conventional film camera type is shown schematically in Figure 9-3 and the 

 counter-recorder type in Figure 9-4. 



Film methods for recording diffraction patterns are less subject to instru- 

 mental variation than counter methods; are more positive in recording weak 

 lines and, likewise, record other effects such as orientation effects. Hence, even 

 if these methods are somewhat slower than the counter methods, they do produce 

 a complete and permanent record which is easily interpreted and much more 

 convenient to store than strip chart records. 



X-ray beam 



, rac tion 

 lines 



Action 



cones 



Figure 9-3. Schematic diagram showing principles of the conventional powder camera. 



SELECTION AND Not enough emphasis can be placed on the 



PREPARATION OF selection of the sample for analysis. Because 



SAMPLE SPECIMEN only a very small fraction of the sample 



placed in the camera is actually exposed to the 

 X-ray beam, it is imperative that all precautions be observed in the choice and 

 preparation of the sample. The sample used must be truly representative of the 

 material being investigated as regards composition, structure or other character- 

 istics for which the sample is examined. 



Single Crystals 



Single-crystal patterns are seldom made when identification of the material 

 is the only objective, as the powder method is usually considerably simpler. 



154 



