Diffraction efficiency and diffraction bandwidth of thermal-gradient and composition-gradient crystals

Measurements were made at the Advanced Photon Source at Argonne National Laboratory on the diffraction efficiency and diffraction bandwidth of a thermal-gradient crystal (Si) and a composition-gradient crystal (Si-Ge) to which a thermal gradient was applied. Gradient crystals are crystals in which the spacing between crystalline planes varies with the position in the crystal. This change in the crystal plane spacing is obtained by applying a thermal gradient to a single crystal or by growing a two-component crystal in which the ratio of the two components changes with position in the crystal. Measurements were made at two energies, 92.6 and 153 keV. Both crystals were 1-cm cubes. Laue diffraction (transmission diffraction) was used in all experiments. The thermal gradient was applied perpendicular to the [111] diffraction planes of the pure silicon crystal and perpendicular the [400] diffraction planes in the composition-gradient crystal (Si-Ge). The thermal gradient applied to the crystals was quite uniform. This resulted in nearly spherical surfaces for the crystalline planes. Changing the value of the thermal gradient applied to the crystals changed the radius of curvature of the spherical surface. The thermal gradients were varied between 0.0 and 114 degrees C/cm, resulting in radii of curvature from near infinity down to 40 m. By adjusting the thermal gradient applied to the crystals, one obtained diffraction efficiencies between 0.90 and 0.95 and bandwidths of up to 100 arc sec. The diffraction efficiency in the Laue geometry is defined as the ratio of the intensity of the diffracted beam to the intensity of the undiffracted beam that passes through the crystal when there is no diffraction. The length of the crystal traversed by the x-ray beam was 1.0 cm in all cases. (c) 2005 American Institute of Physics.