ABSTRACT
As any moving particles, neutrons have a wave character (the “matter waves” first put forward by de Broglie around 1923). Because their mass is close to that of individual atoms, slow neutrons in thermodynamical equilibrium with a medium at ambient temperature (such as the moderator in a fission reactor), called “thermal” neutrons, have altogether a wavelength of the order of interatomic distances in condensed matter (∼0.1 nm), and kinetic energies close to atomic vibration energies (∼10−2 eV). They then give rise, like X-rays, to diffraction by crystalline solids; moreover, within the solid, they also can absorb or emit quanta of collective vibration energy (“phonons”), raising the possibility of spectroscopic studies (e.g. measurement of elastic wave dispersion curves) by inelastic neutron scattering. Both phenomena, diffraction and inelastic scattering, are observed by analysing the change of direction and speed of thermal neutrons “scattered” by a sample of solid or liquid matter.
