The first prototype diamond monochromator at the Institut Laue-Langevin

We report on the performance of the first diamond neutron monochromator built at the ILL. It has been designed for the hot neutron diffractometer D9 with the aim of improving significantly the instrument performance in particular for short wavelengths in the 0.3-0.9 A wavelength range. Diamond crystal plates with dimensions of 1.5 x 1.5 x 0.18 cm3 an average mosaic spread of 0.15° have been synthesized at the University of Augsburg. They exhibited excellent neutron diffraction properties when examined on a neutron double-crystal test setup. Sufficiently thick diamond elements with a controlled mosaic spread of 0.25° have been obtained by stacking several of these crystals. First tests runs carried out at the ILL confirmed the predicted high reflectivity of the diamond stacks. The diamond prototype monochromator uses the (220) reflection in transmission geometry replacing the Cu (220) monochromator on D9 that has the same d-spacing. The final performance studies on D9 showed that the diamond device did not perform better than the original copper crystal. This unexpected result could be explained by significant optical aberrations caused by non- uniformities of both the angular and spatial mosaic distribution in the individual diamond crystals, as revealed by a detailed characterisation study using high-energy X-ray diffraction.