Common basics

HEiDi is the name of the single crystal diffractometer at the hot source of the FRM-II. The diffractometer is installed at the beam line SR-9B. It does a lot of investigations in several areas of diffraction studies with fast neutrons.

The physical principle is based on the Bragg law 2d*sin(θ)=λ. An incoming beam with a fixed wavelength λ becomes diffracted by a crystal with a given lattice plane distance d with a diffraction angle of 2Θ;. The mechanical properties of the diffractometer assure that the diffracted radiation is directed to the detector, where it is collected. Further information about the instrument can be found at Hardware and Technical Data.

Diffraction with neutrons allows solid state investigations in the following areas, especially if using fast neutrons:

• Structural analysis
  Following the Bragg law shorter wavelengths allow the measurement of smaller distances between lattice planes. The upper limit of all measurable Bragg reflections can be described as a sphere with radius |Q|=sin(θ)/λ in reciprocal space. The maximum diffraction angle θ is limited to values smaller than θ<90° due to the given geometry. Therefore, a significant increase of the available Q space can be reached only by a reduction of the wavelength λ, which yields to more detectable diffraction reflections. This gain of information allows a very accurate determination of structural parameters. These are the lattice parameters and atomic positions in the unit cell as well as the mean square displacements (MSD) of the atoms. Because of this, the investigation of the MSD U_ij, which have an influence on Bragg intensities which depends strongly on Q, allows also quantitatively very accurate studies of the relationships between the experimentally determined MSD and certain structural properties (see also Examples).
• Magnetism
  Because of their magnetic moments neutrons can react not only with the core potential of atoms but also with their magnetic moments. The magnetic cross section, described by the scattering length b_m, can be (dipending on the actual atom) in the same order of magnitude as the (core) scattering length b_K Therefore, neutrons can be used to investigate magnetic structures. b_K is independent from the Q value. b_m decreases rapidly with increasing Q. If a large Q space is available, this gives the opportunity to determine separately the core structure using the high Q range and the magnetic structure using the small Q range.
• Extinction
  Multiple bragg scattering at several lattice planes of equal indices can weaken the intensity of a bragg reflection drastically. This phenomenon is called extinction and it can be observed typically on very perfect and large crystals. This effect decreases significantly for shorter wavelengths. Therefore, a more accurate data evaluation becomes available, which is highly desirable for accurate structural investigations.
  Due to the bandwidth from 0.3 to 1.2 Å an improved wavelength dependent description of extinction can be derived which would be very useful for data evaluation from time-of-flight data.
• Absorption
  Certain isotopes (e.g. Sm, Gd) show an extreme absorption if observed with thermal neutrons. Normally, this makes diffraction experiments very time consuming or even impossible. But at shorter wavelengths absorption decreases. This give the opportunity to perform data collections with sufficient signal-to-background ratio.

Examples

Here are some examples: