Gamma-ray Compton scattering studies

The method of measuring ground state electron momentum densities using Doppler broadening of Compton scattered radiation, is reviewed. The principles of Compton profile measurements with gamma-ray sources used with energy dispersive photon detectors, are introduced. Particular emphasis is shown in the design and construction of two gamma-ray Compton spectrometers using a 5 Ci ²⁴¹Am (59.54 KeV) annular source, and a 120 Ci ¹⁹⁸Au (411.8 KeV) source. The application of the gamma-ray techniques to a number of molecular and metallic systems and comparison with theoretical profiles, highlights the sensitivity of the Compton profile to subtle changes in the bonding and conduction electron density. Measurements of the momentum density of isotropic hydrocarbons with ²⁴¹Am provide a method of assessing the reliability and validity of the localised molecular orbital and self-consistent-field descriptions of these molecules. It is shown that an average set of localised bond orbitals obtained theoretically are not sufficiently accurate to describe the Compton profile of highly strained cyclic molecules, conjugated molecules and resonant structures. Directional Compton profiles for niobium and niobium hydride using ¹⁹⁸Au show the sensitivity of the profile to subtle changes in the band electron profile following the introduction of hydrogen into the metallic lattice. Overall qualitative changes in the directional profiles are understood within the rigid band approximation. Momentum density studies with the two gamma-ray techniques show that in general, the usefulness of these methods lies not in a determination of an absolute Compton profile but in measurements where intepretation is focussed on the difference between similar profiles