Development of multilayer monochromators for stripe-free X-ray imaging

Multilayer (ML) monochromators are essential components in synchrotron radiation facilities, widely used for X-ray imaging and X-ray diffraction, as well as select X-ray spectroscopy techniques. However, ML monochromators introduce stripe artefacts to the reflected beam, which degrade the quality of the X-ray images. These stripe artefacts originate from figure errors on the monochromator's surface, which are challenging to minimise for most manufacturers. In this study, we demonstrate stripe-free imaging from ultra-high-quality ML monochromators. We employed a state-of-the-art ion beam figuring (IBF) technique to produce multilayer substrates with a cutting-edge slope error of less than 30 nrad root mean squared (rms). These substrates were coated in an advanced multilayer deposition system, enabling the production of uniform multilayer coatings. The performance of the ML monochromators was tested at the B16 test beamline at the diamond light source. Speckle-based metrology was used to verify the theoretical link between wavefront curvature and the appearance of stripe artefacts. We obtained stripe-free X-ray images from the newly fabricated ML monochromators in both single-bounce and double-bounce configurations, with excellent image clarity and flat field uniformity. This represents a breakthrough in the production of ML monochromators.