What is a neutron?

The neutron path through a material

The neutron path through a material

The neutron is a powerful tool for the study of condensed matter (solids and liquids) in the world around us, having significant advantages over other forms of radiation in the study of microscopic structure and dynamics.

Neutron scattering gives detailed information about the microscopic behaviour of condensed matter, playing a major role in shaping the experimental and theoretical understanding of materials ranging from magnetism and superconductivity to chemical surfaces and interfaces.

A neutron is an uncharged (electrically neutral) subatomic particle with mass 1,839 times that of the electron. Neutrons are stable when bound in an atomic nucleus, whilst having a mean lifetime of approximately 1000 seconds as a free particle. The neutron and the proton form nearly the entire mass of atomic nuclei, so they are both called nucleons.

Why use neutrons for condensed matter studies?

Neutrons scatter from materials by interacting with the nucleus of an atom rather than the electron cloud. This means that the scattering power (cross-section) of an atom is not strongly related to its atomic number (the number of positive protons in the atom, and therefore number of negative electrons, since the atom must remain neutral), unlike X-rays and electrons where the scattering power increases in proportion to the number of electrons in the atom. 

This has three advantages: 

  • it is easier to sense light atoms, such as hydrogen, in the presence of heavier ones
  • neighbouring elements in the periodic table generally have substantially different scattering cross sections and can be distinguished
  • the nuclear dependence of scattering allows isotopes of the same element to have substantially different scattering lengths for neutrons. Isotopic substitution can be used to label different parts of the molecules making up a material.

The interaction of a neutron with the nucleus of an atom is weak, (but not negligible) making them a highly penetrating probe. This allows the investigation of the interior of materials, rather than the surface layers probed by techniques such as X-ray scattering, electron microscopy or optical methods. This feature also makes the use of complex sample environments such as cryostats, furnaces and pressure cells quite routine, and enables the measurement of bulk processes under realistic conditions.

Because of the weak interaction, neutrons are a non-destructive probe, even to complex and delicate biological or polymeric samples.

Neutron energies are similar to the energies of atomic and electronic processes, i.e. in the meV to eV range, allowing energy scales from the µeV of quantum tunneling, through molecular translations, rotations, vibrations and lattice modes, to eV transitions within the electronic structure of materials to be probed.Neutrons are spin-1/2 particles and therefore have a magnetic moment that can couple directly to spatial and temporal variations of the magnetisation of materials on an atomic scale. Unlike other forms of radiation, neutrons are ideally suited to the study of microscopic magnetism, magnetic structures and short wavelength magnetic fluctuations. The cross-sections for magnetic scattering and scattering from the chemical structure are fortunately of the same magnitude, permitting the simultaneous measurement of the magnetic and chemical behaviour of materials.

A little history of neutron scattering...

At the end of the second World War researchers in the USA gained access to the large neutron fluxes that even relatively modest nuclear reactors were capable of delivering. Neutrons had then been known as building blocks in the atomic nucleus for more than a decade (Nobel Prize to Chadwick in 1935 for their discovery). Enrico Fermi showed in 1942 that neutrons from fission of the uranium nucleus could support a controlled chain reaction. He had earlier made the important discovery that slowed-down or thermal neutrons show a much greater inclination to react than fast ones do (Nobel Prize for this discovery, among others, to Fermi in 1938)). It is the special properties of these slow neutrons that make them suitable for detecting the positions and movements of atoms. Even before the entry of the nuclear reactors into the research arena, results of using simple neutron sources had indicated that neutron beams could be used for studying solid bodies and liquids (condensed matter). However, there were many difficulties to be overcome before these possibilities could be realised.

The 1994 Nobel Prize in Physics was awarded to Bertram Brockhouse and Clifford Shull for their pioneering contributions to the development of neutron scattering techniques for studies of condensed matter: for the development of the neutron diffraction and neutron spectroscopy techniques. In simple terms, they helped answer the questions of where atoms "are" and of what atoms "do".

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