Research with Neutrons
Neutrons are neutral particles. Together with protons, they are the building blocks of the atomic nuclei. Neutrons weigh slightly more than the positively charged protons. The wave-particle dualism described by quantum mechanics implies that neutrons also behave like material waves. The wavelength of thermal neutrons is in the range of the atomic distances in condensed matter (crystals, liquids).
Since neutrons are neutral they penetrate deep into matter and they are scattered by atomic nuclei and electron magnetic moments. In this way, the arrangement and motion of atoms (atomic nuclei) or magnetic structures can be determined in a suitable scattering experiment.
As with X-ray scattering, structures on an atomic, molecular or nanoscopic scale become visible. In addition, neutrons allow the simultaneous analysis of motions from fast single atom vibration to slow large-scale fluctuations. Further light elements –in particular hydrogen—have the same visibility as heavy ones and the scattering may vary considerably between isotopes of the same element. The latter enables deeper insights using the “contrast variation” technique. Magnetic moments interact directly with the neutron spins and thereby yield a direct signal from magnetic structures and dynamics in a sample.
The weak interaction of neutrons with matter results in vanishingly small radiation damage of sensitive (e.g. biological) samples. The neutron transparency of even centimetre-thick container walls makes it possible to look at samples in-situ during their function even at extreme conditions e.g. at high pressure or high temperature, or in a special atmosphere. Any of these specific properties or a combination of them makes neutrons a unique and indispensable probe in condensed matter research.
It is a major challenge to prepare suitable beams with a sufficient flux of neutrons with well-defined velocity and direction that are needed for scattering experiments. Neutrons in sufficient numbers for research purposes are produced in large facilities, reactors or spallation sources. After being slowed down in a moderator to thermal or sub-thermal speeds, they can be prepared and guided to dedicated instruments and used in scattering experiments.