Neutron diffraction

Neutron scattering different experimental methods for the investigation of condensed matter be referred to the on the scattering of slow or thermal neutrons on a specimen (English: target) based. Neutrons interact with atomic nuclei and with the magnetic moments of electrons, which is why they are suitable for the investigation of the structure, dynamics and magnetic order of condensed matter at the atomic scale. In the neutron scattering distinguish between inelastic, elastic and quasi-elastic scattering. The inelastic scattering is connected with the presence or de-excitation of a phonon, a magnon or other internal degree of freedom of the target. By measuring the change of the kinetic energy of the neutron can determine the energy of the excitation. In elastic scattering, the interaction is not affiliated with any energy transfer. Since the de Broglie wavelength of thermal neutrons of the order of an atomic diameter, join the elastic scattering of neutrons by condensed matter, interference effects, which can be used for structural studies. This study method is often referred to as neutron diffraction or neutron diffraction. A third method is the quasi-elastic scattering, which is used for the study of diffusion mechanisms, at the atomic level.

Applications

Since neutrons have no electric charge, they penetrate quite deeply into a matter: the mean free path of thermal neutrons in condensed matter is of the order of millimeters ( the exact value depends on the density and composition of the sample from ). Therefore, neutron scattering is suitable for investigating bulk properties of matter - in contrast to electron diffraction, which is limited to areas near the surface.

As all particles are neutrons not only particles, but also wave properties. The wavelength of slow neutrons is about 0.1 to 1 nm and is thus of the same order of magnitude as interatomic distances in molecules and solids. Similarly to the diffraction of light by a grid, it is also used in the scattering of neutrons in a regularly structured sample to mechanical wave interference; the angular distribution of the scattered neutrons has the regularity of a diffraction pattern can be deduced from the atomic structure of the sample under investigation.

The stated up to this point properties - electrical neutrality and wavelength in nm range - have in common neutrons with X-rays. For structural studies, one therefore relies primarily a similar in principle, but in practice simpler and cheaper X-ray diffraction. Neutron scattering is advantageous, however, if one can take advantage of the following further characteristics of the neutron:

• The scattering cross section of neutrons depends on properties of the diffusing nuclei and, therefore, varies from nuclide to nuclide and even from isotope to isotope. In contrast, X-ray radiation is scattered in particular electrons, therefore, the scattering cross-section increases with the atomic number and, for example, hydrogen for X-ray diffraction is almost invisible. In particular, the investigation of biological samples neutron scattering is complementary (complementary ) used for X-ray diffraction to determine the position of hydrogen atoms. By isotope exchange the significance of neutron scattering experiments can be specifically increased.

• Neutrons have a magnetic moment and therefore are dispersed in the magnetic bars. Neutron scattering is therefore a valuable tool for studying magnetic structures.

• The energy of slow neutrons is a few meV and is thus of the same order of magnitude as the excitation energy of phonons and magnons. Inelastic neutron scattering is therefore the standard method for measuring the dispersion of phonons and magnons.

• Neutron scattering light adjacent elements in the periodic table can be measured, such as Na , Mg2 and Al3 , since neutron scattering yields different results depending on the nature of the isotope and the spin. X-ray scattering provides worse results here, since the electron shell is measured, which is only slightly different in the mentioned cases.

Well-known research institutions

For research on a smaller scale you can use as a neutron source, for example an americium -beryllium source or a Californiumquelle. An appropriate major research institutions on the other hand is a research reactor or a particle accelerator with spallation available. Significant research centers for neutron scattering are

• In Europe the Institut Laue -Langevin in Grenoble ( founded in 1967, reactor operation since 1972)
• Laboratoire Léon Brillouin at the Centre d' Etudes de Saclay near Paris nucléaires
• The research neutron source Heinz Maier- Leibnitz in Garching (reactor operation since 2004 )
• The Helmholtz Centre Berlin for Materials and Energy in Berlin
• The research reactor at the GKSS Research Centre Geesthacht (until 2010)
• The Jülich research reactor II at Forschungszentrum Jülich in Jülich ( until May 2006)
• The Paul Scherrer Institute in Villigen AG in Switzerland ( spallation source SINQ )
• The Rutherford Appleton Laboratory near Oxford ( spallation source ISIS)
• The Budapest Neutron Center at the Atomic Energy Research Institute in Budapest in Hungary

• In America NIST in Gaithersburg MD in Washington DC
• The High Flux Beam Reactor and the Brookhaven Graphite Research Reactor at Brookhaven National Laboratory, historically significant, since both shut down
• The Intense Pulsed Neutron Source at Argonne National Laboratory in Argonne, Illinois ( spallation source, operating since 1981)
• The Lujan Neutron Scattering Center at Los Alamos National Laboratory in Los Alamos, New Mexico ( spallation )
• Chalk River Laboratories (Canada), historically significant ( Bertram Brockhouse )

• In Australia The Australian Nuclear Science and Technology Organisation ran the historic reactor HIFAR and 2007 brought one of the most advanced neutron scattering facilities OPAL in operation

• In Russia the pulsed reactor IBR -2 at the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research in Dubna, near Moscow ( reactor operation since 1984)
• The WWR -M reactor at the Petersburg Nuclear Physics Institute in Gatchina near St. Petersburg ( reactor operation since 1959, renewed again and again)

History

The neutron scattering was established in the 1950s as a physical method of investigation. For their pioneering work Clifford Shull and Bertram Brockhouse received the 1994 Nobel Prize in Physics. They lined up so that one in the series of Nobel laureate with the longest gap between discovery (1946) and the Nobel Prize (1994). Under Heinz Maier- Leibnitz neutron guide was invented at the small research reactor Munich in Garching near Munich. Maier- Leibnitz also launched the construction of the high-flux reactor in Grenoble. At least since the 1990s, many small research reactors have been shut down worldwide; neutron scattering focuses on a few large institutions.