Crystal monochromator geometry

The three most common geometries of a crystal monochromator and a sample, used in powder diffraction, are illustrated in Figure 2.15 and their characteristics are compared in Table 2.4. Diffracted beam monochromators Figure 2.15a, b) have a relatively high intensity output (or in other words have low intensity losses) but do not separate the Kaj and Ka2 doublet. The removal of both the KP component and white radiation is excellent. The... 

Figure 2.15. The three different monochromator/sample geometries used in powder diffraction a) flat diffracted beam monochromator, parallel arrangement b) curved diffracted beam monochromator, angular arrangement, and c) flat primary beam monochromator, parallel arrangement. F - focus of the x-ray source, S - sample, M - crystal monochromator, D - detector, Rm - radius of the monochromator focusing circle, Rq - radius of the goniometer focusing circle.

The effects of misalignment and Soller slits can also be included in the calculation as well as a different diffractometer geometry. For the case of the diffractometer equipped with a crystal monochromator it is also possible to provide a solution in the context of the proposed method. For reasons of space, we outline here the possible treatment of these effects without going into details. 

Direct geometry instruments use choppers or crystal monochromators to fix the incident energy and they are found on both continuous and pulsed sources. To compensate for the low incident flux resulting from the monochromation process, direct geometry instruments have a large detector area. This makes the instruments expensive, they are generally twice the price of a crystal analyser instrument. At present, they are used infrequently for the study of hydrogenous materials, so we will limit our discussion to a chopper spectrometer at a pulsed source and a crystal monochromator at a continuous source. 

The bromine K EXAFS spectra were recorded in transmission geometry at beamline ROMO II at the HASYL.AB in Hamburg with a Si(.31 1) double crystal monochromator. Data were collected with ton chambers. The first ion chamber, monitoring... 

The powder X ray diffraction patterns were scanned in transmission technique with a GO-2000 diffractometer (Ital Structures, Riva del Garda, Italy) operating in the Seemann-Bohlin geometry and equipped with a quartz-curved crystal monochromator of the Johansson type aligned on the primary beam. The Cu-K i radiation (X 1.5406 A) was employed, and an instrumental 26 step of 0.1° every 10 s was selected. 

Figure 2-1 of Chapter 2 shows an experimental configuration for depolarization measurements in 90° scattering geometry. In this case, the polarizer is not used because the incident laser beam is almost completely polarized in the z direction. If a premonochromator is placed in front of the laser, a polarizer must be inserted to ensure complete polarization. The scrambler (crystal quartz wedge) must always be placed after the analyzer since the monochromator gratings show different efficiencies for L and polarized light. For information on precise measurements of depolarization ratios, see Refs. 21-24. 

Figure 2.10 Bragg Brentano geometry with a diffracted beam monochromator. The crystal is usually graphite, which has a low degree of crystalline perfection, and hence a large acceptance angle (tenths of a degree). Thus a flat crystal is adequate.

Figure 2.11 Bragg Brentano geometry with a pre sample monochromator. A near perfect crystal, e.g. quartz or germanium, is required to separate Kai and Ka2.

Diffractometer in parallel geometry. Hybrid front monochromator and flat analyzer crystal... 

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