Using a prism to reject or select harmonic reflections in an X-ray monochromator

The angle by which an X-ray deviates from its original direction when it passes through a refractive prism is proportional to the square of the X-ray wavelength. This effect was used to discriminate between different harmonic orders of a two-crystal non-dispersive monochromator. Two cases are discussed. In the first case, a refractive prism allows tuning of the silicon 333 reflection to the peak of its double-crystal rocking curve while the higher-order harmonics are suppressed, with much of the harmonic rejection being controllable by the angle of the prism. In the second case, it is demonstrated that slightly tuning the second crystal of the monochromator allows the selection of a specific harmonic order from among reflections 333, 444 and 555.