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

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.