How Spallation Works
Neutrons are generated in two ways: by means of nuclear fission in research reactors, and with a newer method referred to as spallation. Spallation sources allow investigations to be performed that are not possible with neutron reactors. They achieve a higher power density and a higher intensity of the neutron beam. This is because with spallation, six times less energy is used per neutron than for nuclear fission, thus also generating six times less waste heat.
In addition, spallation sources can create neutron pulses instead of a continuous beam. This type of pulse is more intensive, although the overall neutron capacity is the same. Numerous measurements rely on these types of neutron pulses. Another advantage of spallation is that the process is not based on a chain reaction. As soon as the facility is switched off, the spallation process also stops.
The heart of a spallation source consists of a proton accelerator and the target, in which the neutrons are released. The proton accelerator accelerates proton salvoes at nearly the speed of light and guides them to the target, the atomic nuclei of a heavy metal such as lead or mercury. While the impact itself directly releases only a few neutrons, it charges the atomic nuclei of a heavy metal with energy, so that it breaks apart and 20 to 30 neutrons are released per nucleus. Because the released neutrons, with a speed of 20,000 kilometres per second, are much too fast and contain too much energy for experiments, tanks with water or liquid hydrogen decelerate them to an ideal speed of several hundred to two thousand meters per second. Long tubes, the neutron guides, then guide the neutrons to the various experimental stations surrounding the neutron source.