Double crystal x-ray and gamma ray diffractometry

Abstract

An introduction to the dynamical diffraction theory is given and its results are discussed in relation to double crystal diffractometry in the various arrangements. A review of the work using this technique is also given. The micro-computer controlled diffractometers, the EGG and the COFFIN together with the interface system are described. The software written for the automatic control of the diffractometer is given in detail. The micro computer controlled diffractometer has proved a great advantage in that it reduces tremendously the time of the experiment and made possible the experiment using γ-rays which otherwise is impossible to carry out due to its low intensity and high background. Results are presented of a study of the damage introduced into silicon when cut with a diamond tool on a lathe. The depth of damage introduced is investigated by measuring rocking curves full width at half height maximum as the crystal was etched. The result obtained shows that the bulk of the surface damage introduced is confined to the first 5µm of the crystal which is completely removed after a further 100µm is removed and is independent of the tool type. Therefore diamond turning can replace the time consuming hand lapping process with a reduction in cost and would therefore be advantageous in the production of mono- chromators and mirrors of non-standard shapes and sizes. Previous work using γ-ray energy is reviewed. The double crystal technique using gamma rays is potentially a powerful tool for the measurement of lattice tilts in thick perfect single crystals. The narrow rocking curve (less than 0.5 arc seconds) and the long counting time (up to 6 minutes per point) necessitates the use of micro-computer. Results are presented of experiments to establish the feasibility of the method. The perfection of the triangular monochromator crystal used at the Protein Crystallography Station, Daresbury Laboratory was also investigated from rocking curve measurements. The effects of bending the triangular monochromator crystal on the FWHM and integrated reflecting power were studied and the measurement of curvature was made on the Lang camera. The 6-fold increase in the integrated intensity of an unpolished monochromator over a polished one could be a great advantage for protein crystallography experiments. This has yet to be tested at the Daresbury station. Results are presented on the effect of a magnetic field parallel to the two fold axis on the double crystal rocking curves of hematite crystal which indicate a complex behaviour where multiple splitting occurs and is extremely unstable. Double crystal topographs of Indium phosphide and gallium arsenide crystals indicate the usefulness of synchrotron radiation to crystal growers for a rapid and routine assessment of crystal perfection.