X-RAYS AND ORIENTATION OF QUARTZ CRYSTALS 



327 



plate beyond the crystal. This necessitated either the use of a very thin 

 crystal or a long exposure. 



To avoid the variation due to crystal thickness we adopted the "Back 

 Reflection Laue camera." As shown in Fig. 3.23 the X-ray beam passes 

 through a hole in the photographic film before striking the crystal. It is 

 coUimated by two pinholes, one on each side of the film. Spot reflections 

 from many planes fall on the film and in a few minutes exposure leave a 

 record of their points of impingement. Most of these reflections are not 

 due to the peaks of the radiation curve (Fig. 3.2) but each spot is due to a 



edge; guide 



Fig. 



3.23 — Arrangement of collimator, film and crystal plate for a back reflection Laue 



photograph 



different X from the continuous background of Fig. 3.2. However, each 

 satisfies the equation n\ = 2d sin d. 



Figure 3.24 is such a record with many of its spots marked. The spot 

 (01-1) is recognized as the point of intersection of the greatest number of 

 rows of spots. The spot (01-2) is the second most obvious intersection 

 point. 



On examining such a film we recognize such spot configurations and then 

 mark the indices of the corresponding atomic plane for a few chosen spots. 

 From these we can measure three angle-errors. For example, if the crystal, 

 Fig. 3.25, is rotated about the vector t by amount Ct the spot pattern on the 

 film will be rotated by the same amount e^ . If the crystal is in error by 

 amount e^ being rotated about ivw clockwise the spot pattern of the film 



