Influence of a carbon over-coat on the X-ray reflectance of XEUS mirrors

We describe measurements of the X-ray reflectance in the range 2-10 keV of samples representative of coated silicon wafers that are proposed for the fabrication of the X-ray evolving universe spectrometer (XEUS) mission. We compare the reflectance of silicon samples coated with bare Pt, with that for samples with an additional 10 nm thick carbon over-coating. We demonstrate a significant improvement in reflectance in the energy range ∼1-4 keV, and at a grazing incidence angle of 10 mrad (0.57°). We consider the resulting effective area that could be attained with an optimized design of the XEUS telescope. Typically an improvement of 10-60% in effective area, depending on photon energy, can be achieved using the carbon overcoat.