Instruments
The ILL's diffraction instruments are divided into two main groups: powder diffractometers and single-crystal diffractometers.
In this page you will find a very short description of our instruments. If you are interested in more detailed information, you can consult the official page of the instrument (mantained by each Instrument Scientist).
Powder diffractometers
These two-axis diffractometers are used to investigate the structure of powders, liquids and amorphous materials. Diffraction group machines use relatively short wavelength neutrons (0.3 to 2.0 Å) to resolve structures to atomic resolution, in contrast to diffractometers in the Large Scale Structures group that use long wavelength neutrons for lower resolution of larger structures. Usually a large composite monochromator, up to 300 mm high, is used to select a narrow band of wavelengths and focus it onto the sample.The monochromator may be made from several crystals of pyrolytic graphite, copper or germanium. Large multi-detectors and linear position-sensitive detectors (PSDs) cover a large solid angle for maximum efficiency.
The two high-resolution powder diffractometers D1A and D2B are used mainly for Rietveld refinement, with scans lasting from 30 minutes to several hours. They are complemented by two high-flux medium-resolution powder machines, D1B and D20, which are used mainly for temperature scans and other types of fast experiment, especially on small samples. D4, on the short wavelength hot source, is used for liquids and amorphous materials, together with D20.
Single-crystal diffractometers
Single-crystal diffraction is a powerful method for the investigation of structural details in condensed matter. Hot neutrons are required to uncover the finest details in the nuclear positions and neutron spin polarisation is a handle to separate mixed components (nuclear polarisation, magnetic and electronic scattering).
A characteristic of the four-circle diffractometers is the use of Eulerian cradles for orienting the sample crystals, with the detector moving in a horizontal plane. Normal-beam diffractometers have a mechanism for tilting the counter out of the horizontal plane, thus enabling the installation of heavy equipment for special crystal environments (cryostats, magnets, etc.).
These diffractometers can be used to find:
- average atomic positions. From these we can learn how the atoms are bound together to form molecules, and how the molecules are stacked;
- local atomic distributions. This gives information about the time averaged thermal motion or the local atomic disorder;
- magnetic structures and magnetic moment distributions.
Structural data of this kind are required for a large number of systems, ranging from organic molecules to high temperature superconductors. Often studies are made as a function of temperature, pressure and magnetic field which may lead to important modifications of the crystal structure.