Neutron reflectivity and lattice spacing spread of pyrolytic graphite

The neutron reflectivity and lattice spacing spread was measured at 4.42 Angstrom on two grades of highly oriented pyrolytic graphite (HOPG), each with neutron mosaic spreads in the range 0.6-1.6 degrees. The peak reflectivity of the more disordered grade was 5-10% higher than that of the more ordered grade, and the average interlayer spacing was similar to 0.003 Angstrom higher. The d-spacing spreads of the more disordered grade were 0.08-0.12% and those of the more ordered grade were 0.03-0.07%. By combining the two grades into composites containing one to four layers of each type, it was possible to increase the effective d-spacing spread to 0.15-0.18%. The increased Delta d/d should yield higher neutron beam intensities in certain types of backscattering instruments. (C) 2000 Elsevier Science B.V. All rights reserved.