Neutron diffraction
A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of the structure of the material.
When a beam of neutrons emanating from a reactor is slowed and selected properly by their speed, their wavelength lies near one angstrom (0.1 nanometer), the typical separation between atoms in a solid material.
Impinging on a crystalline sample, it will scatter under a limited number of well-defined angles, according to the same Bragg's law that describes X-ray diffraction.
Diffractograms therefore can show strong, well-defined diffraction peaks even at high angles, particularly if the experiment is done at low temperatures.
[4] Magnetic scattering does require an atomic form factor as it is caused by the much larger electron cloud around the tiny nucleus.
Neutron diffraction can be used to determine the static structure factor of gases, liquids or amorphous solids.
Most experiments, however, aim at the structure of crystalline solids, making neutron diffraction an important tool of crystallography.
One practical application of elastic neutron scattering/diffraction is that the lattice constant of metals and other crystalline materials can be very accurately measured.
The high penetration depth permits measuring residual stresses in bulk components as crankshafts, pistons, rails, gears.
This technique has led to the development of dedicated stress diffractometers, such as the ENGIN-X instrument at the ISIS neutron source.
These experiments, first performed by Clifford Shull, were the first to show the existence of the antiferromagnetic arrangement of magnetic dipoles in a material structure.
Neutron diffraction can be used to establish the structure of low atomic number materials like proteins and surfactants much more easily with lower flux than at a synchrotron radiation source.
Nevertheless, by preparing samples with different isotope ratios, it is possible to vary the scattering contrast enough to highlight one element in an otherwise complicated structure.
Neutron was discovered around early 1930s, and diffraction was first observed in 1936[11] by two groups, von Halban and Preiswerk [12] and by Mitchell and Powers.
[13] In 1944, Ernest O. Wollan, with a background in X-ray scattering from his PhD work[14] under Arthur Compton, recognized the potential for applying thermal neutrons from the newly operational X-10 nuclear reactor to crystallography.
(The other half of the 1994 Nobel Prize for Physics went to Bert Brockhouse for development of the inelastic scattering technique at the Chalk River facility of AECL.
