Point Focus Collimation

The cross-section of the primary X-ray beam is extended and not an ideal point. This fact results in a blurring of the recorded scattering pattern. By keeping the cross-section tiny, modern equipment is close to the point-focus collimation approximation - because, in general, the features of the scattering patterns are relatively broad. Care must be taken, if narrow peaks like equatorial streaks (cf. p. 166) are observed and discussed. The solution is either to desmear the scattering pattern or to correct the determined structure parameters for the integral breadth of the beam profile (Sect. 9.7). 

In addition to point-focus apparatus there are scattering devices with an extremely elongated cross-section of the primary beam. Historically this geometry has been developed as a compromise between ideal collimation and insufficient scattering power. Their practical importance is decreasing as more powerful point-collimated sources become available. Kratky camera (Alexander [7], p. 107-110) and Rigaku-Denki camera (BaltA Vonk [22], p. 83) are the most frequent representatives of slit-focus devices. 

Figure 4.13. Optical scheme of ATR accessory with hemispherical IRE. (a) Collimation of diverging beam from point focus for hemisphere with refractive index of 1.3. (b) Physical layout and (c) geometric optics. All horizontal distances are with respect to accessory center and all vertical distances are with respect to instrument focal plane, (mm). Beam divergence of 6° and focus diameter of 10 mm were used. Reprinted, by permission, from P. W. Faguy and N. S. Marinkovic, Appl. Spectrosc. 50, 394 (1996), pp. 396 (Fig. 3) and 397 (Fig. 4). Copyright 1996 Society for Applied Spectroscopy.

In many cases. X-ray mirrors are also used to focus the beam by using a crystal with graded composition or curvature. Pinholes are also used to collimate point focused beams by using small circular orifices. For cases where a beam with very small size is required (for improved spatial resolution), mono or polycapillary optics are employed (where refiecting/refracting surfaces focus the beam in to small spot sizes). 

Figure 2.21. A, High-intensity point source lamp B, parabolic mirror C, light baffle D, narrow slit E, collimating lens F, Coming filters G, reaction cell or series of cells H, focusing lens I, photomultiplier.

Fraunhofer diffraction phenomena are observed when both the source and the point of observation are effectively at infinite distance from the diffracting ohject, obstacle, or aperture. This condition is sometimes brought about by passing the light from the source through a collimator before it is diffracted, and then focusing the parallel diffracted rays at the point of observation. 

Because a 2-D photon-counting detector was not available at the time of construction of the demonstrator, the detector was realized as a single 1-D vertical detector column based on scintillator crystals and photomultipliers, which can be rotated around the focus point, thereby acquiring one projection. Spatial resolution was ensured by a collimator made of thin tungsten lamellae placed in front of the detector. 

Simultaneous with the publication of Hocker et al., there appeared the results of Yardley and Moore [142] on laser-excited vibrational fluorescence in CH4. A mechanically chopped He-Ne 3.39-micron laser [143, 144] was used to excite the asymmetric stretching [/ = 2948 cm-1 (36.55 X 10-2 eV)] vibration, i>3 (see Figure 3.17). The optical arrangement is shown in Figure 3.18. The He-Ne laser tube, 220 cm in length, is shown on the left. Mx, M2, and Ms are mirrors Bx and B2 are baffles to eliminate stray light Lx and L2 are lenses which focus the laser output into a collimated beam having a diameter of 2 mm, and thence, into a Pyrex fluorescence cell. At the focal point between Li and L2 is a chopper wheel, to produce a nearly perfect square wave modulated at frequencies between 600 and 10,000 Hz. An audio oscillator and a 60-W amplifier are used to drive the synchronous chopper motor. An InSb infrared detector (response time of about 4 nsec) is used to... 

The total reflection of mirrors can be used to focus the radiation. Synchrotron radiation, while collimated in the vertical plane it spreads over the horizontal one. Wavelength resolution requirements normally restrict the vertical aperture to one mm or so, in any case. On the other hand it is desirable to condense the horizontal spread into a focal point. Double focussing with a mirror system is possible and an ideal mirror geometry has been worked out For a point source the mirror has to be shaped like an ellipsoid, and the source and the image have to be placed in the respective foci. The long distances involved in synchrotron work mean that a good approximation to shape is achieved by making use of bent cylindrical mirrors ... 

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