Abstract
Extinction distance is a parameter which, for a given material and reflection, is normally assumed to depend on accelerating voltage and the deviation from Bragg condition of the reflection concerned. The results to be presented here indicate that, under certain conditions, crystal thickness is also important in deter mining the value of the extinction distance obtained. Theoretical values of extinction distance have been determined from measurements of the peak to peak spacing in plots of diffracted beam intensity as a function of crystal thickness. All calculations were carried out by using the ‘multi-beam dynamical theory. Fig. 1 shows the variation of bright field intensity with crystal thickness for fcc cobalt when the (111) reflection is in the exact Bragg position. The systematic reflections 333…555 were taken into account, and an accelerating voltage of 150 Kv was assumed. From these results it can be seen that in thinner parts of the crystal the extinction distance is about 280Å. The results in Fig. 1 also shows that in thicker regions of the crystal the extinction distance decreases to about 180A°.
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