A method for aligning crystals in precession photography using monochromated radiation

Routine alignment procedures based on the position and shift of white radiation streaks in the zero-level Laue cone are unsuitable if monochromated radiation is used in precession photography. Correct orientation can be achieved using the following two step procedure: (1) Owing to misalignment of the crystal some reflections can be shifted off the trace of the zero-level Laue cone. This can result in an unequal distribution of reciprocal-lattice rows within the upper (or lower) semicircle along the horizontal (and/or vertical) diameter of a reference circle generated by a reciprocal-lattice plane truly parallel to the precession axis. The amount of angular rotation necessary to bring an equivalent number of rows into the two semicircles constituting the trace of the zero-level Laue cone is first measured. (2) Since an error of at most 2°20′ will remain after step 1 (for μ = 10°) the correct orientation is obtained by a series of sequential corrections continued until an equal intensity distribution is obtained from symmetry-related reflection spots.