Operational Features of a New Electron Diffraction Unit

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Abstract
A new electron diffraction unit having a three‐lens optical system and a maximum effective specimen‐to‐plate distance of 200 cm is described. The applications of such a system to the study of long interatomic spacings is discussed. Wide variations in the area of specimen irradiated are obtainable. An explanation is given of the design considerations in such an optical system, with special reference to the functioning of the post‐specimen lens as both focusing and magnifying lens. The use of the unit as a shadow microscope is explained, and the advantages are given of this instrument over other shadow microscopes with respect to increased size of image, increased size of specimen field available for observation, and greater specimen protection from damage by the concentrated electron beam. The resolution attainable with the instrument is calculated and a ``resolution index'' is defined which makes possible intercomparisons of resolution under various operating conditions. Factors which limit accuracy, as opposed to resolution, are discussed critically and the accuracy of data obtainable with the instrument is estimated. A number of diffraction patterns are shown to illustrate the results obtained under various operating conditions as diffraction camera and shadow microscope, and also to illustrate the techniques of charge neutralization, spot diameter measurement, and internal calibration of plates using a known specimen.

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