Crystal monochromators

The purest kind of radiation to use in a diffraction experiment is radiation which has itself been diffracted, since it is entirely monochromatic. If a single crystal is set to reflect the strong A a component of the general radiation from an x-ray tube and this reflected beam is used as the incident beam in a diffraction camera, then the causes of background radiation listed under (1) and (2) above can be 

A large gain in intensity may be obtained by using a bent and cut crystal, which operates on the focusing principle illustrated in Fig. 6-15. A line source 

In practice the crystal is not bent and then cut as described above, but the unbent crystal, usually of quartz, is first cut to a radius of 2/ and then bent against a circular form of radius R. This procedure will produce the same net result. The value of 9 required for the diffraction of a particular wavelength A from planes of spacing d is given by the Bragg law  

The source-to-crystal distance SC, which equals the crystal-to-focus distance CF, is given by 

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. 

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SI(220) double crystal monochromator was used with entrance slit (1 mm high 20 m from the source point) chosen to give a bandpass of 2 eV at the Pt edge, 11,563.7 eV.( ) The operation of the catalyst... 

The irradiating X-ray beam cannot be focussed upon and scanned across the specimen surface as is possible with an electron beam. Practical methods of small-spot XPS imaging rely on restriction of the source size or the analysed area. By using a focussing crystal monochromator for the X-rays, beam sizes of less than 10 pm may be achieved. This must in turn correspond with the acceptance area and alignment on the sample of the electron spectrometer, which involves the use of an electron lens of low aberration. The practically achievable spatial resolution is rarely better than 100 pm. A spatial resolution value of 200 pm might be regarded as typical, and it must also be remembered that areas of up to several millimetres in diameter can readily be analysed. 

The typical experimental setup (here the experiment established at beamline G3/ HASYLAB [12] is shown) is outlined in Figure 5. The white synchrotron radiation is monochromatized by a double crystal monochromator using the Ge (311) reflection... 

X-ray Instrumentation. All experiments were performed at the Cornell High Energy Synchrotron Source (CHESS) operated at 5.8 GeV (Stations A-3, B and C-2). Monochromatic radiation was obtained with a Si (220) double crystal monochromator. In order to eliminate higher harmonics, 50% detunning was typically employed. ... 

The x-ray powder diffraction pattern of isoxsuprine HCl has been measured using a Philips PW-1710 diffractometer, equipped with a single crystal monochromator and using copper Ka radiation. The pattern thusly obtained is shown in Figure 1, and the table of scattering angles, interplanar d-spacings, and relative intensities are found in Table 1. 

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 existence of the Periodic Table of the chemical elements and the concept of the atomic number motivated A. van den Broek to assert that the nuclear charge in a neutral atom was exactly equal to its atomic number 20). This cogent speculation was given experimental support by the brilliant experimental work of H.G.J. Moseley 21). He measured the frequency, v, of the characteristic x-rays for most of the known elements using the recently discovered crystal monochromator. A relationship was established ... 

X-ray absorption spectroscopic measurements were carried out at the storage ring DORIS III (HASYLAB DESY, Hamburg, Germany) at the EXAFS II beam line, which was equipped with a Si (111) double-crystal monochromator. All spectra were recorded at room temperature in a step-scanning mode. For data analyses the program WinXAS [17] was used. 

The phase composition of catalysts was studied by X-ray diffraction [XRO) technique. XRD spectra were recorded by using a Phillips 17D0 powder diffractometer equipped with a graphite crystal monochromator and CuK radiation. 

Neutrons with a wavelength of 5 to 10 A are often used. A narrow range of wavelengths is usually selected to give a quasi-monochromatic beam. This can be achieved by a crystal monochromator or, more usually, by a velocity-chopper system of rotating discs which select neutrons of a given velocity. 

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