and consists of a series of concentric rings on a flat film, or arcs of rings on a 

 cylindrical strip of film. 



A careful study of Figure 9-2 shows that a fixed space arrangement of atoms 

 with definite fixed distances between them must always produce precisely the 

 same X-ray pattern. Furthermore, if the space arrangement is retained but the 

 distances between atom centers are changed, the X-ray pattern will retain its 

 same general appearance but will either expand or contract. On the other hand, 

 if the space arrangement is altered, the pattern is changed. Consequently, X-ray 

 diffraction patterns are a sort of fingerprint of crystalline materials. Each 

 individual substance present in a mixture will produce its unique diffraction 

 effects, so that the pattern derived from the mixture is a composite of the patterns 

 of all the materials or compounds in the mixture. Furthermore, the intensities of 

 the lines of the individual patterns are functions of the relative amount of the 

 material present in the mixture, so that the method also has quantitative aspects. 



However, the X-ray-diffraction method is sometimes considered rather 

 limited as an analytic tool because the relative sensitivity requires that an appreci- 

 able amount of a constituent (from 1 to 30 percent) (Brosky, 1945) must be 

 present in a mixture before its presence can be detected. Some materials with 

 patterns having reasonably low background intensities and fairly strong lines 

 can readily be detected in concentrations as low as one-half to 1 percent, whereas 

 other materials with weaker patterns, can only be detected when present in 

 considerably larger amounts. Limitation of the number of detectable con- 

 situents in mixtures due to crowding of lines is considered a drawback by some 

 workers using small-diameter cameras with large pinhole systems (Brosky, 1945) . 

 The use of larger camera (10 to 20 cm) diameters, smaller pinhole systems, and 

 longer radiation wave lengths to spread out the patterns and increase the resolu- 

 tion should increase this number considerably. A very important disadvantage 

 of the method is its inability to detect amorphous phases, such as glasses, when 

 present in only limited amounts, and the fact that solid solutions may not 

 always be observed. 



APPARATUS FOR Two general types of apparatus are commonly 



RECORDS NG THE used for recording the X-ray-diffraction pat- 



DIFFRACTION PATTERNS tern. The difference in these types of appar- 

 atus arises in the manner in which the X-ray- 

 diffraction patterns are recorded; one type uses the conventional diffraction 

 camera with photographic film registering differences in line intensities as line 

 blackness, and the other, a counter tube with a scaling circuit that may be used 

 to measure the intensity of the diffracted rays, records intensity either by the 

 counting technique or by automatic recording apparatus. The principle of the 



153 



