Transcript Poster - high school teachers at CERN

Electron Diffraction Experiment
Setup
Aim
The aim of this experiment is to determine the interplanar spacing of graphite from
the relationship between the radius of the diffraction rings and the wavelength by
using de Broglie equation and Bragg’s diffraction.
Introduction
Electrons have both wave and particle behaviours that will show up depending on
the type of the experiments, i.e., experiments measuring particle properties will not
give the wave behaviour and vice versa.
According to de Broglie equation, fast electrons have high momentum and hence a
wavelength comparable to the spacing between layers in a crystal. These electron
waves can undergo Bragg’s reflection and interfere. Making use of the wave
behaviour of electrons, fast electrons are diffracted from a polycrystalline layer of
graphite and interference rings appear on a fluorescent screen. The interplanar
spacing in graphite is determined from the diameter of the rings and the
accelerating voltage.
Measurements
Results
Once the equipment is setup properly, an electron diffraction rings pattern
appears.
Ring radii vs electron wavelength
0.025
0.023
0.021
0.019
r/
0.017
0.015
0.013
0.011
0.009
0.007
0.005
1E-11
1.2E-11
1.4E-11
1.6E-11
/m
1.8E-11
2E-11
The margin of error may be large due to
the fact that the diameter measurement
is difficult and not very accurate.
Teacher guide
The diameters of the first and second rings are measured for different
accelerating potential.
A graph of the radius of the rings against the wavelength of the electron wave is
plotted and the gradient is used to find the interplanar spacing between graphite
layers.
This experiment involves two important concepts in modern physics – de Broglie
relationship and Bragg’s reflection. It may be used to demonstrate wave-particle
complementary and application of Bragg’s reflection
As the applied voltage increases, there are additional rings to be seen. They
correspond to either higher order or other lattice plane spacing. Teachers may
choose to elaborate on Bragg’s conditions.