Channel cut monochromator

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-... 

SCH 95] SCHUSTER M., GOBEL H., Parallel beam coupling into channel-cut monochromators using curved graded multilayers , J. Rhys. D Appl. Rhys., vol. 28, p. 270-275, 1995. 

The experiments were carried out on the Exafs 4 spectrometer at LURE-DCI, running at 1.85 GeV with an average current of 250 mA. The Exafs data were collected using a conventional step-by-step set up with a channel cut monochromator Si(l 11) for Pt and two ion chambers as detectors, with Pt foil used to calibrate the monochromator. The spectra were recorded above the L ii edge of platinum (11,560 eV). The results are displayed in figure 1. 

Fig. 8. Photograph of the X-ray pnilse selector, the Si(lll) channel-cut monochromator and the high-precision diffractometer.

Figure 3 shows one of our photoacoustic cell for X-ray spectroscopy of solid samples The cylindrical cell has a sample chamber at the center with volume of 0.16 cm which has two windows of beryllium (18 mm x 0.5 mm thickness). A microphone cartridge is commercially available electret type (10 mm ) and the electronics of preamplifier for this microphone is detailed elsewhere Figure 4 shows the typical experimental setup for spectroscopic study X-ray was monochromated by channel cut silicon double crystal (111) and ion chamber was set to monitor the beam intensity. Photoacoustic signal intensity was always divided by the ion chamber current for the normalization against the photon flux. X-ray was modulated by a rotating lead plate (1 mm thick) chopper with two blades. 

Comparison between the core-level X-ray absorption spectroscopy (XAS), emission (XES), and X-ray photoemission spectroscopies (XPS) usually shows that the spectral edges rarely coincide with each other and with the Fermi level. It is common practice, however, to place F at the emission threshold which corresponds to a fully relaxed ion core (16). For defining the structure of the edge, an energy resolution of at least 1-2 eV is required in the range of 5-20-keV X-ray photons. This can be achieved with Bonse-Hart channel-cut silicon monochromator crystals. 

FIG. 1 X-ray diffractogram of TDAE-Cgo compound. Data were taken at wavelength 1,2995 A with a channel-cut Ge(lll) monochromator and Ge(220) analyser, The smooth curve is a Rietveld fit, as described in text. To define the baseline, we have drawn a piecewise linear curve through regions of the spectrum where there are no allowed diffraction peaks. Inset shows a diagram of the TDAE molecule. The lower panel shows residuals, to the same vertical scale. Regions around untreated f.c.c. Ceo peaks excluded from the fit are absent from the model curve and residuals. 

We used channel-cut Si 220 or Si 400, or double crystal Si 311 monochromators, and air ionization chambers detectors. Most of the spectra were recorded with a step of 2 eV on a range of 800 to 1000 eV. 

The monochromator is a Ge(220) channel cut. A focussing double crystal monochromator is under development (Goulon and Lemonnier, unpublished work, see section 5.2.4.2). Multiple wavelength data have been collected on terbium parvalbumin at the Lm absorption edge of terbium (Kahn et al 1985) and the structure solved (see 9.7.4). 

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