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

TL;DR

This study shows that adding a thin carbon over-coat to silicon-based XEUS mirrors significantly enhances X-ray reflectance between 2 and 10 keV, potentially increasing the telescope's effective area by up to 60%.

Contribution

It provides experimental evidence that a 10nm carbon over-coat improves X-ray reflectance of silicon mirrors for the XEUS mission, a novel approach for telescope mirror enhancement.

Findings

Carbon over-coat improves reflectance in 1-4 keV range.

Effective area can increase by 10-60%.

Significant reflectance enhancement at 10 mrad grazing angle.

Abstract

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