基于球面弯晶聚焦结构的桌面型单能X射线源设计及实验

Design and Experimental Study of a Desktop Monochromatic X-raySource Based on Spherical Bent-crystal Focusing Structure

提出了一种采用球面弯晶聚焦结构获得高亮度桌面型单能X射线的方法。在子午方向利用晶体色散实现单能、在弧矢方向通过球面镜聚焦提高光强。理论建模和光学仿真评估了该结构的色散和聚焦性能,验证了球面弯晶聚焦结构相对于柱面弯晶在聚焦特性上的显著优势。针对Al靶Kα1线单能需求,设计和装调了基于低功率Al靶X射线管的单能装置,并实验验证了其性能。结果显示,当X射线管工作在7 W功率条件下,CCD曝光10 min,Al靶Kα1线全视场光谱的探测器计数大于2×105,能谱展宽约为0.592 eV;引入200μm限束光阑,能谱展宽进一步减小至0.493 eV,探测器计数约为2×104。研究结果证实了该装置可以有效获得高亮度Al靶Kα1谱线,也为精确测量光学器件和系统的光谱特性提供了一种新的获得高亮度单能X射线的技术途径。

X-ray optics and systems have been widely used for applications such as plasma diagnostics,X-ray spectroscopy,astronomical observation and material analysis.The spectral characteristics of the optics and systems need to be accurately measured,for which a high-brightness monochromatic X-ray source is indispensable.Synchrotron radiation facilities provide the most reliable metrology,but their measurement timeliness is limited because of the allocated user beam time and long distances to reach the facilities.Commercial X-ray measurement equipment,such as X-ray fluorescence spectrometers,X-ray reflectometers and diffractometers,are only capable of measurements of small-size optics at a few fixed energy points.Desktop monochromatic X-ray sources that combine X-ray tubes with dispersive optics have provided a laboratory-based approach to calibrating X-ray optics,where the dispersive optics usually refers to multilayers,gratings and crystals.Multilayers are distinguished by their high luminous flux and wide working energy band,but their spectral resolution is relatively low;X-ray gratings allow for much higher spectral resolution,but the fabrication processes are quite difficult for extremely dense grooves for X-ray diffraction.Crystals have a naturally unique dispersion capability in the X-ray band that can substantially improve the spectral resolution.Common-used planar and cylindrical crystals have limited effects to enhance the system throughput because of relatively weak focusing capability in one or both dimensions;complex aspheric bent-crystals,such as toroidal,parabolic and hyperbolic crystals,can reduce aberrations and focus in both meridional and sagittal directions so that system throughput and spectral resolution can be significantly improved.However,aspheric bent-crystals are difficult to fabricate and have many limitations for application;and the aberration would deteriorate abruptly if the incident angle deviates from the designed value.Spherically bent-crystals enable two-dimensional focusing and are relatively easy to be fabricated;therefore,they have been widely used in X-ray spectrometers.Conventional spherical bend-crystal spectrometers adopt the Johann scheme,where the light source and the image plane are located on a Rowland circle with a diameter equaling to the radius of the concave bend-crystal.By applying Johann scheme,better focusing and higher spectral resolution are obtained in the meridional direction,but the brightness of the spectral lines acquired in the image plane is largely decreased due to light divergence in the sagittal direction.Therefore,it is necessary to optimize the optical structure to improve the intensity of dispersed monochromatic X-ray.In this paper,a desktop method for obtaining high-brightness monochromatic X-ray is proposed based on a spherical bent-crystal focusing structure.It achieves monochromaticity in the meridional direction using crystal dispersion,and obtains high brightness in the sagittal direction simultaneously based on spherical-mirror focusing.A geometric model is developed to theoretically compare the efficiency of collecting light of the spherical and cylindrical bent-crystals;also,the dispersion and focusing performance of different optical layouts with spherical and cylindrical bent-crystals are simulated and evaluated.Both theoretical and simulation results show that the spherical bent-crystal focusing structure has an excellent performance in terms of focusing characteristics,with the brightness of the spectral line obviously increased,compared with the cylindrical bent-crystal.In view of the monochromatizing requirements of the Al Kα1 line,a device based on low-power Al-target X-ray tube is designed and adjusted,and its performance was tested by spectroscopic experiments.Measurement results show that with the X-ray tube working at 7 W power and CCD exposure for 10 minutes,the count of the full-field spectrum of the Al Kα1 line is greater than 2×105 with a spectral broadening of about 0.592 eV;when introducing a 200μm beam-limiting slit,the spectral broadening is further reduced to 0.493 eV and the CCD count is about 2×104.The results have verified that the device can effectively obtain the high-brightness Al Kα1 line,and also provides a new technical approach for accurately measuring the spectral characteristics of X-ray optics and systems.