Diffraction

We generally use a large crystal monochromator to select a particular neutron wavelength, just as the different wavelengths of light can be separated using a prism or fine grating. The material to be studied is placed in this monochromatic neutron beam, and the scattered neutrons are collected on a large 2D detector. The sample can be a liquid, a bunch of fibres, a crystal or a polycrystal.
A polycrystal is the usual form of solid matter, such as a lump of metal or ceramic, and is made up of millions of tiny crystals.

To understand how neutron diffraction works, imagine how light is diffracted by a regular grating or grid. Scattering from the different lines of the grid interferes to give diffraction ‘spots’ with spacing inversely proportional to the spacing of the lines. X-ray and neutron diffraction work the same way, but the grid is now the array of atoms in the material. By measuring the intensities and positions of the scattered X-ray or neutron spots,we can deduce the atomic structure.

Neutron diffraction experiments at ILL are thus really quite simple, and available to a wide variety of users – materials scientists, chemists, physicists and biologists.The simplest is called ‘powder diffraction’, when a polycrystalline lump of material, often ground to a fine powder, is placed in the beam. Neutrons are scattered at specific angles, corresponding to the spacing between atomic planes, and by measuring these angles and intensities the atomic structure of the material can be deduced. If instead of a crystalline powder an amorphous or liquid sample is used, there are only broad peaks at specific angles corresponding to average interatomic distances.
To obtain more data, short neutron wavelengths are used, and sometimes one type of atom is replaced by its isotope – chemically identical, but with a different nucleus and different neutron-scattering power – this difference then gives information specific to that atom.

So-called inelastic scattering is a little more complicated. Here the change in neutron energy is measured as well, and this gives information about the energies of vibrations and other excitations in the sample.

Strain scanning

Because neutron diffraction determines interatomic distances, it can be used to monitor minute changes in these distances caused by the deforming of a material.
Furtermore, since neutrons can penetrate deep into matter they can map stresses in bulky objects such as engine components. The ILL has a strain-imager - SALSA - devoted to such studies.

Monitoring the changes in the structure of a metal or an alloy over time provides information about their durability. These methods are also applied to archeological artefacts, supplemented by new imaging (neutron radiograpgy and tomography) techniques.