Guide hall n°1, cold guide H15 | | Monochromator crystal | K intercalated pyrolytic graphite (002) | resolution δλ/λ | 2 % | wavelength λ | 7.56 Å | λ/2, λ/3 - filter | 5 PG crystals | Collimation | pinholes | Ø 0.5; 1.0; and 1.5 mm | source-to-sample distance | 1 m | Sample | flux at specimen at λ = 7.56 Å | 1.7 x 106 n cm-2 s-1 | Detector | distances | 16.5 ... 50.0 cm | rotation | -45° <2θ <20° | area | 20 x 20 cm2 | pixel size | 1.75 x 1.53 mm2 | background | 7 cps for the whole multidetector |
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DB21 is a diffractometer for the study of the structure of crystalline biological complexes at low resolution (lower than 10Å). The method exploits the principles of contrast variation in which particular components of a macromolecular complex can be highlighted by varying the H2O/D2O content of the solvent. This is done by soaking crystals in the appropriate mixture. Typical objects studied are viruses, consisting of protein and nucleic acid, or the protein-detergent complexes of solubilised membrane proteins. Typical crystal sizes are 0.01 - 1.0 mm3.
Because of the limited crystal sizes available (rarely > 1.0 mm3) the instrument is very small and compact rather like an X-ray diffractometer. To obtain a good spatial resolution between diffraction spots and to separate the lowest diffraction orders from the beam-stop an incident wavelength of 7.56Å is selected using a rather novel K-intercalated graphite. For smaller unit cells a pyrolytic graphite monochromator giving neutrons of 4.6Å wavelength is also available.
The detector is of the Anger camera type allowing a resolution of 1.75 x 1.53 mm. Due to the γ-sensitivity of this detector no cadmium is used in beam defining apertures that are instead made of LiF backed with B4C. A single crystal of Bi removes in-beam γ-radiation.
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