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Quick reference guide
Introduction
The electron diffraction tube consists of an electron gun that accelerates electrons towards a graphite foil. In contrast to the cathode ray tube and the fine beam tube a much higher voltage is used, why the wave behaviour of the particles outcrop: the electrons are diffracted at the inner structure of the graphite.
Functional principle
- The source of the electron beam is the electron gun, which produces a stream of electrons through thermionic emission at the heated cathode and focuses it into a thin beam by the control grid (or “Wehnelt cylinder”).
- A strong electric field (10 kV!) between cathode and anode accelerates the electrons, before they leave the electron gun through the spaces of the anode-grid.
- Afterwards the accelerated electrons hit a thin graphite foil. The electrons are diffracted there so they fly towards the screen in different directions.
- When electrons strike the fluorescent screen, light is emitted so that the diffraction picture gets visible.
- The whole configuration is placed in a vacuum tube to avoid collisions between electrons and gas molecules of the air, which would attenuate the beam.
U A
Cathode
Control grid Anode
Flourescent screen
Graphite foil
Safety precautions
- Don’t touch fine beam tube and cables during operation, high voltages of 10 kV are used in this experiment!
- Do not exert mechanical force on the tube, danger of implosions!
- The bright spot just in the center of the screen can damage the fluorescent layer of the tube. To avoid this reduce the light intensity after each reading as soon as possible.
Experimental procedure
- Set the voltage of G1 to -50 V using the second knob on the dc power supply (or whatever value the beam is bright enough).
- Set the voltage of G4 to about 0 V using the third knob on the dc power supply (or whatever value that make the beam is sharp enough).
- Slowly increase the high voltage supply until the ring structure appears on the fluorescent layer on the electron diffraction tube. The visibility of high order rings depends on the light intensity in the laboratory and the contrast of the ring system which can be influenced by the voltages applied to G and G4. Rings should appear when the voltage is about 4 kV.
Electrons behaving like particles would not case a diffraction picture when passing matter like the graphite foil. Since a diffraction picture gets visible, there is diffraction – electrons behave like waves.
After Einstein introduced the duality of wave and particle behaviour of light first in 1905, deBroglie proposed 1924 that not only light has both wave and particle behaviour: matter, so far seen as consisting of particles, should behave like waves as well, which can be verified with this experiment in the electron diffraction tube
- Measure the radii r 1 and r 2 of the first and the second ring using the vernier callipers for the acceleration voltages U = 5 kV and U = 10 kV!
r (^1) r (^2)
Particle physics: scattering experiments
Inside the electron diffraction tube, electrons were accelerated and shot onto a graphite foil. This procedure is one of the basic principles used in particle physics experiments. Such scattering experiments resulted in the discovery of the atomic nucleus (Rutherford 1908) and the quark structure of hadrons (Friedman, Kendall, Taylor 1962).
To analyse the inner structure of matter, the bullet particles have to be as small as possible compared to the analysed structure. Light with a wavelength of λ=500 nm e.g. is not appropriate to analyse the planes of a graphite foil whose distance is only d = 213pm. That’s why light-optical microscopes are not useful to analyse the planes of graphite because the wavelength is much bigger than the structure. The deBroglie wavelength of 10 kV electrons is about 12 pm (see above) so they allow the study of the inner structure of graphite.
In summary bullet particles for scattering experiments have to be very small to get a good resolution. Since deBroglie wavelength λ= h/p is inversely proportional to the impulse p, scattering experiments need strong particle accelerators. This is the reason why the electron diffraction tube needs a high voltage of at least 10 kV.
Well-known scattering experiments
Year Experiment Bullet particles Cognitions
1908 Rutherford α-particles Discovery of the atomic nucleus
1956 Hofstadter Electrons Size of a proton
1962 Friedman, Kendall, Taylor Electrons Proof of the existence of quarks
1992 HERA Electrons, muons, neutrinos Structure of protons
Proton
Electron
Scattering experiment to prove the existence of quarks
