Fixed-Exit Monochromators for High-Energy Synchrotron Radiation

Current developments in X-ray optics for synchrotron radiation beamlines are briefly reviewed. Reference is made to recent work on adaptive mirrors, cryogenic cooling of monochromators, use of thin diamond crystals, and active correction of the crystal shape for distortions caused by beam heating. The use of bent Si crystals as monochromators is discussed in detail. At high energies Si crystals become transparent to X-rays allowing new monochromator constructions. Bending of the crystal increases the energy bandpass and allows focusing. Different combinations of bent crystals that provide a fixed exit beam are discussed. These include vertically diffracting meridionally focusing Laue-Laue crystals, a Laue crystal combined with a sagittally focusing Bragg crystal, and a Laue-Bragg pair of crystals which provides meridional focusing at two stages.