Description: Electron diffraction patterns for single crystal and polycrystalline materials are displayed on a CRT screen.

A Welch model 2639 'Electron Diffraction tube' is used in an experiment in the second year lab. The tube has graphite and aluminum foils mounted on a grid between the electron gun and the screen of the tube. The path of the electron beam may be varied so that the beam passes through one of the foils or misses both. The aluminum is polycrystalline so that the diffraction pattern consists of rings. The graphite consists of small crystallites with the c-axis perpendicular to the surface of the foil so that the pattern consists of an array of spots with six-fold symmetry.
The accelerating voltage may be varied between 0 and 10 kV. The change in the spacing of the diffraction features with a change in voltage may be easily observed. It is necessary to adjust the position of the electron beam as the voltage is varied.

It is worthwhile to contrast the diffraction of electrons with the diffraction of light by a two dimensional grid. It is also worthwhile to take along the model of the graphite crystal structure.

Cautions: The instrument is getting old and fragile. Do not bump it. Please turn down the high voltage when not in use.

Concepts demonstrated: Electron diffraction

Setup Time: Short

Difficulty/Commitment: Straightforward

Visibility: Use a video camera with a monitor or video projector. The room needs to be dark to see the pattern.

Related demonstrations:

References: PIRA #7A60.10
A different tube is used to demonstrate electron diffraction in a polycrystalline material in Video Encyclopedia 24-23.
The manual for the Welch 2639 is on file in the second year lab. The early development of the tube is described in a short note in AJP 30,549(196?).A photograph of a diffraction pattern obtained with this apparatus appears in "Quantum Mechanics" by Leslie Ballentine.