Microbeams and microcameras

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 camera shown in Fig. 5-3 has a motor to rotate the specimen for such integration. The sectored disc on the cassette of this camera is designed for recording two or more partial patterns on one film for comparative purposes. After the first pattern is made, the specimen is changed, the disc is rotated about the collimator by an amount sufficient to cover the previously exposed portion of film and uncover an unexposed portion, and the second exposure is made. Or the disc may be removed, if one wishes to record complete Debye rings from a single specimen. 

Sometimes one has only a minute amount of material from which a diffraction pattern has to be obtained. Or one may wish to obtain a pattern from only a very small region 

The central problem is the design and construction of the collimator and its placement relative to the focal spot of the x-ray tube. Hirsch [6.3] has discussed these matters. Collimators are often made of glass capillary tubing. If such collimators are used with ordinary x-ray tubes, the collimated beam will be weak and exposure times very long, because most of the x-rays coming from the tube are wasted. The proper procedure is to use a microfocus tube (Sec. 1-7). 

While it may be possible to adapt an ordinary pinhole camera to some microbeam work simply by changing the collimator, better results will be obtained with a specially designed microcamera [6.3, G.39]. Such a camera will usually have a small specimen-to-focal-spot distance (to increase intensity and improve collimation), a small specimen-to-film distance (to reduce exposure time), and some arrangement for accurately positioning the specimen in the beam. Diffraction patterns of specimens amounting to as little as 10 micrograms have been obtained in such cameras. 

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