1.1. History of neutron scattering.

In 1935 Professor James Chadwick was awarded the Nobel Prize in Physics for the discovery of the neutron. In 1942 Enrico Fermi showed that neutrons from fission of the uranium nucleus could support a controlled chain reaction. Earlier in 1938 he won the Nobel Prize in Physics for the discovery that slowed-down neutrons more readily interact with matter and could be used to detect the positions and motions of atoms. At the end of the Second World War the researchers in the USA gained access to the large neutron fluxes delivered by first nuclear reactors. The first neutron diffraction experiments were carried out in 1945 by Ernest Wollan at the Graphite Reactor in the Oak Ridge National Laboratory, USA. Together with Clifford Shull they established the basic principles of this experimental technique and successfully applied it to study different materials. Cliff Shull and Bertram Brockhouse demonstrated that the directions in which neutrons are “elastically” scattered without changing energy provide information on the position and arrangement of atoms. In 1994 Shull and Brockhouse received the Nobel Prize in Physics for their pioneering ideas and contributions to the development of neutron scattering techniques.

Over the past fifty years a constantly increasing number of scientists from the fields of physics, chemistry, biology, materials science, geology and others have been turning to neutron scattering methods to find the answers to the most complicated problems in their fields of research.

1.2. Neutron sources.

At present, neutron scattering techniques have practically ceased to be used solely for investigating the atomic and magnetic structure and the dynamics of simple crystals. The emphasis has been increasingly placed on the studies of nanostructures, disordered systems, complex chemical reactions and catalytic processes. The research activities have extended into the areas of study of complex liquids, self-organizing systems and exotic electronic states as well.

All these problems can be solved only with the use of modern high-flux neutron sources: nuclear reactors utilizing controlled fission chain reaction of uranium or plutonium nuclei or proton accelerator-based spallation sources producing neutrons by bombarding heavy nuclei with high-energy protons. Neutron fluxes may be either continuous or pulsed. In such processes the produced neutrons have very high energies, which requires additional installation of neutron moderators on a source. As a result, moderated 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. As a rule, moderators are made from aluminium and filled with liquid hydrogen or liquid methane (depending on the required parameters of a neutron beam).

High flux neutron sources are very expensive to build and to operate and therefore are few in number in the world. In 1950 the first research reactor intended specifically for scientific investigations was constructed. Its prime objective was to produce as high neutron flux as possible. Over the years neutron sources have evolved into multi-purpose research facilities applied in a broad spectrum of experimental investigations. At present, hardly more than 30 laboratories in the world are equipped with medium- and high-flux neutron facilities. Research neutron sources serve only as sources of neutrons and are inapplicable for other purposes.

1.3. Properties of neutrons.

A neutron is an electrically neutral elementary subatomic particle with mass almost 2000 times that of the electron. The neutron lifetime as a free particle is about 15 min in spite of the fact that neutrons are stable when bound in an atomic nucleus.

Basic neutron properties used in neutron scattering:

• The energy of moderated neutrons is comparable to the energy of atomic and molecular motions and lies within the MeV to eV energy range
• The moderated neutron wavelength is comparable to interatomic distances, thus making it possible to study structures in the range from 10^-5 to 10⁵ Å.
• Since neutrons are neutral particles, they interact with the nucleus of an atom rather than with the diffuse electron cloud. The neutron scattering cross sections from nuclei of neighbouring elements in the periodic table may be substantially different, which makes it possible to “see” light nuclei in the presence of heavy ones, to effectively apply isotopic substitution technique and to easily distinguish the neighbouring elements. This peculiarity is a great advantage over the X-ray scattering technique, since x-rays are scattered by the electron cloud.
• Neutrons have a magnetic moment and therefore can be used to study microscopic magnetic structure and magnetic fluctuations, which determine macroscopic parameters of matter.
• Neutron radiation penetrates deep into materials, thus making it possible to study microscopic properties of bulk samples. Such investigations cannot be performed by means of optical methods, X-ray scattering or electron microscopy.
• Neutron radiation is completely non-destructive therefore neutrons can be used to study even delicate biological systems.

The main difference between neutron radiation and x-rays is that neutrons are scattered by the atomic nuclei. Consequently, there is no need for an atomic form factor to describe the shape of the electron cloud of the atom and the scattering power of an atom does not decrease with the scattering angle as it does for X-rays. Diffraction patterns in the neutron scattering show well-defined diffraction peaks even at high scattering angles.

It might be well to point out one more important peculiarity of neutron radiation. X-ray scattering is practically insensitive to the presence of hydrogen in a structure, whereas the nuclei of hydrogen and deuterium are strong scatterers for neutrons. This means that using neutrons we can determine the position of hydrogen in a crystal structure and its thermal motions far more precisely. What’s more, the neutron scattering lengths of hydrogen and deuterium have opposite signs, which makes it possible to apply “contrast variation” technique. By changing the isotopic composition of sample buffer (by varying the amount of hydrogen and deuterium), the experimenter can change the contribution of various components of the studied object into the scattering. In practice, nevertheless, it is not desirable to work with high concentrations of hydrogen in a sample, since neutron scattering by hydrogen has large inelastic component. This results in a large background, which only weakly depends on the scattering angle, and consequently the elastic scattering peaks are drowned in the inelastic background. This problem arises and becomes particularly pronounced when water-based liquid samples are studied. The variation of other isotopes apart from hydrogen and deuterium is possible but, as a rule, very expensive. Hydrogen is relatively inexpensive and at the same time particularly interesting because it plays an exceptionally large part in biochemical structures.