A middle energy‐bandwidth X‐ray monochromator for high‐flux synchrotron diffraction: revisiting asymmetrically cut silicon crystals

To supply the growing demand for high photon flux in synchrotron science including surface diffraction, a middle energy‐bandwidth monochromator covering the 10−4 to 10−3 range has been adapted by applying an asymmetric diffraction geometry to a cryogenically cooled silicon 111 double‐crystal monochromator used as a standard for the undulator source at SPring‐8. The asymmetric geometry provides a great advantage with its ability to configure flux gains over a wide energy range by simply changing the asymmetry angle, while the angular divergence of the exit beam remains unchanged. A monolithic design with three faces has been employed, having one symmetrically cut and another two asymmetrically cut surfaces relative to the same atomic plane, maintaining cooling efficiency and the capability of quickly changing the reflection surface. With the asymmetric geometry, an X‐ray flux greater than 1014 photons s−1 was available around 12 keV. A maximum gain of 2.5 was obtained relative to the standard symmetric condition. Using an asymmetric diffraction geometry, a silicon double‐crystal monochromator can produce a higher photon flux than the standard symmetric geometry for low‐emittance synchrotron radiation X‐rays, while the outgoing beam from the monochromator maintains its narrow angular spread. These features are well suited to synchrotron diffraction and scattering.