Soft x-ray reflectivity-enhancement in astronomical telescopes via overcoatings: alternative materials and deposition methods

Low density overcoatings (mainly based on materials containing Carbon) onto usual high-density coatings (based i.e. on materials like e.g. like Ir, Au or Pt) have been proposed since many years ago in order to enhance the X- ray reflectivity at low energy (between 0.5 and 4 keV) of X-ray astronomical optics. The trick is to make use of the total reflection from the thin low-density material (which does not suffer much the photoelectric absorption) at low X-ray energies; the reflection of photons at higher energies (> 4 keV) occurs thanks to the much denser material under the overcoating. For several future projects, like e.g. ATHENA, LYNX and eXTP, it is foreseen the use of low-density overcoatings that will importantly increase the effective area at low X-ray energies. In this paper we will introduce the use of overcoatings based on materials different from the usual ones considered so far like C, B4C and SiC. In particular, we will discuss about a novel approach based on the use of thin layer of a Carbon-like materials deposited using a wet deposition process. A possible combination with an intermediate thin layer of Chromium deposited e.g. via sputtering onto the usual high density material (Ir, Au or Pt) before the application of the Carbon-like material is also considered in the study, because it can further greatly enhance the soft X-ray effective area of future X-ray telescopes.