Impact of beryllium microstructure on the imaging and optical properties of X-ray refractive lenses

Beryllium is one of the most transparent materials to hard X-ray radiation and, as a direct consequence, it is the main material for the fabrication of X-ray refractive optics and instrumentation for synchrotron radiation sources and free-electron laser facilities. However, it is known that almost all beryllium currently in use is polycrystalline material. In this paper, the influence of the microstructure of different beryllium grades on the optical properties of X-ray refractive lenses is studied. The experiments were performed at the ESRF ID06 beamline in X-ray coherent transmission microscopy mode in the near- and far-fields. Two sets of refractive lenses made of beryllium O-30-H and IS-50M grades with different internal microstructure were used. It was found that both beryllium grades have a strongly inhomogeneous structure, which inevitably produces speckle patterns under coherent illumination in imaging experiments. It was shown that fine-grained beryllium O-30-H is better suited for imaging applications, whereas beryllium IS-50M with a relatively large grain microstructure is more appropriate for focusing and collimation of X-rays. A discussion on the requirements for X-ray optical materials used at the third- and fourth-generation synchrotrons is also presented.