Reflection plane crystal monochromators

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

FIGURE 2.22 Bragg reflections from a double-crystal monochromator. From the Bragg equation, nX=2ds,m6, t/for the planes of the crystal stays constant, so changing the angle changes the wavelength of the X-rays reflected. 

The existing instruments for resonance scattering experiments may be classified according to the monochromator system. As synchrotron radiation is highly polarized in the plane of the orbit, the vertical reflection by the monochromator crystals is preferred. Under these conditions the polarization factor remains nearly constant. With reference to this design feature, the following arrangements exist or are under construction ... 

The channel-cut monochromator is the simplest type employed experimentally. A channel is cut in a perfect crystal (e.g.. Si) to provide two parallel reflecting surfaces that have a particular crystal plane [e.g., the Si (220)] parallel to the surface. The Bragg condition is used to select a particular wavelength and the reflected beam emerges parallel to the incident beam but is vertically displaced by 2D cos d, where D is the distance between the two faces and 0 is the angle between the beam and the Bragg planes. The accuracy of data collected using channel-cut crystal monochromators may be limited due to harmonic con-... 

The measurements of Campbell show the effect very clearly (Fig. 8-36). Crystals M and C are both calcite (CaCOj) M serves as a flat crystal monochromator set to reflect Abe radiation, and C is the crystal being studied. Two fixed counters with wide slits are arranged to receive the transmitted beam T and the beam D diffracted by the transverse planes. Crystal C is then rotated through a small angle about 6g, and the intensities of T and D are measured as a function... 

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...

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. 

These are the monochromator crystal materials that are used extensively for macromolecular crystallography at SR sources. The wavelength of the reflected beam from the monochromator can be calculated from the specific lattice plane spacing and the angle of reflection. 

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 ... 

The Bragg equation provides a simpler physical relationship between the directions of incident and scattered rays by a crystal plane, i.e. a monochromic incident beam of wavelength A will be reflected by a family of parallel crystal planes (h, k, 1) if the incident angle is 0 ... 

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