Subsurface Laboratory Methods 225 



with the Geiger-counter apparatus seem to be lacking. H. F. Carl has 

 described a method which he found to yield satisfactory quantitative accu- 

 racy.^^ Whatever method is selected, it should be remembered that differ- 

 ent materials pack differently into the holder, and the operator should 

 first check his technique on a series of synthetic samples of known compo- 

 sition before attempting to use it quantitatively or on unknown specimens. 



Position and Type of Film 



As indicated in figure 92, diffraction lines (rings) can be produced 

 over the entire region from practically 0° to 175° of the 29 angle. Some 

 materials such as metals and inorganic compounds produce patterns rang- 

 ing over the entire region from 0° to 175°, whereas organic compounds 

 produce practically an entire pattern at small angles. Again, certain sec- 

 tions of the region from 0° to 175° may be selected for detailed study, 

 as for example in back-reflection work or studies where extreme accuracy 

 is involved, when the region from 130° to 175° is used (See fig. 96). Con- 

 sequently, the type of camera and film selected depends upon the objec- 

 tive of the investigation. Thus, for example, a camera with the film in the 

 form of a cylinder with the specimen located at the axis is to be much 

 preferred for the identification of rocks, minerals, and soils. 



Most, if not all. X-ray-film emulsions available were developed pri- 

 marily for radiographic work and consequently have a rather high degree 

 of contrast ^" or reveal relatively small differences in absorption by the 

 materials studied. For diffraction work, especially for studying mixtures, 

 a film showing a straight-line function with a moderate slope over a con- 

 siderable range, when the line density, log f -^ j , is plotted against expos- 

 ure, {lot), is desirable (See fig. 97), At present, such film is not generally 

 available, and one must use the emulsions developed for radiographic 

 work, compromising between exposure time and pattern quality. The 

 fastest emulsions usually show considerable background blackening, where- 

 as slower films producing good, clean backgrounds require considerably 

 more exposure. Thus the choice of film rests on a number of conditions. 

 For rapid and only approximate identifications, the fast films are pre- 

 ferred, whereas slower films are used if all possible information is to be 

 gleaned from the pattern. Films as a rule are duplitized; that is, they 

 have emulsions on both sides. To a slight extent, the double emulsion 

 causes diffuseness in the lines, but rarely sufficiently to justify use of single- 

 layer-emulsion film. All films should be developed according to the time, 

 temperature, and processing conditions recommended by the manufac- 

 turer.^^ 



Intensifying screens have been used for cutting down exposure time, 

 but this practice is not recommended for mixtures of minerals because the 



" Carl, H. F., op. cil. 



Radiography of Materials, Rochester, N. Y., Eastman Kodak Co., X-ray Division. 

 Radiography of Materials, Rochester, N. Y., Eastman Kodak Co., X-ray Division. 



