Structural analysis of diamond mosaic crystals for neutron monochromators using synchrotron radiation

•1-mm-thick diamond mosaic crystals have been grown by heteroepitaxy.•The crystals have been characterized by neutron diffractometry.•Excellent neutron reflectivities have been obtained.•Deviations from theoretical data for ideal mosaic crystals have been observed.•Synchrotron radiation measurements show that the deviations are due to a non-uniform distribution of the mosaic spread.

The beams extracted from thermal neutron sources such as nuclear reactors are monochromatised by Bragg diffraction using imperfect single crystals with an angular mosaic spread of typically 0.2–0.8°. For neutron wavelengths below 1.5 Å, the highest reflectivity of all crystalline materials is expected for diamond. Nowadays diamond single crystals with an appropriate mosaic spread exceeding a thickness of 1 mm can be grown by heteroepitaxy on an Ir/yttria-stabilised zirconia bilayer deposited on a Si(001) single crystal.To explain the observed neutron reflectivity being below the theoretically expected value, we have studied the spatial distribution of the mosaic structure of two crystals by high resolution X-ray diffraction using a laboratory X-ray source and synchrotron radiation. The first sample (A) showed a uniform mosaic spread of 0.18° ± 0.02° across the 1 cm wide sample. The peak shift of the X-ray rocking curves of 0.08° indicated a weak curvature of the crystal lattice. The measured absolute neutron peak reflectivity of 34% corresponded to 90% of the value predicted by theory. The peak width of the neutron rocking curve for the second sample (B) was twice as big, but here the peak reflectivity of 13% corresponded to only half of the theoretical value.This unfavourable behaviour could be assigned to a substantial spatial variation of the mosaic spread deduced from the synchrotron X-ray studies. X-ray diffraction with high spatial resolution indicated a mosaic block size below 50 μm for sample A. This was consistent with chemical etching experiments on the surface of a comparable sample which showed both randomly distributed dislocations and others that are arranged in boundaries of several 10 μm large domains.