Electron Diffraction Apparatus

an instrument used to investigate the atomic structure of solids and gas molecules by means of electron diffraction.

An electron diffraction apparatus is a vacuum device similar in design to an electron microscope. In a column, which is the main assembly of the apparatus, electrons emitted by a cathode—a glowing tungsten filament—are accelerated by a high voltage (20–1,000 kilovolts for fast electrons and up to 1 kilovolt for slow electrons). A narrow electron beam formed by apertures and magnetic lenses is directed into the object chamber onto a specimen, which is mounted on a special holder. The scattered electrons enter a camera, and a diffraction pattern (an electron diffraction picture) is produced on a photographic plate or screen; the image may be examined with the naked eye or with a microscope mounted in the camera. An electron diffraction apparatus may be fitted with various devices for heating, cooling, vaporizing, or deforming the specimen.

An electron diffraction apparatus also includes a vacuum system and an electric power supply unit; the latter contains a cathode heating source, a high-voltage source, and power supplies for the electromagnetic lenses and various devices in the object chamber. The supply system provides stepped changes in the accelerating voltage (for example, the model ER-100 electron diffraction apparatus has four voltage steps: 25,50,75, and 100 kilovolts). The resolving power of an electron diffraction apparatus reaches thousandths of an angstrom and depends on the electron energy, the size of the electron-beam cross section, and the distance from the specimen to the screen, which in modern designs may vary over a range of 200–600 mm. The design of an electron diffraction apparatus provides for a system for direct registration of the intensity of scattered electrons by means of a Faraday cylinder or an open-type secondary-electron multiplier.

In a device designed to investigate the diffraction of slow electrons, a vacuum of 10^–8–10^–9 mm Hg must be maintained in the column.