The diffraction physics of X-ray interference phenomena is also of great theoretical and experimental interest. Imperfections can reveal the modes of growth of crystals, both natural and synthetic. They also affect the physical properties of materials. For example, the presence of nitrogen in diamond greatly reduces the otherwise exceptionally high thermal conductivity. The facilities of the College Electron Microscope Unit are also employed for microprobe analysis and for scanning electron microscopic studies of surfaces.
Work being done at low temperatures reveals electric charge states of vacancies.
Another exciting area being pursued is positronium production at condensed monolayers of gases on graphite which is yielding new insights into two-dimensional behaviour, an approach originating in this laboratory. Positron annihilation is investigated with high-resolution photon detectors and we plan to extend the work to liquids. Positronium production at condensed layers of hydrogen on graphite P C Rice-Evans, R Morton and D P van der Werf Mat. D P van der Werf, M Nathwani, A C Towner, J W Taylor, R Morton, A P Knights, P C Rice-Evans and A Szekeres Appl. Twinning in natural diamond.