the basis for progress as well as the ultimate limitation in electron microscopy of organic materials. Gabor noted that the highest intensity available for light optics comes from sunlight, which produces an energy density of 2,000 watts/cm2-steradian. The electron sources in early microscopes could produce a million times that amount, and modern sources even more. While the high intensity made good images possible (because numerical apertures used for electron microscopes are less than 1% of the size used in light microscopy) early microscopists feared that such a high flux of charged particles would destroy most specimens, especially organic ones. Although it was soon found that biological specimens could survive observation by electron microscopy, the introduction of double-condenser illumination systems revealed the problem of specimen contamination. In time it became clear that radiation damage was more fundamental than the gross increases or decreases in specimen mass observed in contamination and etching.