Brauer S, Stephenson G B, Sutton M
J Synchrotron Radiat. 1995 Jul 1;2(Pt 4):163-73. doi: 10.1107/S0909049595003190.
Perfect crystals in the asymmetric Bragg geometry are evaluated as optical elements for manipulating coherent X-ray beams. Such optics can be used to modify the transverse coherence length of a synchrotron X-ray beam, with the intention of increasing the usable coherent flux. The wavelength range, angular divergence and flux of X-rays passing through a pinhole aperture are examined in detail, as functions of source and pinhole size, crystal-to-pinhole separation and the asymmetry factor. In developing this analysis, the behavior of asymmetrically cut crystals is explained in reciprocal space, with reference to the crystal truncation rod associated with the reflection. The results show that, for synchrotron beams that are collimated to a small fraction of the incident Darwin width, the wavelength range accepted by the crystal is typically dispersed into an angular spread in the exit beam. This chromatic aberration greatly reduces the transverse coherence length in a manner that does not conserve the coherent flux. The calculations are in agreement with measurements of the divergence and flux through a micrometer-sized pinhole using a synchrotron wiggler X-ray source.
在非对称布拉格几何结构中的完美晶体被评估为用于操控相干X射线束的光学元件。这种光学器件可用于改变同步加速器X射线束的横向相干长度，目的是增加可用的相干通量。详细研究了穿过针孔孔径的X射线的波长范围、角发散和通量，它们是源尺寸、针孔尺寸、晶体到针孔的间距以及不对称因子的函数。在开展此分析时，通过与反射相关的晶体截断棒，在倒易空间中解释了非对称切割晶体的行为。结果表明，对于准直到入射达尔文宽度一小部分的同步加速器光束，晶体接受的波长范围通常会在出射光束中分散为角展宽。这种色差极大地减小了横向相干长度，且相干通量不守恒。计算结果与使用同步加速器摆动器X射线源通过微米级针孔的发散和通量测量结果一致。
