The thermal diffuse scattering around reflection 220 of a thick, perfect silicon crystal has been studied quantitatively by means of a triple-crystal diffractometer and 100 keV synchrotron radiation. The necessary fitting procedures were simplified by deriving an analytic solution to the instrumental resolution function for nondispersive setting of three perfect crystals. The temperature and e t dependence of the thermal diffuse scattering are very well described, taking only acoustical phonons into account; the contribution of optical phonons has been calculated explicitly.

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