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Anomalous neutron scattering `halo' observed in highly oriented pyrolytic graphite...

Publication Type
Journal Name
Journal of Applied Crystallography
Publication Date
Page Numbers
296 to 303

Highly oriented pyrolytic graphite (HOPG) has been used as monochromators, analyzers and filters at neutron and X-ray scattering facilities for more than half a century. Interesting questions remain. In this work, the first observation of anomalous neutron `halo' scattering of HOPG is reported. The scattering projects a ring onto the detector with a half-cone angle of 12.4°, which surprisingly persists to incident neutron wavelengths far beyond the Bragg cutoff for graphite (6.71 Å). At longer wavelengths the ring is clearly a doublet with a splitting roughly proportional to wavelength. Sample tilting leads to the shift of the ring, which is wavelength dependent with longer wavelengths providing a smaller difference between the ring shift and the sample tilting. The ring broadens and weakens with decreasing HOPG quality. The lattice dynamics of graphite play a role in causing the scattering ring, as shown by the fact that the ring vanishes once the sample is cooled to 30 K. A possible interpretation by multiple scattering including elastic and inelastic processes is proposed.