Tandem-phase zone-plate optics for high-energy X-ray focusing

An optical system consisting of two phase zone plates closely arranged in tandem was constructed for focusing high-energy X-rays. The phase zone plates were made from tantalum and their combined thickness was 4.8 μm. An ideal diffraction efficiency of 30% is expected at 30 keV, which is about 3 times higher than that of a single zone plate. The focusing properties at 30 keV were studied both numerically and experimentally. The coaxial tandem arrangement was precisely achieved by observing Young's interference patterns in the far-field produced by the two point foci. A focus size of ̃4 μm was obtained. The photon flux density was 2.2 × 10¹³ photons/s/mm², which is 2.4 and 85 times higher than that obtained with a single zone plate and without focusing, respectively. The focused beam was used for scanning X-ray fluorescence microscopy and the residual tin distribution on a float glass surface was imaged.