Beams of free neutrons are obtained by extracting neutrons from neutron sources.

The source can be considered as a volume filled with a very low pressure neutron gas (density up to 10²⁴ n/m³), which has an energy distribution corresponding to the temperature of the volume. By inserting a tube (called beam tube) into the volume one observes a current of neutrons flowing out through the tube (much like water flowing out from a tube which is stuck into a volume of wet sand). The current density (in n/m²s) depends on the solid angle formed by the aperture and the length of the tube, and also on the reflecting properties of the inner wall of the tube. Neutrons are indeed reflected at a wall, but usually at an extremely low rate, which is a function of the wall's nature and surface quality, the neutron energy, the angle of incidence.

The characteristics of neutron beams and reflection properties are outlined below.

The moving neutron has a kinetic energy E = m•v² (m = mass of the neutron, v its velocity). The SI unit for energy is the Joule (J), but for particles the energy unit of electronvolts (eV) is usually used (1 J = 6.2410¹⁸ eV). The energy can also be expressed in temperature (unit K) by the relation E = k•T (k = 1.3810^-23 J/K). Furthermore a moving particle can be considered a matter wave with the de Broglie wavelength λ = h/(3k•T•m)^1/2 (h = 6.610^-34 Js). The corresponding wavelength for the neutron is generally expressed in nanometers (nm) (or sometimes in Angstrom units, where 1 Å = 0.1 nm).

A "thermal" neutron, for instance, with a temperature of 300 K (corresponding to E = 25 meV) travels with a speed of 2200 m/s, and has a wavelength of λ = 0.17 nm.