7A60.10 Electron Diffraction

Concepts

Electron diffraction

Overview

Electron diffraction patterns for single crystal and polycrystalline materials are displayed on a CRT screen. Note: with a video camera, this demonstration is appropriate for large lecture halls.

Details

Equipment

  • [1] Electron diffraction tube
  • [1] Video camera
  • [1] Projector

Classroom Assembly

  1. Turn on the power supply
  2. Allow a short time (about 5 minutes) for the filament to warm up with the anode voltage at around 5kV.
  3. Once warmed up, turn up the voltage to 7kV.
  4. Scan the screen for crystals using the horizontal and vertical controls.
  5. Fine tune the beam with one control at a time and adjust the intensity and focus controls after you have located a crystal.

Important Notes

  • The room needs to be dark to see the pattern.
  • The electron diffraction tube is old. Handle with care. Turn down the high voltage when not in use.
  • 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.

Script

  1. Show the diffraction pattern consisting of a central bright spot of undeflected electrons and two concentric rings to the students using a video camera and a projector.

 

Additional Resources

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.

Disclaimer

  • Don't attempt this at home!

Last revised

  • 2018

Technicals

  • We use a Welch model 2639 electron diffraction tube.
  • 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.

Related AV

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.