Asymmetrically grooved monolithic crystal monochromators for suppression of harmonics in synchrotron X-radiation

To suppress unwanted harmonics in crystal-diffracted synchrotron X-radiation, monolithic grooved-crystal monochromators have been studied in theory and experiment. Simply by building non-parallel groove walls in a perfect crystal, the overlap of dynamical diffraction ranges on the two crystals is canceled for higher harmonics, while a partial overlap is preserved for the fundamental, leading to a substantial improvement in fundamental-to-harmonics ratio in the diffracted beam. A good silicon 111 monochromator with wall angles of 0 and 70 delivers a beam of harmonic contamination better than 3.3% at fundamental wavelengths between 1.2 and 1.6 A with a beam intensity exceeding 50% of that available from a standard channel-cut crystal. Silicon 331 and 511 versions can achieve harmonic contaminations well below 0.1%. Asymmetrically grooved monochromators can work as stable wavelength scanners free from harmonics requiring no delicate control of angle. (Auth.)