Seemann-Bohlin camera

This focusing principle is exploited in the Seemann-Bohlin camera shown in Fig. 6-7. The slit 5 acts as a virtual line source of x-rays, the actual source being the extended focal spot on the target T of the x-ray tube. Only converging rays from 

The ends of the film strip are covered by knife-edges M and N, which cast reference shadows on the film. The value of 0 for any diffraction line may be found from the distance U, measured on the film, from the line to the shadow of the low-angle knife-edge N, by use of the relation 

In practice, 6 is found by calibrating the camera with a standard substance of known lattice parameter, such as NaCl, rather than by the use of Eq. (6-4). 

The resolving power, or ability to separate diffraction lines from planes of almost the same spacing, is therefore twice that of a Debye-Scherrer camera of the same radius. In addition, the exposure time is much shorter, because a much larger specimen is used (the arc AB of Fig. 6-7 is of the order of 1 cm) and diffracted rays from a considerable volume of material are all brought to one focus. The Seemann-Bohlin camera is, therefore, useful in studying complex diffraction patterns, whether they are due to a single phase or to a mixture of phases such as occur in alloy systems. 

On the debit side, the Seemann-Bohlin camera has the disadvantage that the lines registered on the film cover only a limited range of 26 values, particularly on the low-angle side. The Seemann-Bohlin camera, in itself, is now virtually obsolete (the diffractometer has greater resolution), except in combination with a monochromator. The combination is then called a Guinier camera (Sec. 6-14). 

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It employs the same focusing principle as the Seemann-Bohlin camera, but the film straddles the slit and the specimen is placed diametrically opposite the slit. Means are usually provided for slowly oscillating the specimen through a few degrees about the camera axis in order to produce smooth diffraction lines. A typical film, punched in the center to allow the passage of the incident beam, is shown in Fig. 6-9. The value of 6 for any diffraction line may be calculated from the relation... 

The resolving power of this camera is therefore the same as that of a Seemann-Bohlin camera of the same diameter. 

Fig. 6-7 Seemann-Bohlin focusing camera. Only one hkl reflection is shown.

The powder pattern of the unknown is obtained with a Debye-Scherrer camera or a diffractometer, the object being to cover as wide an angular range of 20 as possible. A camera such as the Seemann-Bohlin, which records diffraction lines over only a limited angular range, is of very little use in structure analysis. The specimen preparation must ensure random orientation of the individual particles of powder, if the observed relative intensities of the diffraction lines are to have any meaning in terms of crystal structure. After the pattern is obtained, the value of sin 9 is calculated for each diffraction line this set of sin 9 values is the raw material for the determination of cell size and shape. Or one can calculate the d value of each line and work from this set of numbers. 

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