Dispersive sandwich-type monochromators based on bent perfect crystals (BPCs) for high-resolution neutron scattering experiments

Dispersive double reflections realized by means of two bent perfect crystal (BPC) slabs of different cuts used as a sandwich can provide a monochromatic beam of excellent resolution parameters. The dispersive sandwich monochromator/analyzer provides the freedom to combine crystal slabs of different cuts, that is, different crystal reflections for the double diffraction process. For some combination of the individual crystal slab, it is possible to achieve the back-scattering resolution for a rather low monochromator take-off angle. Therefore, by using a suitable combination of two slabs, one can practically obtain a monochromatic neutron beam of any wavelength in the thermal region. Depending on the bending radius of the sandwich the resolution Δλ/λ and the Δ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">α</mml:mi> </mml:math> collimation can be continuously adjusted in the range of 5 × 10 −5 –1 × 10 −3 . Such dispersive BPC elements can also be used for the high-resolution analysis of the scattered beam, as well as for the high-precision λ-calibration of the time-of-flight neutron scattering devices. Another advantage of this dispersive monochromator realization is saving one axis of the neutron scattering instrument.