Editing Neutron diffraction (section)

===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 [[hydrogen]] (H) in a structure, whereas the nuclei <sup>1</sup>H and <sup>2</sup>H (i.e. [[Deuterium]], D) are strong scatterers for neutrons. The greater scattering power of protons and deuterons means that the position of hydrogen in a crystal and its thermal motions can be determined with greater precision by neutron diffraction.  The structures of [[metal hydride complex]]es, e.g., [[Magnesium iron hexahydride|Mg<sub>2</sub>FeH<sub>6</sub>]] have been assessed by neutron diffraction.<ref>Robert Bau, Mary H. Drabnis "Structures of transition metal hydrides determined by neutron diffraction" Inorganica Chimica Acta 1997, vol. 259, pp/ 27-50. {{doi|10.1016/S0020-1693(97)89125-6}}</ref>

The neutron scattering lengths ''b''<sub>H</sub> = −3.7406(11) fm <ref name="Sears">{{citation|author=Sears, V. F.|title=Neutron scattering lengths and cross sections|journal=Neutron News|date=1992|volume=3|issue=3|pages=26–37|doi=10.1080/10448639208218770}}</ref> and ''b''<sub>D</sub> = 6.671(4) fm,<ref name="Sears" /> 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.