Neutron diffraction is a crystallography technique that uses neutrons to determine the atomic structure of a material.
Neutrons are particles found in the atomic nucleus. In a nuclear reactor, neutrons can be set free when nuclei decay (fission, radioactivity). All quantum particles can exhibit wave phenomena we typically associate with light or sound. Diffraction is one of these phenomena; it occurs when waves encounter obstacles whose size is comparable with the wavelength. If the wavelength of a quantum particle is short enough, atoms or their nuclei can serve as diffraction obstacles. When neutrons from a reactor are slowed down and selected properly, their wavelength lies near one angstrom, the typical separation between atoms in a solid material.
A neutron diffraction measurement requires a neutron source (e.g. a nuclear reactor), a target (the material to be studied), and a detector. Other components may be needed to select the desired neutron wavelength. Some parts of the setup may also be movable.
Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray and electron diffraction). The neutrons will only interact with the nucleus of the atom. Thus neutron diffracton reveals the atomic structure only, and not charge distribution around the atom (although the two are usually very similar).
Neutron diffraction then reveals structural details of the target material, which are measured by recording the way in which neutrons are deflected. Neutrons can also change their speed during the scattering experiment; this is used to study the types of vibrations that can occur in the solid. An important difference between neutron and X-ray diffraction is that neutrons are sensitive to magnetic forces in the material.
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