Four-reflection monochromators

The divergence of a beam originating from a Bartels four-reflection monochromator is typically 12 arcseconds, or 2.7 thousandths of a degree. Note that, in the case of parabolic multi-layer monochromator, the divergence is 10 times that. 

A four-reflection monochromator comprised of plane single crystals is placed between the source and the sample (see Figure 2.36). The beam irradiates the sample, which is placed on a three axes sample holder. The sample holder makes it possible to orient the normal to any family of ctystal planes in the diffraction plane defined by the axis of the incident beam and the directions of the diffracted beams. Since the beam is strictly parallel, it is not necessary to inclnde a slit between the sample and the monochromator. The intensity of the diffracted beams is meastrred by using a detector which moves along a circle, centered on the sample or with a curved position sensitive detector. This last featrrre makes this system a distant relative of the Debye-Scherrer diffractometers, for which the sample is, by definition, the center of the detection circle. 

Figure 3.7. Calculation of the instrumental resolution function of a diffractometer equipped with a four-reflection monochromator and a curved position sensitive deteetor...

Fig. 1.15 High resolution XRD schematics (a) and typical instrument configuration (b). In (a) the probed volume can be decreased by reducing the primary beam angular divergence A(co) (done in (a) by the use of a two-reflection monochromator) and reducing the detector acceptance A(20) (done in (b) by using an analyzer crystal). The high resolution configuration in (b) uses a two-(shown) or four-reflection monochromator. For overview scans the analyzer crystal can be removed and only the detector with a narrow slit is used in the secondary optics. High-speed line detectors can also be used (without the analyzer crystal)...

Figure 2.16 The CCC with a monochromating crystal, developed by Tanner and Bowen. The four-reflection Si 220 with single Ge 200 monochromating crystal is shown, for CuK 1...

Fig. 17a. Double monochromator camera for SAXS studies. The orientation of the reflecting planes is schematically indicated, b. Single monochromator camera for SAXS studies, c. Four-crystal monochromator setup...

Most available infrared instruments use the Littrow mount for the prism, the beam being reflected from a plane mirror behind the pnsm and thus returning it through the prism a second time. This doubles the dispersion produced. Actually, a double-pass system is also used so that the beam goes through the pnsm four times. Other design modifications include those with single beam and double monochromator, double beam and double monochromator, and related combinations, See also Infrared Radiation. 

The basic idea is to measure both the transmission and the reflectivity of a thin slab of material, which is usually carried out in a device known as a spectrophotometer. Figure 16.5 schematically illustrates the four major components of such a device a source of radiation, a monochromator, the sample, and a number of detectors. In a typical experiment, both T and R are measured, preferably simultaneously. Because of multiple reflections at the various planes of the crystal, T is not given by Eq. (16.14), but rather by ... 

Figure 7. Comparison of two measured muscovite rocking curves for the (006) Bragg reflection acquired by using (a, open circles) an undulator synchrotron beamline with a Si(l 11) monochromator at Er = 7.4 keV, and (b, solid circles) Cu Kax X-rays from a Cu fixed-anode source (Er = 8.04 keV ) followed by a Si(lll) four-bounce high-resolution monochromator and high-resolution diffractometer. The difference between the two-curves is primarily due to the size of the incident beam slit, which is 0.2 mm x 0.05 mm for (a) and 2 mm x 1 mm for (b). Only a source with the brightness of the undulator could be slitted down to such a small size as for (a) and still has sufficient flux on the sample for XSW measurements. Also shown is the best fit to the rocking curve of (a) (solid line). At 7.4 keV, the Darwin width of Si(l 11) is 37 / -ad and that of the muscovite (006) is 20 /rrad.

Figure 17. Guinier method, showing the four possible relative positions of specimen and monochromator A) Symmetrical transmission B) Asymmetrical transmission C) Symmetrical reflection D) Asymmetrical reflection...

Z = 4 Zb = 1.502 g cm A specimen, approximately 0.27 x 0.29 x 0.35 mm was used for X-ray data collection on an automated four-circle diffractometer with graphite-monochromated Mo Ka radiation (X = 0.7107 A). A total of 1287 independent reflections with 20 less than 50 were measured in the a)-20 scanning mode, and of these 1116 reflections with Fo > 3a(F) were used for the structure determination. 

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