Back-reflection pinhole camera

Back-reflection pinhole camera. If the incident beam is normal to the sheet specimen and therefore parallel to the fiber axis, and a projection like that shown in Fig. 9-10(b) is made (projection plane parallel to sheet), then both the incident beam and the fiber axis coincide with the center of the projection. If the texture is ideally sharp, the (hkl) pole figure will consist of one or more concentric circles centered on the center of the projection, and the chance that one of these pole circles will coincide with the concentric reflection circle is essentially zero no reflection will occur. But if the texture has enough scatter, one of the pole circles will broaden into a band wide enough to touch the reflection circle at all points a Debye ring of uniform intensity will be formed. See Prob. 9-7. Thus a uniform Debye ring is not always evidence for randomly oriented grains. 

For given values of B and n, which results in a greater effective depth of x-ray penetration, a back-reflection pinhole camera or a diffractometer ... 

An electroplated layer of copper on sheet steel is examined in a back-reflection pinhole camera with Cu Ka radiation incident at right angles to the sheet surface. Assume the copper has a fiber texture with the fiber axis [ww] scattered by an angle p in every direction about the sheet normal. How large must p be for the 420 Debye ring (see Table 4-2) to appear on the film if the fiber axis [//yw] is (a) [110], (b) [100] ... 

In this case it may be shown that the fractional error in d is proportional to sin 4(f) tan = (90° — 0). With either of these extrapolation functions a fairly precise value of the lattice parameter can be obtained in addition, the back-reflection pinhole camera has the particular advantage that mounted metallographic specimens may be examined directly. This means that a parameter determination can be made on the same part of a specimen as that examined under the microscope. A dual examination of this kind is quite valuable in many problems, especially in the determination of phase diagrams. 

When monochromatic radiation is used to examine a powder specimen in a Laue (flat-film) camera, the result is often called, for no particularly good reason, a pinhole photograph. (There is no general agreement on the name of this method. Klug and Alexander [G.39], for example, call it the monochromatic-pinhole technique. ) Either a transmission or a back-reflection camera may be used. A typical transmission photograph, made of fine-grained aluminum sheet, is shown in Fig. 6-11. 

The pinhole method is used in studies of preferred orientation, grain size, and crystal quality. With a back-reflection camera, fairly precise parameter measurements can be made by this method. Precise knowledge of the specimen-to-film distance D is not necessary, provided the proper extrapolation equation is used (Chap. 11) or the camera is calibrated. The calibration is usually performed for each exposure, simply by smearing a thin layer of the calibrating powder over the surface of the specimen in this way, reference lines of known 0 value are formed on each film. 

The pinhole camera, used in back reflection, is not really an instrument of high precision in the measurement of lattice parameters, but it is mentioned here because of its very great utility in metallurgical work. Since both the film and the specimen... 

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