Neutrons are very useful to material probes and can be created in reactors or by accelerators. The most intense accelerator-based neutron sources are called spallation sources and use a process that sends high-energy ions into a heavy metal target. They are currently the brightest neutron sources in the world.
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Spallation sources
Due to several intrinsic characteristics, neutrons are fundamental for the probing of materials:
- Neutrons are neutral particles and are scattered from the nuclei of atoms. This makes neutrons far more penetrating probes than ion beams or X-rays. One of the applications of this penetration is the 3D mapping of stresses deep inside large engineering objects, such as engine blocks.
- Neutrons also have a nuclear spin, making them very sensitive to the location and orientation of magnetic moments in materials and the probe of choice for many studies of magnetism.
- The mass of the neutron means it can transfer momentum and energy to a sample, making it very useful for the study of fundamental vibrations within materials, which gives information on the strengths of chemical bonds and magnetic interactions.
Neutrons can be created by nuclear reactions in reactors and accelerators. A spallation source consists of a high-powered accelerator that brings protons with energies of greater than 0.5 GeV (gigaelectronvolt) into a heavy metal target, such as mercury or tungsten. These metals “spall off” free neutrons in response to the impact. They are the world's brightest source of neutrons for research. As accelerators can be readily pulsed, spallation sources are generally pulsed neutron sources, unlike most reactors that generate neutrons constantly.
Where appropriate, IAEA supports countries in the development of neutron sources (research reactors or accelerators) and provides a coordinating role in technology development. One example is a project on advanced cold moderator design. The fast neutrons created in the initial spallation or fission reaction carry too much energy for many materials studies and so their energy must be reduced before they are directed to a sample. Moderators are typically liquid water (a thermal source) or a refrigerated liquid or solid that is rich in hydrogen (a cold source). The IAEA is running a coordinated research project on advanced cold moderator designs for both reactors and spallation sources, with the aim of improving their design and performance worldwide.
An upcoming application of spallation sources is in future hybrid reactors. This would use the unmoderated spallation neutrons to burn nuclear waste or breed nuclear fuel. Several projects around the world are being developed in this area.
A global list of spallation sources can be found on the IAEA's Accelerator Knowledge Portal.