Abstract
The inelastic scattering of neutrons from polycrystalline reactor grade graphite has been measured at graphite temperatures of 20 degrees C, 1000 degrees C and 1920 degrees C, and for incident neutron energies of 0-0015 eV, 0.00246 eV and 0.00445 eV. With these incident energies, step changes in the cross section are clearly visible, and it can be shown that these originate from the momentum conservation condition associated with scattering from longitudinal phonons travelling normal to the graphite net planes; the position of the steps thus yield points on this dispersion curve. The data produced are consistent with sinusoidal dispersion curves, and the corresponding force constant between adjacent planes, C33, can therefore be determined. This force constant decreases rapidly with increasing temperature indicating the importance of anharmonic terms in the analysis of the thermal properties of graphite.