The neutron scattering is a major area of research with neutrons . It comprises various experimental methods for the investigation of condensed matter , which are based on the scattering of slow or thermal neutrons on a test body (English: target ). Neutrons interact with atomic nuclei and with the magnetic moments of electrons, which is why they are suitable for studying the structure, dynamics and magnetic order of condensed matter on an atomic scale . In neutron scattering, a distinction is made between inelastic, elastic and quasi-elastic scattering. The inelastic scattering is associated with the excitation or de-excitation of a phonon , a magnon or another internal degree of freedom of the target. The energy of the excitation can be determined by measuring the change in the kinetic energy of the neutron. With elastic scattering, the interaction is not associated with any energy transfer. Since the De Broglie wavelength of thermal neutrons is in the order of magnitude of the diameter of an atom, interference effects occur in the elastic scattering of neutrons on condensed matter , which can be used for structural investigations. This investigation method is often referred to as neutron diffraction (or diffractometry). A third method is quasi-elastic scattering, which is used to study diffusion mechanisms at the atomic level.
Since neutrons have no electrical charge, they penetrate very deeply into matter: the 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). Therefore, neutron scattering is suitable for investigating the volume properties of matter - in contrast to electron diffraction , for example , which is limited to areas near the surface.
Like all particles, neutrons have not only particle but also wave properties. The wavelength of slow neutrons is approximately 0.1 to 1 nm and is thus of the same order of magnitude as the distances between atoms in molecules and solids. Similar to the diffraction of light on a grating, the scattering of neutrons on a regularly constructed sample also leads to wave mechanical interferences; the angular distribution of the scattered neutrons has the regularity of a diffraction image, from which conclusions can be drawn about the atomic structure of the examined sample.
The properties mentioned up to now - electrical neutrality and wavelength in the nm range - have neutrons in common with X-rays . For structural investigations, therefore, primarily the basically similar, but practically simpler and cheaper X-ray diffraction is used . However, neutron scattering is beneficial if one can take advantage of the following additional properties of the neutron:
- The scattering cross-section of neutrons depends on the properties of the scattering atomic nuclei and therefore varies from nuclide to nuclide and even from isotope to isotope. In contrast, X-rays are mainly scattered by electrons , which is why the scattering cross-section increases with the atomic number and, for example, hydrogen is almost invisible to X-ray diffraction. In the investigation of biological samples in particular, neutron scattering is used as a complement to x-ray diffraction to determine the position of hydrogen atoms. The informative value of neutron scattering experiments can be increased in a targeted manner through isotope exchange.
- Neutrons have a magnetic moment and are therefore scattered on magnetic grids. Neutron scattering is therefore an important method for studying magnetic structures.
- The energy of slow neutrons is a few meV and is therefore 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.
- By neutron scattering in the can periodic table of elements adjacent elements such. B. Na , Mg and Al can be clearly distinguished, since the scattering depends on the isotope and its nuclear spin. X-ray scattering delivers poorer results without contrast, since the electron shell is measured, which differs only slightly in the cases mentioned.
Neutron scattering was established as a physical investigation method in the 1950s. Clifford Shull and Bertram Brockhouse received the 1994 Nobel Prize in Physics for their pioneering work . They join the ranks of Nobel Prize winners with the longest gap between discovery (1946) and the award of the Nobel Prize (1994). Under Heinz Maier-Leibnitz , the neutron guide was invented at the small Munich research reactor in Garching near Munich . Maier-Leibnitz also directed the construction of the high-flow reactor in Grenoble. Since the 1990s at the latest, many small research reactors have been shut down worldwide; neutron scattering is concentrated in a few large institutes.
Methods of neutron scattering
- Inelastic neutron scattering (spectroscopy):
- Clifford G. Shull: Early Development of Neutron Scattering . In: Reviews of Modern Physics . tape 67 , no. 4 , October 1995, p. 753-757 , doi : 10.1103 / RevModPhys.67.753 .