Contrast Variation

It is very fortunate that hydrogen and deuterium nuclei scatter neutrons very differently. Hydrogen containing compounds scatter neutrons strongly, but the scattered neutrons reflect little structural information, and are relatively uniformly scattered. Deuterium compounds, on the other hand, scatter less, but most of the scattered neutrons reflect the structure and are scattered quite non-uniformly. Thus, if we subtract a constant background from the scattering pattern of a system containing regions of different chemical composition then we can change which components are highlighted in the neutron scattering and selectively pick out different structures. As an example consider an emulsion of water droplets in oil, where the droplets are stabilised by a layer of surfactant on the outside. As illustrated below, if we only have protonated surfactant, a common case, we can get three useful contrast variations – the whole droplet highlighted, surfactant only highlighted, and water core highlighted.

For the more technical reader only can rephrase the paragraph above by saying that the proton has a very high incoherent scattering length and a smallish but negative coherent scattering  length. The deuteron has a small incoherent scattering length and a large positive coherent scattering length. Thus the scattering from hydrogen compounds is strong, but highly dominated by the incoherent signal which reflects single particle proton motion only. By contrast the smaller scattering from deuterated compounds is dominated by coherent scattering which arise from two particle correlations, and thus contains detailed structural information.