Richard Gillilan schrieb:
There's a simple rule of thumb I have heard for predicting if spot overlap is likely to be a problem for a given beam divergence: 500 A of unit cell corresponds to 1 mr of divergence. So, for example, 250 A would be a unit cell limit for a 2 mr beam. Has anyone heard of this rule before? I heard it from someone who heard it from someone, so I have no original reference and don't know how valid it is.There are a number of factors which influence spot overlap: beam divergence, mosaic spread of crystal, point spread function of detector, and resolution range of interest. I would love to find some references to simple estimates based on these parameters. Best I have seen so far is a paper by Sarvestani et. al. (J. Appl. Cryst. (1998) 31 899-909, but it is a detailed simulation rather than a single formula.Richard Gillilan MacCHESS
Richard,Blundell-Johnson (1976) write on page 278: "The maximum permissible angular range delta-phi which avoids overlap is given by
delta-phi = P*/dm* - deltawhere p* is the relevant reciprocal lattice spacing and dm* is the maximum resolution in reciprocal space".
but they mean overlap resulting from rotation of crystal ("rotational overlap"), whereas your rule of thumb applies to "spatial overlap" of simultaneously occuring spots on the detector.
Your rule may arise from the following consideration: let's assume 1 A wavelength, and 500 A cell axis (for a,b,c). Then Bragg's law will give an angle between the diffracted x-rays of close to 1/500, which is 2 mrad. If your beam has 1mrad divergence, you might just be able to resolve the spots on the detector.
best, Kay
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