A conceptual model for ray tracing calculations with mosaic crystals

Abstract

Mosaic crystals provide an interesting choice for synchrotron radiation monochromators under certain conditions. They show a wider and lower reflectivity curve than perfect crystals, but a higher integrated reflectivity. Some mosaic crystals such as graphite or beryllium could be considered as monochromators or premonochromators for third generation synchrotron radiation machines (Sincrotrone Trieste, European Synchrotron Radiation facility, etc.). In order to assess these possibilities, we have implemented a new mosaic crystal part in the ray tracing code SHADOW. The effect of the random distribution of the crystallites in a mosaic crystal can be analyzed efficiently with a Monte Carlo method. Taking into account the random distribution of the crystal planes, modeled as a Gaussian of standard deviation κ, it is possible to reproduce the well known focusing and defocusing properties of these crystals. For reflectivity calculations we have implemented in the computer code the Mosaic Crystals Theory of Zachariasen. As secondary extinction is not negligible in mosaic crystals, we have also included the penetration effect of the x-ray beam inside the crystal.