Primary beam monochromator

A disadvantage of the back end monochromator is its inability to separate Ka 2 from Kai. For very precise unit cell determinations this separation is desirable. For this purpose a front end or primary beam monochromator is used (Figure 12c). This monochromator is the only t)q)e that can be used with area detectors. Their disadvantages include a high intensity loss, the need for precise alignment and nonremoval of fluorescent radiation. 

Figure 12 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...

Figure 5.2 Part of a whole powder pattern fit of the NAC reference sample. Same reflections (211) (a) and (921) (b) as on Figure 5.1, measured with a conventional laboratory diffractometer (Bruker D8 Advance), in Bragg Brentano geometry, using Cu Kq radiation, without a primary beam monochromator.

As area detectors (other than multiwire systems) are not energy discriminating devices, apotential source of error lies in the contamination of the data with harmonics of the assumed wavelength of the primary beam. The importance of this effect has been estimated for molybdenum Ka radiation using a graphite monochromator [1],... 

The amplitude of I m for Si or Ge is close to zero, therefore the contribution of 12 to the 111 reflection is zero. Hence, the 111 reflection from a Si or Ge monochromator is used to obtain 12 free radiation. However, these monochromators also drastically reduce the intensity of the primary beam compared to the graphite monochromators found in most commercial diffractometers. 

Optical devices are placed in the light path in order to shape the primary beam. Beam-position monitors, shutters, slits, monochromators, stabilizers, absorbers, and mirrors are utilized for this purpose. The effective beam shape and its flux are defined by these components. In particular, if mirrors are cooled, vibration must be avoided and thermal expansion should be compensated. 

Laboratory X-ray sources emit highly divergent radiation. With conventional optics the major part of this radiation is discarded by a slit system and a monochromator. Both components can be replaced by a Gobel mirror [73,74], Figure 4.5 shows its construction and application. As a result a parallel and highly monochromatic primary beam is received. Replacement of conventional incident beam optics (cf. Fig. 2.2) by a Gobel mirror increases the primary beam intensity by a factor of 10-50. 

This is most easily done at a laboratory source where the current of the X-ray tube is decreased to the lowest possible value. At a synchrotron beamline this is more complicated, because the measurement of the primary beam requires special adjustment. So, technically this should be done before the final optical adjustment of the device, as long as the slits can be narrowed for the purpose of intensity attenuation and as long as the primary beam stop is not yet mounted. It is not advised to use absorbers that are mounted behind the monochromator, because they change the spectral composition of the X-ray beam. 

Electroluminescence was excited by v oltage i mposed to the ELT samples by stripe contacts. Photoluminescence (PL) and lasing were excited at T=18-500 K by the radiation of a N2 laser ( A v = 3.68 eV, /= 1 kHz, Tp = 8 ns) and a CW HeCd laser (Av=3.81 eV). The X-ray diffraction was measured with a Philips X Pert Materials Research Diffractometer. The system uses CuX radiation and a four-crystal Ge monochromator in the (220) setting. It is also equipped with an X-ray mirror in order to increase the intensity of the primary beam. The beam size was limited to 1.4x3 mm, co - 20 scans were performed using a triple bounce Ge (220) analyser. 

Two philosophies have been developed for the fast registration of X-ray absorption spectra [73-76]. One of these can be characterized as a brute-force variant of the normal transmission experiment, where the energy of the primary beam is increased stepwise. It takes about 20-30 min to take a full spectrum, and much of this time is used to perform corrections in the adjustment of the beam and to calm down mechanical vibrations and instabilities, which occur after every movement of the monochromator crystal. The QEXAFS (Quick-Scanning EXAFS) method [75,76] uses a step-wise, rather than a continuous motion of the motors driving the movement of the crystals, so that such extra times are not necessary. If the counting times of the detectors are minimized as well, a full XAFS spectrum of sufficient quahty can be obtained in a few seconds. Spectra with only minor losses in quahty as compared to conventional step-by-step scanning can be obtained in a few minutes. 

Let the intensity distribution at the focus be represented by the function G(o ), the reflectivity of the monochromator crystal by R(J, a, X), and the spectral intensity distribution of the primary beam by i(X - Xg) " 16 reflection intensity from the monochromator is proportional to the product of these functions [6]. The total intensity of the monochromator beam when the monochromator is placed at the Bragg angle (t> = t>g) is then defined by the following expression ... 

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