Final state effects in neutron Compton scattering measurements

The single atom momentum distributions of condensed matter systems can be derived using the technique of neutron Compton scattering (NCS). The electron Volt spectrometer (eVS) which is situated at the world's most intense pulsed neutron spallation source, ISIS, has been configured to perform NCS measurements. Interpretation of NCS data requires the use of the impulse approximation, however even at the high energy and momentum transfers obtainable on the eVS deviations from the impulse approximation occur. These deviations are generally known as final state effects (FSE) which manifest themselves as an asymmetry in the measured momentum distribution. The aim of the work reported in this thesis is to demonstrate how final state effects can be accounted for in a simple way using the expansion method described by Sears. An advantage of the Sears method is that the first asymmetric term in the expansion is proportional to the mean Laplacian of the potential, <#nabla#"2V>, thus giving access to further information on the single atom potential. The Sears expansion has been incorporated into data analysis routines and applied to measured data on three systems that were chosen to represent the systems that are regularly investigated using the eVS. Measurements have been carried out on the deuteron in ZrD_2, a light atom in a heavy lattice, beryllium, a polycrystalline solid and pyrolytic graphite, an aligned crystalline sample with an anisotropic momentum distribution. The study shows how the new analysis method gives more reliable values for the mean kinetic energy <E_k>, which can be derived from the measured momentum distribution. A comparison of measured data with simulated data calculated within the harmonic approximation reveals how <#nabla#"2V> can be a sensitive probe of anharmonicity of the interatomic potential. An anisotropy in the derived <E_k> and <#nabla#"2V> of pyrolytic graphite has been measured indicating the dependence of final state effects on bonding strength. The derived <#nabla#"2V> imply that the potential for vibrations of atoms perpendicular to the planes is anharmonic. (author)Available from British Library Document Supply Centre-DSC:DXN032173 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo