Abstract
The aspect of high voltage electron microscopy which makes it inviting to solve both biological and physical problems is the ability of this technique to probe thicker specimens and at higher resolution than is currently possible with conventional electron microscopes. Naturally, there will be a thickness limit above which an electron beam will cease to be an effective means of obtaining useful microscopical data. The ultimate limit will be determined by the specimen induced chromatic aberration, resolution, contrast processes, and transmitted intensity available for image production.Recently Uyeda and Nonoyama have determined the maximum usable thickness of molybdenite for operating voltages up to 1.2 MeV. They showed that this thickness is approximately proportional to kβ2, where k is a small constant and β is the ratio of the velocity of an electron to that of light. Aluminum has also been observed to follow this β2 relationship. Even thicker specimens have been viewed with fair clarity, indicating that anomalous transmission effects can occur in crystalline material. These are thought to be due to directional dependence of the absorption coefficient. For aluminum, the thickness of specimens showing anomalous transmission would fall above curve A in Figure 1.
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