Neutron diffraction allows us to find out the structure of materials at an atomic level – ‘where atoms are’.
We can also use neutrons to examine what the atoms are doing.
In most materials the atoms do not move in a random way. They move in coherent, correlated ways because there are interactions and forces between them. It is these forces and interactions that give materials their properties.
At an atomic level, materials can be thought of as arrays of balls (atoms) held together by springs. If one of the atoms is moved the 'springs' (the inter-atomic forces) will pull it back into position. This will cause a wave to propagate through the array, like ripples across a pool. The wave is characterised by its wavelength and the energy required to create it.
In complicated materials, with several different types of atoms and different strengths of forces between them, waves of many different wavelengths and energies can be created.
To measure these waves we use the fact that a neutron passing though a material will interact with the atoms of the material and create a wave by transferring some of its energy to them.