When beams of neutrons are used to probe small samples of materials they have the power to reveal what cannot be seen using other types of radiation. Neutrons appear to behave either as particles or as waves or as microscopic magnetic dipoles. It is these specific properties which enable them to yield information which is often impossible to obtain using other techniques.
Why use neutrons?
Neutron wavelengths range from 0.1 Å to 1000 Å, making them an ideal probe of atomic and molecular structures, atomic and molecular structures, both single atomic species or complex biopolymers.
Neutron energies are of the same magnitude as the diffusive motion in solids and liquids, the coherent waves in single crystals (phonons and magnons), and the vibrational modes in molecules. Any exchange of energy of the order of 1 μeV (even 1 neV with spin-echo) to 1 eV between a sample and an incoming neutron can be detected.
As neutrons are electrically neutral, they can penetrate deep into matter without doing damage. This makes them an ideal probe for biological materials or engineering components under extreme conditions of pressure, temperature, magnetic field or within chemical reaction vessels.
Sensitivity and selectivity
In contrast to X-rays, the neutron scattering power varies from one nucleus to another in a quasi-random manner. This means that light atoms (like Hydrogen) are visible despite the presence of heavier atoms, and neighbouring atoms may be distinguished from each other. This makes it possible to use isotopic substitution in order to vary the contrast in certain samples and thus highlight specific structural features.
Neutrons possess a magnetic dipole moment which makes them sensitive to magnetic fields generated by unpaired electrons in materials. Precise information on the magnetic behaviour of materials at atomic level can be collected.
In addition, the neutron scattering power of an atomic nucleus depends on the respective orientations of the neutron and the nucleus spins. This makes the neutron a powerful instrument for detecting the nuclear spin order.
The power of polarisation
The possibility of producing beams of polarised neutrons (with all their spins parallel) permits to pinpoint magnetic information much more precisely, facilitating the deciphering of complex magnetic structures. The use of polarised neutrons has permitted measurement of such quantities as the magnetic moments of the individual components of alloys and the magnitude and sign of magnetic scattering amplitudes. The inelastic scattering of polarised neutrons is of great importance for the investigation of the dynamic properties of the lattices of magnetic crystals.
The way neutrons scatter off gases, liquids and solid matter gives us information about the structure of these materials (elastic neutron scattering).
The neutron excitation of atoms gives information about the binding energy within matter (inelastic neutron scattering).
Their ability to act as 'small elementary magnets' makes neutrons an ideal probe for the determination of structures and dynamics of unknown magnetic matter.
Heavy nuclei can be split with neutrons. This can shed light on a number of still unknown processes in atomic fission. Neutrons can also be captured by nuclei. The process releases secondary radiation which can be used to determine the inner structure of these nuclei.
Neutrons are used in different fields of research.
Condensed-matter physics, materials science and chemistry
- Examination of the structure of new materials, for example new ceramic high-temperature superconductors or magnetic materials (important for electronic and electrical applications).
- Clarification of still unknown phenomena in processes such as the recharging of electric batteries.
- Storing of hydrogen in metals, an important feature for the development of renewable energy sources.
- Analysis of important parameters (for example elasticity) in polymers (for example plastic material).
- Colloid research gives new information on such diverse subjects as the extraction of oil, cosmetics, pharmaceuticals, food industry and medicine.
Biological materials, naturally rich in hydrogen and other light elements, are ideal samples for analysis with neutrons.
- Cell Membranes
- Virus Investigations
- Photosynthesis in Plants
Nuclear and elementary particle physics
- Experiments on the physical properties of the neutron and the neutrino.
- Production of extremely slow neutrons down to 5 m/s (the velocity of the neutrons which leave the reactor is generally about 2200 m/s). This enables completely new experiments to be performed with such particles.
- Experiments on atomic fission and the structure of nuclei.
Since neutron diffraction is non-destructive, it is ideal for the analysis of different technical phenomena in materials.
- Visualisation of residual stress in materials (example: railway rails).
- Hardening and corrosion phenomena in concrete.
- Inhomogeneity and trace elements in materials.