Ray Monochromators

The broad spectral bandwidth of synchrotron radiation makes it necessary to employ monochromating devices in most spectroscopic studies which transmit only a small part A , AA of the radiation spectrum. The spectral resolution required for inner-shell spectroscopy or Compton scattering studies is A / = -AA/A = as determined by the ratio of 

X-ray crystal monochromators may be operated in the reflection mode (Bragg case) or in the transmission mode (Laue case). The condition for diffraction to occur is given by the familiar Bragg s law 

The resolution is thus determined by the angular collimation Ad as well as the Bragg angle 0. Two factors contribute to Ad angular spread of the incoming radiation (which depends on the experimental geometry) and the reflection width of the crystal. Both factors will now be discussed in more detail. 

For the choice of a proper diffraction crystal one has to take several aspects into account e.g., high reflected intensity, small intrinsic reflection width, and small content of heavy atoms which might fluoresce. Also the thermal conductivity of the crystal may be important since for synchrotron radiation of high intensity the heat load of the first optical element is large. 

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Figure 8.3 An X-ray monochromator using a bent quartz crystal Q T is the target chamber...

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Large high purity crystals ate cut into windows and refracting components for use in x-ray monochromators (14), and in the vacuum uv, uv, visible, and it ranges. 

Jin x-ray monochromator. A monochromator is a large single crystal (usually graphite) that is oriented so that a very iatense reflection is directed toward the sample. AH wavelengths are absorbed by the monochromator except a small range of wavelengths used for the diffraction experiment. Usually only the characteristic radiation is used if an x-ray tube is the x-ray source. 

A further improvement is possible with the help of an x-ray monochromator 29> 30>, however, the inherent loss in sensitivity and the problems of probe adjustment create a number of difficulties which still have to be overcome in practise. Also the spectra obtained with the help of a monochromator clearly indicate that the gain in resolution for the test standard graphite is around 0.2 eV and the remaining 0.7 eV are obviously due to other parameters 30). 

The primary use of lithium fluoride is in the ceramic industry. It reduces the firing temperature and improves the resistance to abrasion, acid attack and thermal shocks. It is essential component of the fluorine cell electrolyte. An addition of small amount (1-1.5%) to KHF2 HF electrolyte improves the wettability of the carbon anodes and lowers the tendency of the cell to polarize. Another important use of LiF is in flux compositions containing chlorides, borates and other fluorides. Lithium fluoride windows made from high purity crystals are used for X-ray monochromators, UV, visible or IR regions [18]. 

T. Tanaka, Y. Ishizawa, J. Wong, Z. U. Rek, M. Rowen, F. Schafers, and B. R. Muller, Development of a YBee Soft X-ray Monochromator for Synchrotron Radiation, in Proceeding of the 11th International Symposium on Boron, Borides and Related Compounds, Jap. J. Appl. Physics, Series No. 10, eds. R. Uno and I. Higashi, Tokyo, 1994, 110. 

Sayers DE, Stem EA, Lytle FW (1971) New technique for investigating nonciystalline structures Fourier analysis of the extended X-ray absorption fine stmeture. Phys Rev Lett 27 1204-1207 Schaefers F, Muller BR, Wong J, Tanaka T, Kamimura Y (1992) YBge a new soft X-ray monochromator crystal. Synchrotron Rad News 5[2] 28-30... 

Figure 14.3 Schematic diagram of an ESCA instmment using an X-ray monochromator and a multichannel detector.

X-ray Satellites [2]. When conventional ESCA is utilized with direct focus X-ray units (usually A1 Ka or Mg Ka) the resulting XPS or ESCA spectra will produce and register photoelectrons from all aspects of the X-ray line source. In the case of the two major sources employed, this means that not only are photoelectrons ejected by the principal lines (centered at 1486.6 eV for A1 Ka and 1253.6 eV for Mg Ka), but they are also produced by any satellite lines associated with these X-ray sources. A typical example is shown in Figure 3, where the A1 Ka satellites at 9.6 and 11.5 eV (with respect to the position of the principal line) have been produced. These two satellite peaks have 5 and 3.5% of the intensity of the principal photoelectron line. Often these satellite lines do not create any problems, and they may even be viewed as an additional useful feature for calibration processes however, they may introduce some measurement problems, particularly if these satellites overlap weak photoelectron peaks. One must, therefore, always be cognizant of their presence. It should be noted that one of the advantages in employing an X-ray monochromator is the removal of these X-ray satellites. 

The WD spectrometer (Figure 1) behaves as an X-ray monochromator. Selected crystalline materials are used to diffract lines in the fluorescence X-ray spectrum. When a beam of X-rays is directed at certain crystalline materials. X-ray photons are reflected off the various atomic layers in the crystal. Destructive interference occurs between almost all reflected photons, except those that satisfy the Bragg equation ... 

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