The calibration device discussed here has been developed from that described by Pettifer and Hermes . The device allows absolute calibration of the X-ray energy from a synchrotron source. From Bragg's law the energy or wavelength of a diffracted beam from a perfect crystal may be calculated with knowledge of the lattice spacing of the crystal and the Bragg angle. Accurate values of the lattice spacing of silicon may be achieved from interferometry measurements but accurate measurement of diffraction angles is more difficult. The calibration device described here uses the known angles between lattice planes in a perfect silicon crystal as a reference on which to make angle measurements. The apparatus records Bragg peaks in back reflection, excited in a perfect silicon crystal of known lattice constant, as a function of the monochromatic X-ray energy. The orientation of the silicon crystal with respect to the incident beam vector is found by refining the calculated positions in energy space of the Bragg peaks for a specific crystal orientation against the experimentally measured positions. A smoothly varying offset is usually observed between the experimental and calculated diffraction line energies and is used to form a calibration curve so that all energies over the scanning range may be calibrated.
The design of the original apparatus was changed so as to allow it to be used in diffraction experiments requiring energy calibration, e.g anomalous scattering studies. Secondly a novel method of using the apparatus as a X-ray energy monitoring device has been developed which has the potential of allowing the energy to be monitored and corrected if necessary while a diffraction experiment is being performed whereas most optimised MAD experiments to date have involved sporadic calibration of the X-ray energy at various stages of the data collection but never during the measurement of diffracted intensities.