Neutron scattering

Neutron scattering, the scattering of free neutrons by matter, is a physical process and an experimental technique using this process for the investigation of materials.

Neutron scattering as a physical process is of primordial importance in nuclear engineering.

Neutron scattering as an experimental technique is used in crystallography, physics, physical chemistry, biophysics, and materials research. It is practised at research reactors and spallation neutron sources that provide neutron radiation of sufficient intensity. Neutron diffraction (elastic scattering) is used for determining structures; Inelastic neutron scattering is used for the study of atomic vibrations and other excitations.

Fundamentals

Neutron energies, moderators

Main article: Neutron temperature

Free neutrons produced by nuclear fission in a reactor or a spallation source initially have kinetic energies of a few MeV (Mega-electronvolt). These neutrons must be slowed down to maintain a fission chain reaction or/and to be a useful research tool. This is achieved by inelastic scattering in a neutron moderator. In research neutron sources, the moderator usually consists of light or heavy water at temperatures well below 100°C. When neutrons have approached thermal equilibrium with the moderator, they are called thermal neutrons; their kinetic energy has a Maxwell distribution centered around 25 meV (milli-electronvolt).

For many research applications, it is advantageous to have cold neutrons with even lower energies. They can be produced in a cold neutron source, a relatively small volume (few dm³) of liquid H₂ or D₂ at about 20 K, corresponding to a kinetic energy of the order 2 meV; boiling methane yields somewhat higher energies. Cold neutrons have wavelengths that are comparable to the atomic spacing in solids and liquids, and kinetic energies that are comparable to those of dynamic processes in materials.

Neutron-matter interaction

Since neutrons are electrically neutral, they penetrate matter more deeply than electrically charged particles of comparable kinetic energy; therefore they are valuable probes of bulk properties.

Neutrons interact with atomic nuclei and magnetic fields from unpaired electrons. The neutrons cause pronounced interference and energy transfer effects in scattering experiments. Unlike an x-ray photon with a similar wavelength, which interacts with the electron cloud surrounding the nucleus, neutrons interact with the nucleus itself. The interaction is described by Fermi's pseudopotential. Neutron scattering and absorption cross sections vary from isotope to isotope.

Also depending on isotope, the scattering can be incoherent or coherent. Among all isotopes, hydrogen has the highest neutron scattering cross section. Also, important elements like carbon and oxygen are well visible in neutron scattering. This is marked contrast to X-ray scattering where cross sections systematically increase with atomic number. Thus neutrons can be used to analyse materials with low atomic numbers like proteins and surfactants. This can be done at synchrotron sources but very high intensities are needed which may cause the structures to change. Moreover, the nucleus provides a very short range, isotropic potential varying randomly from isotope to isotope, making it possible to tune the nuclear scattering contrast to suit the experiment.

The scattering almost always has an elastic and an inelastic component. The fraction of elastic scattering is given by the Debye-Waller factor or the Mössbauer-Lamb factor. Depending on the research question, most measurements concentrate on either the elastic or the inelastic scattering.

Magnetic scattering

The neutron has an additional advantage over the x-ray photon in the study of condensed matter. It readily interacts with internal magnetic fields in the sample. In fact, the strength of the magnetic scattering signal is often very similar to that of the nuclear scattering signal in many materials, which allows the simultaneous exploration of both nuclear and magnetic structure. Because the neutron scattering amplitude can be measured in absolute units, both the structural and magnetic properties as measured by neutrons can be compared quantitatively with the results of other characterisation techniques.

Neutron Scattering Instruments

• Neutron diffraction
  • Crystallographic Instrumentation
  • Small angle neutron scattering
  • Neutron Reflectometry
• Inelastic neutron scattering
  • Neutron triple-axis spectrometry
  • Neutron time-of-flight scattering
  • Neutron backscattering
  • Neutron spin echo
  • Neutron resonance spin echo

The Practice of Neutron Scattering

History

First neutron-scattering instruments were installed at beam tubes at multi-purpose research reactors. In the 1960s, high-flux reactors were built that were optimized for beam-tube experiments. The development culminated in the high-flux reactor of the Institut Laue-Langevin (in operation since 1972) that achieved the highest neutron flux to this date. Besides a few high-flux sources, there were some twenty medium-flux reactor sources at universities and other research institutes. Starting in the 1980s, many of these medium-flux sources were shut down, and research concentrated at a few world-leading high-flux sources.

Performing a Neutron Scattering Experiment

Main article: Neutron facilities

Today, most neutron scattering experiments are performed by research scientists who apply for beamtime at neutron sources through a formal proposal procedure. Proposals are assessed for feasibility and scientific interest. Results of successful experiments are published in scientific journals. Short reports on all performed experiments are collected and published by the neutron sources.

Applications

Neutron scattering has been used to study various vibration modes,

External links

• Neutron scattering - a case study
• Neutron Scattering - A primer (LANL-hosted black and white version) - An introductory article written by Roger Pynn (Los Alamos National Laboratory)
• Podcast Interview with two ILL scientists about neutron science/scattering at the ILL
• YouTube video explaining the activities of the Jülich Centre for Neutron Scattering