Crystal monochromators focusing

There are three accessories used to produce monochromatic radiation metal foil filters, crystal monochromators, and focusing mirrors. An element with atomic number Z can be used as a selective filter for radiation produced by an element of atomic number Z+ 1. For example, a nickel (Z=28) absorption filter, may be used to cut out the Cu KjS (Z=29 for Cu) radiation, leaving only Cu Ka radiation.Not all white radiation, however, is eliminated by this method. Alternatively a single-crystal monochromator may be used. An intense Bragg reflection from the monochromator crystal is used as the incident beam for X-ray diffraction studies. Focusing mirrors, designed to produce a beam that is not only monochromatic but also convergent, may be used. In this case the incident beam is doubly deflected by two perpendicular mirrors. 

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.

Figure 3.33. The schematic of monochromatization of the diffracted beam using a curved crystal monochromator. RS - receiving slit, M - curved monochromator, MS -monochromator scatter slit, D - detector. The dash-dotted arc represents the goniometer circle. The dashed arc shows the focusing circle of the monochromator.

The X-ray beam from the source is monochromated, focused, and collimated to deliver a parallel beam of defined size and wavelength to the crystal. Because of the intrinsically superior optical qualities of synchrotron beams, the radiation delivered to the crystal is also superior to that from conventional sources. The crystal is mounted on a goniostat, which allows the crystal to be rotated. The crystal is usually flash-cooled to a temperature of 100 K by a cold stream of nitrogen gas to reduce radiation damage. X-rays are ionizing radiation and the free radicals produced as they pass through the protein destroy the crystal. Without flash cooling, protein crystals last only seconds on a synchrotron beamline. 

The spatial resolution of the composition maps mainly depend on the diameter of the beam. Auger imaging is a well developed technique because focusing the electron beam is readily achieved by the use of an electromagnetic lens. The diameter of the electron beam can be as small as about 10 nm when a field emission gun is used. Focusing the X-ray beam in XPS is difficult because the X-rays are electrically neutral and cannot be focused with an electromagnetic field. For modern XPS instruments, a monochromatic X-ray beam can be focused down to a diameter of about 10 /rm using a special X-ray gun and a special crystal monochromator. 

The chief value of the focusing monochromator lies in the fact that all the monochromatic rays in the incident beam are utilized and the diffracted rays from a considerable area of the crystal surface are all brought to a focus. This leads to a large concentration of energy and a considerable reduction in exposure time compared to the unbent-crystal monochromator first described. However, the latter does produce a semiparallel beam of radiation, and, even though it is of very low intensity, such a beam is required in some experiments. 

The use of a monochromator produces a change in the relative intensities of the beams diffracted by the specimen. Equation (4-19), for example, was derived for the completely unpolarized incident beam obtained from the x-ray tube. Any beam diffracted by a crystal, however, becomes partially polarized by the diffraction process itself, which means that the beam from a crystal monochromator is partially polarized before it reaches the specimen. Under these circumstances, the usual polarization factor (1 - - cos 26)12, which is included in Eqs. (4-19) through (4-21), must be replaced by the factor (1 + cos 2a cos 20)/(l -I- cos 2a), where 2a is the diffraction angle in the monochromator (Fig. 6-16). Since the denominator in this expression is independent of 6, it may be omitted the combined Lorentz-polarization factor for crystal-monochromated radiation is therefore (1 + cos 2a cos 20)/sin 6 cos 6. This factor may be substituted into Eqs. (4-19) and (4-20), although a monochromator is not often used with a Debye-Scherrer camera, or into Eq. (4-21), when a monochromator is used with a diffractometer (Sec. 7-13). But note that Eq. (4-20) does not apply to the focusing cameras of the next section. 

With a diffractometer one has the option, which does not exist with a powder camera, of placing a crystal monochromator in the diffracted, rather than the incident, beam. Figure 7-28 shows such an arrangement. The diffracted beam from the specimen comes to a focus at the receiving slit S, diverges to the focusing monochromator M, and comes to a focus again at the counter slit S2. Counter, crystal, and slits are mounted on one support and rotate as a unit about the diffractometer axis. 

Figure 3.36 gives a schematic layout of such a system. The white beam from a synchrotron source is focused by a bent-crystal monochromator,... 

Photon beam position monitors are essential to ensure that after an injection the electron beam position is adjusted to allow the SR to strike the beam line optical components in a constant way. The wavelength output from a double crystal monochromator is especially sensitive to the vertical beam position. Also, the quality of the focus, from a toroid mirror, is especially sensitive to the horizontal beam position (figures 5.18(c) and (e)). On existing machines it is necessary to recalibrate the wavelength and the focussing of a beam line optical system after each injection. 

The XRF intensity maps were obtained at beamline 20-ID-B (PNC-CAT) of the Advanced Photon Source (Argoime National Laboratory) at room temperature. The beamline had a chaimel cut Si(lll) crystal monochromator (3-27 keV) and a pair of Kirkpatrick-Baez mirrors, which focused the beam to 4 pm square at the sample. A 13-element Ge detector at 90° of the incident beam was employed to capture the emiPed fluorescence. A step size of 4 pm was used for an image of 324 X 504 pm at an energy of 10 or 7 keV. 

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