New developments in light material overcoating for soft x-ray reflectivity enhancement

X-ray Wolter focusing telescopes concentrate the light by means of reflection on smooth surfaces at small grazing angle (below a couple of degrees). The traditional coatings for these kind of applications are heavy materials that, due to their high density, present a high critical energy for total reflection. Recent works have shown how a thin layer of a light material, like carbon, on top of a traditional reflecting coating, can enhance the reflectivity in soft x-ray spectral region (below 5 keV), without degrading the performances for higher energies. We presented at SPIE 2007 some experimental results about the reflectivity measurement at very low energies (200 eV) and rather large angles (1-2 deg). In the present work we extend the former study, by the realization of a new set of samples with coatings made of different materials (Pt, Au, W, Ir) and the measurement of their reflectivity for the typical angles (< 1°) and energies (1-10 keV) employed in astronomical grazing incidence telescopes.