Scatter slit

Fig. 7-12. The curved-crystal spectrometer of Adler and Axelrod, showing a polished ore specimen in position. (1) Microscope stage (2) polished ore sample (3) crystal support block (4) Geiger counter and scatter slits. (Courtesy of Adler and Ayelrod and the U. S. Geological Survey.)...

Mispositioned slits result in discrete slit scattering. Slit scattering is recognized by thin and intense streaks in horizontal and/or vertical direction that extend outward from the beam stop. Take out the sample or rotate it in order to make sure that the streaks are not resulting from an interaction of the beam with the sample. Horizontal streaks can be affected by moving horizontal slit edges. 

Fig. 63. X-ray optical system of a Geiger-counter diffractometer by Xorth American Philips Co. Inc. A, X-ray tube target B, Soller slits <7, scatter slit D, specimen Et diffractometer axis F, Soller slits G> counter entrance slit.

The second slit box is located on the detector arm between the sample and the detector. The slit nearest to the sample serves as a scatter slit. It is followed by another Soller slit and a receiving slit positioned just before the detector. The detector in this case is a solid-state detector, which is cooled by a built-in Peltier refrigerator enabling to adjust and maintain the detector sensitivity at extremely narrow width to allow only x-ray photons of specific energy to be registered. Monochromatization of the diffracted x-ray beam is, therefore, achieved electronically rather than by physical means (e.g. by a P-filter or a crystal monochromator), which increases the registered diffracted intensity by eliminating losses in the filter or in the monochromator. 

Figure 3.33. The schematic of monochromatization of the diffracted beam using a curved crystal monochromator. RS - receiving slit, M - curved monochromator, MS -monochromator scatter slit, D - detector. The dash-dotted arc represents the goniometer circle. The dashed arc shows the focusing circle of the monochromator.

Figure 3.34. The set of x-ray powder diffraction patterns collected from the nearly spherical LaNi4 gsSno.is powder (see Figure 3.32, inset) on a Rigaku TTRAX rotating anode powder diffractometer using Mo Ka radiation. Goniometer radius R = 285 mm receiving slit RS = 0.03° flat specimen diameter d = 20 mm. Incident beam apertures were 0.05, 0.17, 0.25, 0.38, 0.5, 0.75, 1, 1.5, 2° and completely opened ( 5°), respectively. An automatic variable scatter slit was used to reduce the background. The data were collected with a fixed step A20 = 0.01°, and the sample was continuously spun during the data collection.

As noted above, a scatter slit can be used to reduce the background noise before it reaches the detector. The aperture of the scatter slit should be selected to enable unobstructed passage of the monochromatic diffracted beam at any Bragg angle, see Figure 3.40. In this example, the scatter slit ScS is wide enough to transmit the beam without affecting its intensity. On the contrary, the scatter slit ScS is too narrow, and only a fraction of the diffracted intensity will reach the detector. 

Figure 3.40. Examples of proper (ScS) and improper (ScS ) selection of scatter slit aperture. DS - divergence slits, RS - receiving slits.

Figure 3.41. The schematic of goniometer optics during data collection employing variable divergence and scatter slits apertures, which enables one to maintain the irradiated area of the sample constant at any Bragg angle. DS - divergence slit, ScS - scatter slit, RS - receiving slit.

Chart Speed Divergence Slit Receiving Slit Scatter Slit ... 

A second divergence slit Urtrits the in-plane divergence of the beam. The diffracted beam can also pass through a second Soller slit or monoehrorrtator before striking the detector. Scatter slits may be used to reduee baekgrourrd. 

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