228 Subsurface Geologic Methods 



copper is chosen almost invariably because it favorably combines sample 

 penetration with a reasonably expanded pattern of good quality. 



Since the K X-ray spectrum always contains characteristic radiation 

 of several wave lengths, suitable filters ^^ ^^ or a crystal monochromator 

 should be employed to produce reasonably monochromatic radiation and 

 thus avoid superposition of lines from a second pattern derived from Kp 

 radiation. In patterns of pure substances or very simple mixtures, the 

 position of Kp lines can be calculated and the lines disregarded in the 

 interpretation of the data. However, the Kp radiation should be removed 

 when making patterns of mixtures, as such patterns are always very 

 complex and the presence of K^ lines serves only to cause errors and 

 confusion. 



Measurement of Lines in Pattern and Conversion to d Values 



The X-ray pattern usually must be measured and the data used to 

 determine interplanar spacings or unit-cell dimensions. Consequently, all 

 precautions must be taken in the processing of the film to avoid film 

 shrinkage or reduce it to negligible amounts. Film shrinkage will be 

 negligible if the camera is calibrated against a pattern obtained from a 

 known substance such as NaCl, the film of the standard pattern having 

 been developed according to a standard procedure, which thereafter is 

 followed explicitly in the development of all patterns obtained with that 

 camera. Film shrinkage has been found to increase with washing time, 

 especially if prolonged and the shrinkage is not uniform throughout the 

 entire film.^^ Developing procedures can be checked for film shrinkage 

 by exposing or marking on the film fixed lengths before processing. Cor- 

 rection for shrinkage is also frequently made through the use of an internal 

 standard such as NaCl, the line positions of which are accurately known, 

 the pattern for NaCl being superposed directly on the pattern of the 

 unknown. 



A number of measuring devices are offered by manufacturers of 

 X-ray apparatus with which either the diameters or radii of the powder 

 rings can be rapidly and accurately measured in units of length, usually 

 centimeters. These devices could be calibrated directly in KX or A, units 

 (A. = 1.00202 KX units) , but calibration in this way restricts their use to 

 a single type of camera with a fixed radius. Consequently, the measure- 

 ments in centimeters must be converted into interplanar spacings or unit- 

 cell dimensions by calculation or calibration curves indicating ring diame- 

 ters or radii (in centimeters or millimeters) as a function of interplanar 

 spacing (in angstrom units) . For rapid and approximate measurements, 

 a direct-reading scale on transparent plastic can be prepared from calcu- 

 lated or plotted data, interplanar distances equivalent to each ring being 

 read directly when the scale is superimposed on the pattern. 



^ Clark, G. L., op. cit. 

 '^Bunn, C. W., op. cit. 



" Claassen, H. H., and Bow, K. E., Correction of X-ray Powder Diffraction Patterns: Sci. Inst. Rev., 

 vol. 17, pp. 307 £f., 1946. 



