Neutron Scattering Halo Observed in Highly Oriented Pyrolytic Graphite

TL;DR

This paper reports the first observation of an anomalous neutron halo scattering in highly oriented pyrolytic graphite, revealing a persistent ring pattern that varies with wavelength, sample orientation, and temperature, indicating complex underlying scattering mechanisms.

Contribution

It introduces the first observation and analysis of a neutron halo scattering phenomenon in HOPG, expanding understanding of neutron interactions with layered graphite structures.

Findings

The scattering ring persists beyond the Bragg cutoff wavelength.

Ring position shifts with sample tilting and wavelength.

Ring intensity diminishes at low temperatures.

Abstract

Highly oriented pyrolytic graphite (HOPG) has been widely used as monochromators, analyzers and filters at neutron and X-ray scattering facilities. In this Letter we report the first observation of an anomalous neutron Halo scattering of HOPG. The scattering projects a ring onto the detector with half cone angle of 12.4 degree that surprisingly persists to incident neutron wavelengths far beyond the Bragg cutoff for graphite (6.71 A). At longer wavelengths the ring is clearly a doublet with a splitting roughly proportional to wavelength. While the ring centers at the beam position if the beam is normal to the basal planes of HOPG, sample tilting results in the shift of the ring towards the same direction. The angle of ring shift is observed to be less than the sample tilts, which is wavelength dependent with longer wavelengths providing larger shifts. Additionally, upon tilting, the ring broadens and splits into a doublet at the low angle side with short wavelength neutrons whereas only subtle broadening is observed at longer wavelengths. The ring broadens and weakens with decreasing HOPG quality. We also notice that the intensity at the ring positions scales with the sample thickness. The ring vanishes as the sample is cooled down to 30 K, suggesting that the lattice dynamics of graphite is one of the factors that cause the scattering ring. A possible interpretation by combining inelastic scattering and Yoneda scattering is proposed.