Elimination of dispersive line broadening due to a second-axis crystal with an (n, -n, n) diffraction spectrometer

A triple-axis double-beam X-ray spectrometer has been used for high-precision differential lattice-parameter measurements of perfect Si and Ge crystals. When two Ge crystals (the standard and the sample) were mounted on the second axis and two Si crystals (the monochromator and the analyzer, respectively) were on the first and third axes, the second-axis rocking curve was wide because of significant dispersion effects. The third-axis rocking-curve width was instead much smaller and equal to the convolution of the Darwin widths of the monochromator and of the analyzer. The two Ge crystals acted as mirrors and did not contribute to the width of the third-axis rocking curve. This finding is explained by means of the Du Mond diagrams.