Neutron diffraction

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Neutron diffraction or elastic neutron scattering is the application of

X-ray diffraction but due to their different scattering properties, neutrons and X-rays provide complementary information: X-Rays are suited for superficial analysis, strong x-rays from synchrotron radiation are suited for shallow depths or thin specimens, while neutrons having high penetration depth are suited for bulk samples.[1]

Instrumental and sample requirements

The technique requires a source of neutrons. Neutrons are usually produced in a

spallation source. At a research reactor, other components are needed, including a crystal monochromator (in the case of thermal neutrons), as well as filters to select the desired neutron wavelength. Some parts of the setup may also be movable. For the long-wavelength neutrons, crystals cannot be used and gratings are used instead as diffractive optical components.[2]
At a spallation source, the time of flight technique is used to sort the energies of the incident neutrons (higher energy neutrons are faster), so no monochromator is needed, but rather a series of aperture elements synchronized to filter neutron pulses with the desired wavelength.

The technique is most commonly performed as powder diffraction, which only requires a polycrystalline powder. Single crystal work is also possible, but the crystals must be much larger than those that are used in single-crystal X-ray crystallography. It is common to use crystals that are about 1 mm3.[3]

The technique also requires a device that can detect the neutrons after they have been scattered.

Summarizing, the main disadvantage to neutron diffraction is the requirement for a nuclear reactor. For single crystal work, the technique requires relatively large crystals, which are usually challenging to grow. The advantages to the technique are many - sensitivity to light atoms, ability to distinguish isotopes, absence of radiation damage,[3] as well as a penetration depth of several cm[1]

Nuclear scattering

Like all

Bragg's law
that describes X-ray diffraction.

Neutrons and X-rays interact with matter differently. X-rays interact primarily with the

atomic number (Z). On the other hand, neutrons interact directly with the nucleus of the atom, and the contribution to the diffracted intensity depends on each isotope; for example, regular hydrogen and deuterium contribute differently. It is also often the case that light (low Z) atoms contribute strongly to the diffracted intensity, even in the presence of large Z atoms. The scattering length varies from isotope to isotope rather than linearly with the atomic number. An element like vanadium
strongly scatters X-rays, but its nuclei hardly scatters neutrons, which is why it is often used as a container material. Non-magnetic neutron diffraction is directly sensitive to the positions of the nuclei of the atoms.

The nuclei of atoms, from which neutrons scatter, are tiny. Furthermore, there is no need for an

Fourier maps (and to a lesser extent difference Fourier maps
) derived from neutron data suffer from series termination errors, sometimes so much that the results are meaningless.

Magnetic scattering

Although neutrons are uncharged, they carry a magnetic moment, and therefore interact with magnetic moments, including those arising from the electron cloud around an atom. Neutron diffraction can therefore reveal the microscopic magnetic structure of a material.[4]

Magnetic scattering does require an atomic form factor as it is caused by the much larger electron cloud around the tiny nucleus. The intensity of the magnetic contribution to the diffraction peaks will therefore decrease towards higher angles.

Uses

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
.

Neutron diffraction is closely related to X-ray powder diffraction.[5] In fact, the single crystal version of the technique is less commonly used because currently available neutron sources require relatively large samples and large single crystals are hard or impossible to come by for most materials. Future developments, however, may well change this picture. Because the data is typically a 1D powder diffractogram they are usually processed using Rietveld refinement. In fact the latter found its origin in neutron diffraction (at Petten in the Netherlands) and was later extended for use in X-ray diffraction.

One practical application of elastic neutron scattering/diffraction is that the

ISIS neutron source
.

Neutron diffraction can also be employed to give insight into the 3D structure any material that diffracts.[6][7]

Another use is for the determination of the solvation number of ion pairs in electrolytes solutions.

The magnetic scattering effect has been used since the establishment of the neutron diffraction technique to quantify magnetic moments in materials, and study the magnetic dipole orientation and structure. One of the earliest applications of neutron diffraction was in the study of magnetic dipole orientations in antiferromagnetic transition metal oxides such as manganese, iron, nickel, and cobalt oxides. 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.[8] Now, neutron diffraction continues to be used to characterize newly developed magnetic materials.

Hydrogen, null-scattering and contrast variation

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. This is because some low atomic number materials have a higher cross section for neutron interaction than higher atomic weight materials.

One major advantage of neutron diffraction over X-ray diffraction is that the latter is rather insensitive to the presence of

metal hydride complexes, e.g., Mg2FeH6 have been assessed by neutron diffraction.[9]

The neutron scattering lengths bH = −3.7406(11) fm [10] and bD = 6.671(4) fm,[10] for H and D respectively, have opposite sign, which allows the technique to distinguish them. In fact there is a particular isotope ratio for which the contribution of the element would cancel, this is called null-scattering.

It is undesirable to work with the relatively high concentration of H in a sample. The scattering intensity by H-nuclei has a large inelastic component, which creates a large continuous background that is more or less independent of scattering angle. The elastic pattern typically consists of sharp

Bragg reflections
if the sample is crystalline. They tend to drown in the inelastic background. This is even more serious when the technique is used for the study of liquid structure. 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. The variation of other elements is possible but usually rather expensive. Hydrogen is inexpensive and particularly interesting, because it plays an exceptionally large role in biochemical structures and is difficult to study structurally in other ways.

History

The first neutron diffraction experiments were carried out in 1945 by

Peyton Rous
and his award of the Nobel Prize in 1966.

See also

References

  1. ^
    IAEA
    , 2003
  2. S2CID 249056691
    .
  3. ^
    doi:10.1080/02603590701394741
  4. ISBN 0-306-11030-X
  5. ISBN 0-19-850091-2
  6. doi:10.1016/j.diamond.2006.09.019
  7. doi:10.1021/cm100440p
  8. ISSN 0031-899X
    .
  9. doi:10.1016/S0020-1693(97)89125-6
  10. ^
    doi:10.1080/10448639208218770
  11. ISSN 0034-6861
    .

Further reading

External links
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