Neutron Moderation

Neutrons produced in the spallation process, and with most other methods of production, have a large range of energies and therefore also wavelengths since these depend on each other. The typical energies of the neutrons when they leave the spallation target are in the order of MeV. The inter-atomic distances in materials are on the length scale of Angstrom (10^-10m). A neutron with a wavelength of 1 Angstrom has an energy of ~0.08 eV. This energy is 8 orders of magnitude lower than the produced neutrons, so we need to slow down the neutrons in order to use them to study materials.

To slow down neutrons we use what is called a moderator. In the moderator the neutrons collide with atoms that roughly have the same mass as a neutron, such as hydrogen. In such a collision the loses half its energy on average and it thus takes about 25-30 collisions for the neutrons to cool from energies in the MeV (mega electron volt) range to the ten meV (milli electron volt) range.

Illustration of how the neutrons can be slowed down by a moderator. The neutron has a high amount of energy before it collides with the moderator, which in this case is hydrogen. After colliding with hydrogen atoms, the neutron loses some of its energy and therefore gets a lower velocity

Figure. Neutron production and moderation.

At ESS there is both a thermal moderator which uses water (H₂O) to cool the neutrons to room temperature, equivalent to a neutron energy of about 0.26 eV, and a cold moderator that uses liquid hydrogen (H₂) with a temperature of 25 K(equivalent to a neutron energy of about 0.002 eV). Rather than emitting neutrons with a single energy, each moderator however sends out neutrons with a range of energies, and this spectrum is typically described by the intensity of neutrons as a function of the wavelength. Some neutron experiments will need the short wavelength (high energy) neutrons, others will need the long wavelength (low energy) neutrons,and some will use the entire spectrum.