Detectors image plate

Fig. 7. A typical X-ray diffraction pattern of the Fepr protein fromZJ. vulgaris (Hil-denborough). The pattern was recorded on station 9.6 at the Synchrotron Radiation Source at the CCLRC Daresbury Laboratory using a wavelength 0.87 A and a MAR-Research image-plate detector system with a crystal-to-detector distance of 220 nun. X-ray data clearly extend to a resolution of 1.5 A, or even higher. The crystal system is orthorhombic, spacegroup P2i2i2i with unit cell dimensions, a = 63.87, b = 65.01, c = 153.49 A. The unit cell contains four molecules of 60 kDa moleculEu- weight with a corresponding solvent content of approximately 48%.

A second type of image-plate detector employs gadolinium oxide, which absorbs a neutron and emits a gamma ray, which in turn exposes the image plate. Image plates have higher spatial resolution but lower efficiency than multiwire detectors. 

The alternative is to vary the size of the Ewald sphere (i.e. the wavelength) until it touches a reciprocal lattice point. In practice, this is achieved by using an incident beam of white radiation with a continuous spectrum of wavelengths. Historically, this was the first X-ray technique (Lane method), later eclipsed by methods using monochromatic radiation but recently resurgent with the introduction of image plate detectors (see below). 

The images of diffracted crystal lattices can be observed with specialized precession photographic equipment, although the modern day image plate detectors used in most laboratories produce a diffraction image that can be analyzed by computer to provide the indices of the lattice diffraction spots (Fig. ll.l,a-c). 

X-ray diffraction data were collected at room temperature on a MAR 300 mm image plate detector, using a Rigaku RU200 rotating anode operated at 50 kV and 100 mA (Table I). Data were processed with DENZO and scaled with SCALEPACK (8). Variations in the unit cell parameters for different crystals were less than 1.25 A, even between metal complexes and low temperature native structures. 

Both flat and cylindrical transmission samples are commonly used in combination with position sensitive or image plate detectors. The major disadvantage of the transmission geometry arises from the fact that self-focusing of the diffracted beam is not as precise as in the Bragg-Brentano... 

Figure 3.15. The schematic of a powder diffractometer with the vertical goniometer axis, cylindrical sample in the transmission mode and image plate detector (IPD). Solid arrows show the incident beam path. Rings indicate intercepts of Debye cones with the IPD. F -focal point of the x-ray source, M - monochromator, C - collimator, DR - Debye rings.

All monochromatization options discussed above have been used successfully in powder diffractometry. With point detectors, i.e. with those detectors, which register, diffracted intensity at a specific angle, one point at a time, either or both the incident and diffracted beam can be monochromatized. When position sensitive or image plate detectors are used, the only feasible option is to use a P-filter or a crystal monochromator to achieve monochromatization of the incident beam. As shown in Figure 3.16, for some materials the background becomes too high, which makes diffraction data nearly useless in the determination of the atomic parameters of the material. 

Using position sensitive or image plate detectors results in a lower resolution and higher background (see Figure 3.16). 

Figure 14.4 An example of the resolution contributions of an ideally aligned typical image plate detector placed 100 mm from the sample. The footprint of a 0.3 mm diffracted beam and the PSF of 0.3 mm of the detector contribute varying amounts to the resolution over the diffraction angle range.

Since the 1990s the combination of diamond anvil cell (DAC) techniques with imaging plate detectors has turned the investigation of compressibility and... 

---