Applications Requirements Affecting Optics Selection

35.1 INTRODUCTION X rays have a century-long history of medical and technological application. Optics have come to play an important role in many capacities. This chapter examines how various X-ray optics enable typical applications, and the factors that influence optics choices. It is not feasible in this chapter to discuss every possible utilization of X-ray optics. Rather, a representative selection has been included. Some applications, such as X-ray astronomy, have been thoroughly addressed in previous chapters. 1,2 There is, in addition, a class of applications for which the primary requirement is adequate intensity in an extremely narrow frequency band-width. These include the absorption spectroscopy tools, such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES). These measurements are typically performed with multiple crystal monochromators 3 on a synchrotron beam line; 4 however, the discussions of intensity gain from X-ray optics in the following fluo-rescence, diffraction, and microscopy sections can be applied to these applications as well, which in some cases are then made feasible using laboratory sources. X-ray optics are used over an extremely large range of X-ray fluxes, from the detection of flu-orescence emission from part-per-billion impurities to beam shaping for third-generation synchrotrons. For a number of synchrotron and other high-flux applications, radiation hardness and thermal stability are important considerations. A large body of experience has been developed for high-heat-load synchrotron mirrors and crystals. (See, for example, Chap. 26.) 5,6 In addition, testing has been performed on a variety of other optics. Many optics such as zone plates, microscopy objectives, and glass capillary tubes are routinely used in synchrotron beam lines and are stable over acceptable flux ranges. Another consideration with synchrotron sources is transverse coherence. Using classical geometric optics, the requirement in slit separation or beam radius, a, to observe diffraction effects is a∆θ << λ (1) where λ is the wavelength of the radiation, and ∆θ is the angle subtended by the source. For typical laboratory sources, the coherent beam radius is too small to have sufficient count rate for measurements. However, for synchrotron sources, ∆θ is much smaller and the radius a is large enough to encompass optics such as zone plates, which require coherent superposition. On third-generation synchrotron beam lines, coherence effects such as speckle are routinely seen. Speckle patterns can be employed to measure time-dependent correlations in fluctuating samples. 7 When effects such as speckle are undesirable, coherence can be …