High resolution X-ray diffractometry

Better conventional collimation will not do, except for the largest synchrotron radiation installations to obtain snb-arc-second collimation in the laboratory would require a collimator some 100m long with a sealed-tube source, and at this distance the intensity would be impracticably low. The problem is solved by the use of a beam conditioner, which is a further diffracting system before the specimen The measnred rocking cnrve is then the correlation of the plane wave rocking cnrves of the beam conditioner and the specimen crystals, from which most of the diffracting characteristics of the specimen crystal may be deduced. 

The simplest conditioner is a perfect crystal of the same type as the specimen, using the same reflecting planes, with the deviation of the diffracted beam in the opposite sense to that at the specimen. This is the classic +, - symmetrical double crystal method , as shown inFigrrre 1.5, which gives excellent and easily interpreted resrrlts. Many variations are, however, possible, for example to maximise the sensitivity to strain, or to emphasise the contribution of near-srrrface layers to the diffraction, and we shall treat these in detail in this book. 

The great ntihty of rocking ctrrve measnrements is based upon two frmdamental properties  

1 The details of the rocking ctrrve are extremely sensitive to the strains and strain gradients in the specimen. 

2 For a given stractmal model, the rocking curve may be computed to high accttracy using fundamental X-ray scattering theory. 

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