Description

The diffractometer RESI is designed for using a maximum of thermal neutron intensity at the FRM II, allowing optimum measurement of weak diffraction phenomena in a large portion of the reciprocal space on single crystalline samples.

Typical Applications

Structure analysis with thermal neutrons (λ = 0.8 Å to 2 Å) is complementary to structure analysis with X-rays. The measurement possibilities provided by this instrument are crucial for many scientific questions:

• Structure analysis, bonding theory, electron densities: Due to the interaction with atomic cores and the diffraction angle independence of the atomic form factor, it is possible to measure Bragg scattering up to high diffraction angles.
• Real crystals and compounds of interest for material science are often not perfectly ordered. The elucidation of these real structures requires the analysis of the corresponding diffuse scattering. The diffuse scattering - off the Bragg reflections - is normally differentially weak and distributed continually (anisotropic) in the reciprocal space.
• Partially crystalline compounds, like fibre structures, show a specific scattering, which is highly anisotropic and continously distributed in the reciprocal space. Therefore, diffractometers with area detectors like RESI are best suited for this kind of problems.
• A new class of aperiodic crystals (“quasi crystals”) show dense, but discrete reflex patterns, where more than 90 % of the reflexes are very weak. Additionally, due to the fact that quasi crystals often contain two or more transition metals (which are almost isoelectronic), neutrons offer much higher contrast than X-ray methods.
• Structural phase transitions can be accompanied by continuous reflection shifting.
• Modulated structures show satellite reflections at “incommensurable” positions. Both areas require analysis of large portions of the reciprocal space.
• Twinned crystals and multi-domain/multi-phase crystals are often difficult to measure on single-counter instruments. The area detector at RESI allows for easy detection and in many cases separation of reflections in such systems.

The advantages of the high-resolution area detector can be utilized best, if the reciprocal space is not too empty. That means, that RESI is optimal for cells of ca. 1000 Å3 to ca. 20000 Å3. Typical crystal sizes range from 5 mm3 to 25 mm3.