Application of composite-layer mirror to soft x-ray telescopes

摘要

Recent development of cryogenic detectors enabled high resolution diagnostics of emission line and absorption line/edge structures. These studies are quite important to investigate physics of interstellar/intergalactic plasma and dynamics of matters around black holes, for example. Soft X-ray telescopes for such purpose, sensitive up to 10 keV, are usually designed using Au, Pt, or Ir for their large electron density and large critical angle. However in fact, these materials are not very suitable in a few to 8 keV region, because they have wide and deep M-shell absorption edge structures in 2-4 keV region. Considering absorption edges and also thin-film characteristics, C or Ni can be alternatives. However, these materials alone cannot be a good mirror, again from absorption edges in the energy region of interest (0.1-10 keV). We designed composite structure consisting of C, Ni and Pt with thickness of 30 to 300 A, to get smooth and high reflectivity all across the energy region. Test fabrication showed interfacial roughness is as low as expected, indicating match of materials is also good. We applied this result to the baseline design of NeXT/Soft X-ray Telescope, by substituting current material (Au) with C-Ni-Pt composite, and obtained 20% larger effective area in 2-8 keV region.