Neutron scattering cross section is the probability of a neutron being scattered from a target energy absorbed by a nucleus. In this article, the focus will be on the cross section of graphite, a common material used for neutron scattering experiments.
Graphite is a crystalline form of carbon and consists of layered hexagonal crystals arranged in a lattice-like array. Each layer of graphite consists of planes of sp2 hybridized carbon atoms joined together by strong σ bonds. It is a highly anisotropic material, meaning that properties like crystal structure and thermal conductivity vary depending on the direction of the crystalline lattice. Graphite is a lightweight, low density material with a high melting point, making it ideal for use in neutron scattering experiments.
Graphites neutrons scattering cross section is strongly dependent on the lattice orientation and crystal structure of the material. These different orientations make it a very versatile material, able to be tailored to the specific application required. In general, the cross section of graphite increases with increasing atomic number, as neutrons are more likely to be scattered by the higher number of atoms. The longitudinal and transverse scattering cross sections also increase when moving from the most common orientation, hexagonal, to cubic or rhombohedral.
At low energies, the cross section of graphite exhibits an energy dependence, with the peak cross section occurring at energies between 5 and 10 MeV. At higher energies, the cross section decreases slowly and the average cross section for graphite is relatively stable and does not exhibit the same energy dependence.
The thermal neutron scattering cross section of graphite is relatively small compared to other materials, such as beryllium or boron. This is due to the small number of nuclei in the lattice and their low atomic mass. However, graphite still has a very useful application in neutron scattering experiments as it is less expensive than many other materials, and its lower relative backscatter makes it easier to detect signals from low-energy neutron scattering.
The neutron scattering cross section of graphite is also used to measure the temperature and stress around a given material. This is done by measuring the degree of backscattering of neutrons from the different crystalline lattices of the material. This information can then be used to determine the stress and temperature levels in the material as well as the elastic properties of the material.
Graphites versatility contributes to its popularity as a material for use in neutron scattering experiments. Its ability to have different lattice orientations and its low thermal neutron scattering cross section make it a useful tool for researching materials properties. The peak energy scattering cross section, combined with its relatively stable cross section at higher energies makes it an ideal material for measuring various properties of materials at different energy levels.
