Cameras for special conditions

Various kinds of special cameras have been devised for obtaining diffraction patterns from specimens subjected to unusual conditions of temperature or pressure [G.39]. These cameras, usually of the Debye-Scherrer type and usually homemade, have designs that vary almost from laboratory to laboratory. 

These cameras are usually designed for specimen temperatures down to about — 150°C. Diffraction studies at still lower temperatures can be more easily made with the diffractometer. 

The simplest cooling method is to run a thin stream of coolant, such as liquid air, over the specimen throughout the x-ray exposure. The diffraction pattern of the coolant will also be recorded but this is easily distinguished from that of a crystalline solid, because the typical pattern of a liquid contains only one or two very diffuse maxima in contrast to the sharp diffraction lines from a solid. Scattering from the liquid will, however, increase the background blackening of the photograph. 

A better method is to cool the specimen with a stream of cold gas. Liquid nitrogen, for example, boils at 77°K ( = - I96°C) and can easily produce nitrogen gas at the specimen at about — 150°C. Gas produces much less background scattering than a liquid coolant. 

Post [6.2] has prepared a bibliography on low-temp)erature techniques. 

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