2D-diffraction

As mentioned before the ID15B end-station was reconstructed to have place for a diffraction setup. Another recommendation was to use area detectors for amorphous materials and liquids experiments on solidification and phase transitions in alloys by measuring complete scattering pattern. The 2D-detector and sample towers are on penta-pod optical table (see Figure 13). The setup can be used on studies of amorphous materials, liquids, powders and single crystals. The sample stage is such that several different sample environments can be interchanged depending upon experimental needs. For example, tensile testing rig (see Figure 13), displex and bath cryostats, furnaces, cryomagnet, etc.. Furthermore, MPI/Stuttgart has constructed large vacuum chamber for the diffuse scattering studies using this diffraction setup. The detector stage, which is translatable along the beam direction in order to cover a range of momentum transfers to match experimental needs, can carry a variety of 2D detectors (MARCCD133, MARCCD165, MARIP345, Pixium, and the image-intensifier).

2D-diffraction setup on penta-pod table. In this experimental configuration the detector stage holds Mar133 CCD-detector and sample stage holds ETMT-rig. Al-plate before the detector cuts low energy fluorescence background and holds the beam-stop.

The setup also has the possibility to use a so-called spiral-slit for the strain measurements. (see Figure 14). This is a novel non-destructive strain and phase mapping technique and provides depth-resolved information about intensity and position of all the Bragg reflections emanating from the sample within the angular acceptance given by the experimental setup. The depth resolution is achieved by the use of the spiral slit. It consists of twelve equidistant concentric spiral apertures cut into a W-plate. All paths through the 2mm thick plate lie on cones, which have one common apex i.e. the focal point of the slit system. Spiral-slits for 50 and 100mm distances are available. The azimuth-selective depth-resolved signal is recorded with an online image plate scanner.

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With the spiral slit the angle and depth of the diffraction signal can be resolved. The strain resolution of this setup is 2x10^-4. The depth-resolution depends on the size of the incident beam and spiral gap, current setups have achieved 250µm resolution. At right is a 2D-diffraction pattern from a friction stir welded Al sample and an Fe-powder reference. Due to depth resolution the signal from different samples can be resolved. Up to three samples with a reference has been measured simultaneously.