Neutrons: Revealing Particles

Thermal neutrons are the single most powerful probe of condensed matter systems, able to give information on atomic structures and atomic motions of importance in materials science and engineering, in biology, chemistry, physics, geology and in many other scientific and engineering disciplines. Thermal neutrons, the kind produced by the NRU reactor, are a useful and in many cases unique probe of materials at the atomic level because they possess a unique combination of properties.

Neutrons have wavelengths in the range 0.5 Å - 5 Å, which matches typical interatomic separations in condensed matter systems. So thermal neutrons are able to probe the atomic arrangements in these systems.

Neutrons have energies in the range 3 meV - 300 meV, which matches the energies of many of the excitations existing in condensed matter systems. So thermal neutrons are able to measure the energies of these atomic motions.

Neutrons have a magnetic moment and interact with the spatial variation of the magnetization in materials on the atomic scale. Thus they are also used to study magnetic structures and excitations. The cross-sections for the neutron scattering from chemical and magnetic structures are of similar magnitudes.

Neutrons interact with the nucleus of an atom rather than with its electron distribution and the strength of the interaction is specified by a single quantity, the neutron scattering length. This is independent of the chemical environment in which the nucleus is situated, is independent of scattering angle and of the neutron energy. The scattering length is not a monatomic function of atomic number. Neighbouring elements in the periodic table and even isotopes of the same element may have very different cross-sections. Neutrons are thus sensitive to light atoms in the presence of heavy ones and they are suited to application of the technique of isotopic substitution. In particular, H and D have very different scattering lengths and substitution of one for the other, contrast matching, is a powerful technique in fields such as biology and polymer chemistry.

Neutrons are non-destructive and highly penetrating, so they probe the interior of bulk materials, largely independent of the surface condition. Consequently, the use of complex sample environments such as cryostats, furnaces, pressure cells etc. is routine. However, neutron scattering techniques are now developed that permit investigation of surfaces and interfaces and this is a growing field of neutron scattering research.

Neutrons are a weak probe of matter. As a consequence, the cross-section contains terms dependent on the neutron interaction, which are known, and terms dependent on the properties of the system under investigation. This greatly facilitates the theoretical interpretation of neutron scattering data, for the experimental results thus give direct information about the microscopic properties of the system of interest.

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