Monolithic double-grooved-crystal monochromators with tunable harmonic suppression for neutrons and X-rays

The principal methods of employing double-crystal reflection for harmonic rejection are briefly reviewed. The effect of using non-equal d spacings (dispersion) is considered. A new type of perfect-crystal monochromator consisting of a monolithic combination of a pair of grooved crystals a and b is described. The monochromaticity (degree of harmonic suppression) M(β) is tuned by tilting groove a with respect to groove b by an appropriate angle β. The tilting is achieved in a simple way through a pair of helical springs. With three reflections in the first and two reflections in the second groove M(β₀) values between about 300 and 100 have been measured for X-rays at energies ranging from 6.72 to 9.86 keV and at a bandwidth ΔE/E≃1.5%. β₀ is the tilting angle for which M(β) is optimized. Once aligned the monochromator is a very simple unit, which can be mounted on any standard goniometer head and which can be used for both X-rays and thermal neutrons.