Si1−xGex gradient crystals: a new monochromator material for hard X-rays

The diffraction properties of a large Czochralski grown Si1−xGex (0.02≤x≤0.07) gradient crystal have been studied by means of a triple-crystal diffractometer using 120 keV synchrotron radiation. Over a crystal length of 40 mm a homogeneous variation of the lattice parameter by 1.7×10−3 has been observed. The diffraction pattern measured in transmission geometry at low Ge content showed a FWHM of 14.8 arcsec at a peak reflectivity of 96% disregarding absorption. With increasing Ge content the FWHM of the diffraction pattern increases up to 49 arcsec with the peak reflectivity decreasing to a value of 72%. Taking absorption into account the effective reflectivity for 120 keV photons varies between 68% and 50%, which is surprisingly high. This material opens up new opportunities of optimising optics in synchrotron radiation instrumentation, it is expected that it will also be of advantage in neutron scattering experiments.