Carbon overcoatings for soft x-ray reflectivity enhancement

TLDR: In this article, the first results concerning the effect of high-density materials on the reflectivity of X-ray observations were presented at the photon energies of 200 eV (i.e. below the carbon K absorption edge).

Abstract: In X-ray astronomical telescopes, the focalization of the radiation is achieved by means of grazing incidence Wolter I (parabola + hyperbola) optics in total reflection regime. In general, high density materials (e.g. Au, Pt, Ir, W) are used as reflecting coatings, in order to increase as much as possible the cut-off angles and energies for total reflection. However these materials present an important reduction of the reflectivity between 0.2 and 5 keV, due to the photoabsorption, and this phenomenon is particularly enhanced in correspondence of the M absorption edges (between 2 and 3.5 keV). In general, this determines a strong decrease of the telescope effective area. To overcome the problem we suggested in previous works the coating of the mirror surface by a low-density material such as carbon. Mirror samples with different coatings made by high density materials: Au, Ir, Pt, and W with a carbon overcoating were manufactured and reflectivity data in the soft X-ray band (100-2000 eV), performed both at the XACT facility in Palermo (Italy) and at BSRF synchrotron in Beijing (China), are showed. In this paper we present some of the first results concerning the measurements carried out at the photon energies of 200 eV (i.e. below the carbon K absorption edge) and 1280 eV (i.e. the region just below the heavy material M absorption edge).