Space applications of Si/B_4C multilayer coatings at extreme ultra-violet region; comparison with standard Mo/Si coatings

In the extreme ultra-violet region, multilayer coatings are the only technique to obtain high reflectivity in normal incidence optical configurations. The interference process which regulates periodic multilayers behavior offers narrow-band spectral filtering without the use of additional filters, fact that makes these coatings particularly suitable for lines emission observations. Despite the large amount of possible materials combinations, Mo/Si multilayers are the standard choice for space research on plasma physics in the 13 - 30 nm spectral region. In this work Si/B4C is presented as an alternative material couple for the 30.4 nm selection. Attractive features are the better spectral purity and the second order reflectivity reduction. A possible application to the Sounding CORonagraph Experiment is described as an example. B4C thin films have been used to characterize this material in terms of optical constants in the 40 nm - 150 nm spectral region where, currently, only few data are available.