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- Fischer, M., et al.
(författare)
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Structural analysis of diamond mosaic crystals for neutron monochromators using synchrotron radiation
- 2013
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Ingår i: Diamond and Related Materials. - : Elsevier BV. - 0925-9635. ; 37, s. 41-49
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Tidskriftsartikel (refereegranskat)abstract
- 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 degrees. For neutron wavelengths below 1.5 angstrom, 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 degrees +/- 0.02 degrees across the 1 cm wide sample. The peak shift of the X-ray rocking curves of 0.08 degrees 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 pm 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 mu m large domains. (C) 2013 Elsevier B.V. All rights reserved.
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