Long-term stability of Mg/SiC multilayers

Mg/SiC multilayers have been selected to coat the primary and secondary mirrors of the SCORE (Sounding-rocket CORonagraphic Experiment) telescope, a part of the NASA HERSCHEL program. Their experimental reflectance at the He II 30.4 nm wavelength is twice as much that of standard Mo/Si multilayer coatings, with a large improvement of the instrument effective area. Nevertheless, their long-term stability still needs to be evaluated in order to consider them as a valuable candidate for instrumentation of a long term space mission. A study is carried out on different Mg/SiC samples designed and fabricated varying the structural parameters and/or the capping layer material and thickness, which have been monitored for four years after deposition