Adaptable monochromators to optimize intensity gain and resolution for experiments with high-energy synchrotron radiation

Abstract

We present investigations of bendable comb-shaped Si crystals and Si<SUB>1-x</SUB>Ge<SUB>x</SUB> gradient crystals, tested in Laue geometry, to be used as monochromator and analyzer for triple crystal x-ray diffractometry with high (around 100 keV) energy synchrotron radiation. At these energies, both the high q- space resolution of synchrotron radiation and the high penetration power of neutrons are present. The overall resolution element in q-space is usually dominated by the properties of the sample. A comb crystal structure can provide a gain in integrated intensity of up to a factor of 70, compared to perfect Si crystal monochromator and analyzer, by adjusting the instrumental resolution to the needs of a sample. However, this intensity gain can further be increased by using Si<SUB>1-x</SUB>Ge<SUB>x</SUB> gradient crystals. The continuously- changing lattice parameter in these crystals yields an increase in maximum peak reflectivity of up to 100% in comparison to theoretical limit of 50% for normal crystals in Laue geometry. Recently good Si<SUB>1-x</SUB>Ge<SUB>x</SUB> gradient crystals of sufficiently large size have been grown by means of the Czochralski technique with geranium content x between 0.02 and 0.08. We find peak reflectivities of these crystals of up to 96%, accompanied by a large increase of the FWHM of the reflection. Combining these properties and the principle of the comb structure, the intensity gain should be easily increasable to factor of 1600 as compared to a set-up with perfect single-crystal monochromator and analyzer.