Next-generation materials for future synchrotron and free-electron laser sources
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Introduction X-ray synchrotron radiation and free-electron laser (XFEL) sources (also referred to collectively as x-ray light sources) are important tools in materials science for probing the atomic and electronic structure of materials and their surfaces, as well as the dynamics of atomic- and molecularlevel processes.1 In turn, materials research is critical for the advancement of the fundamental technologies underpinning these sources. In this issue of MRS Bulletin, we explore recent advances in materials for the optics, detectors, and other components that are essential parts of these research tools. X-ray synchrotron radiation is generated by a beam of electrons (or positrons) that are accelerated to nearly the speed of light. While passing through a linear periodic magnetic field created by specialized magnet devices (or insertion devices) called undulators, these electrons (or positrons) oscillate and emit x-rays at each bend. X-rays emitted at consecutive bends add up to form a powerful laser-like, quasi-monochromatic x-ray beam billions of times brighter2,3 than light from conventional laboratory x-ray tubes (Figure 1), that can probe matter at the atomic scale. In addition to brightness, these x-ray light sources exhibit natural collimation, which leads to greatly improved resolution, as well as the ability to select a single photon energy that interacts with the material of interest, yielding an optimum signal-to-noise ratio. These characteristics
have enabled detailed studies ranging from materials behavior and properties under extreme conditions4 (Figure 2) to the behavior of biological systems at the cellular and molecular levels. Experiments at synchrotron sources have resulted in important discoveries, as evidenced by the Nobel prizes received for such work. X-ray sources have seen an exponential improvement over the last 40–50 years, a trend that is continuing today (Figure 1). New XFEL sources are being developed worldwide that promise high repetition rates with unprecedented brilliance (also called spectral brightness or simply, brightness, which is the number of photons/second/millimeter2/milliradian2/0.1% energy bandwidth), and pico- to femtosecond timing over a broad photon energy range with full, or a high degree of, spatial and temporal coherence;5 these sources will enable revolutionary advances in x-ray science and new discoveries in the fields of physics, biology, materials science, and engineering. Also, “ultimate” storage rings will allow powerful new experiments that take advantage of the full coherence and brightness of the diffraction-limited radiation. These new sources present major challenges and new opportunities to develop advanced optical elements and detectors. All optical elements in the beam path must be manufactured to a high degree of precision, and their materials must maintain their characteristics during use to preserve the integrity of the source brightness. The detector materials
Lahsen Assoufid, Advanced Photon Source, Argonne National Laboratory, USA; assoufi[email protected] H
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