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Why are "Laue Spots" spots instead of rings? Or is there an effective way to estimate the number of Laue spots? I can't find any formulas or theory concerning this issue. Thanks for your attention!

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Laue spots / rings are just the transmission mode analogue of Bragg diffraction spots / rings. A crystalline sample will show a spot pattern while a powdered sample (lots of small randomly oriented crystallites) will show rings.

See any entry level solid-state textbook (e.g. First chapters of Kittell) for a derivation of the "Laue condition" that determines where these spots appear.

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Thank you very much! I need some efficient way to estimate the magnitude of order of the number of spots, is there some effective way to estimate? –  Golbez Jul 10 '12 at 7:14
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It will depend on how finely the powder is ground. Assuming random orientation of the crystals, each crystal illuminated by the X-rays will generate a pattern of spots. If the crystals are large there will be few of them and you'll get a pattern of discrete spots. If the crystals are small there will be so many of them that the spots merge to create the rings. –  John Rennie Jul 10 '12 at 9:10

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