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The intensity changes produced in single-crystal diffraction reflections when one or more secondary reflections occur simultaneously are discussed both theoretically and experimentally. The theory is an extension of the usual treatment of secondary extinction, based on the mosaic crystal model. An approximate solution, valid in the thin crystal limit, is in good agreement with neutron diffraction experiments on single crystals of iron. Both theory and experiment demonstrate the importance of sample geometry on the magnitude and sign of the simultaneous reflection effects. The effects may be minimized by controlling the sample geometry in addition to the usual precautions taken to reduce secondary extinction.

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