Technique
During a SANS experiment a beam of neutrons is directed at a sample, which can be an aqueous solution, a solid, a powder, or a crystal. The neutrons are elastically scattered by nuclear interaction with the nuclei or interaction with magnetic momentum of unpaired electrons. In X-ray scattering, photons interact with electrical cloud so the bigger element the bigger effect but in neutron scattering, neutron interacts with nuclei and interaction depends on isotope and some light elements like deuterium show similar scattering cross section as heavy elements like Pb.
In zero order dynamical theory of diffraction the refractive index is directly related to the scattering length density and is a measure of the strength of the interaction of a neutron wave with a given nucleus. The following table shows the scattering lengths for various elements (in 10−12 cm).
| H | D | C | N | O | P | S |
|---|---|---|---|---|---|---|
| -0.3742 | 0.6671 | 0.6651 | 0.940 | 0.5804 | 0.517 | 0.2847 |
Note that the relative scale of the scattering lengths is the same. Another important point is that the scattering from hydrogen is distinct from that of deuterium. Also, hydrogen is one of the few elements that has a negative scatter, which means that neutrons deflected from hydrogen are 180° out of phase relative to those deflected by the other elements. These features are important for the technique of contrast variation (see below).
Read more about this topic: Small-angle Neutron Scattering
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