Electron diffraction tube
Practical Activity for 14-16
This shows that an electron beam is diffracted when it passes through graphite, suggesting that electrons have a wave-like character.
Apparatus and Materials
- Electron diffraction tube and stand
- EHT supply, 0-6 kV variable
- Connecting wires, including some with shrouded terminals
- Bar magnet
Health & Safety and Technical Notes
Ensure that the high voltage anode circuit incorporates a protective resistor.
Some EHT supplies have this built-in so that it is in series with the high voltage terminal; others may require you to explicitly wire it in to the circuit.
Using leads with shrouded plugs will ensure that you cannot accidentally come into contact with the high voltage.
Watching the voltmeter which indicates the voltage across the output of the EHT supply will also allow you to judge when the voltage has dropped to a safe level at the end of the demonstration.
Notes on the equipment:
- The electron diffraction tube is an evacuated glass tube and is therefore fragile and should be handled with care. It is best not to move the tube while the filament is hot. Avoid switching the filament heater on and off unnecessarily.
- Check that your EHT supply includes a protective high-resistance resistor.
- It helps to used connecting wires of different colours for the two circuits, e.g. black wires for the filament circuit, red for the high voltage anode circuit and green for the earth connections.
This demonstration shows how a beam of electrons is diffracted as it passes through a graphite film. The film below shows how to set up the diffraction tube so that it can be used safely.
- The demonstration uses a 6 kV supply; teachers may feel that this is daunting and potentially dangerous. However, the film shows that, if you understand the circuits involved, there is no significant danger.
- The wiring may seem complex. There are in fact two simple series circuits: a low voltage circuit to the heated filament and a high voltage circuit to provide the accelerating potential between the cathode and the anode.
- The diffraction pattern is likely to be faint so you may wish to set the demonstration up in a darkened room, or in a separate darkroom.
- Why do we see a diffraction pattern of rings? This is because the graphite is polycrystalline. The graphite film has regions in which the planes of carbon atoms are in different orientations. The electron beam is quite broad and so it passes through many different regions of the graphite. (If the graphite were a single crystal, a diffraction pattern of discrete spots would be formed.)
- Why do the rings shrink as the accelerating voltage is increased? The electron beam has a wave character – this is why the beam is diffracted. The wavelength of the beam is shorter than the distance between the atomic planes of the graphite. If the accelerating voltage is increased, the electrons have greater energy and hence shorter wavelength ( E = hc / l ). With a shorter wavelength, the waves are diffracted less and so the diameters of the diffraction rings decrease.
- Knowing the accelerating voltage and the diameter of a diffraction ring, it would be possible to estimate the atomic spacing in the graphite.
- Why is the electron beam deflected by a magnetic field? This is an example of the motor effect. The electron beam represents an electric current (in the opposite direction to the beam, since the electron charge is negative). Fleming’s left-hand rule will give the direction of the force on the electrons. You could use Helmholtz coils to produce a uniform and calculable field in the evacuated tube.
- Once you have mastered this type of electron beam tube, you should feel confident to go on and use others such as the deflection tube and the e / m tube.