In order to explain the workings of the precession camera it is first necessary to develop some ideas about the reciprocal unit cell and reciprocal lattices.
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When an X-ray photograph is taken of a crystal, the relationship between the X-ray picture and the
crystal unit cell is a reciprocal one. In the diagram on the right, the direct lattice of the crystal
produces a photograph that displays the reciprocal crystal lattice. This particular direct lattice has
c < a and β > 90° ;the reciprocal lattice has a* < c* and β* < 90°.
(Reciprocal cell constants are always “starred”). Bragg's Law (2dsinθ=λ)
hinted at this reciprocal relationship: d (the perpendicular spacing between Bragg planes) is inversely proportional
to the Bragg reflection angle, θ. As d decreases, θ increases. In order to explain the workings of the precession camera it is first necessary to develop some ideas about the reciprocal unit cell and reciprocal lattices. |
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