Seiji TAKAHASHI

Abstract

Hard X-ray imaging observations of astronomical objects using focusing optics have not been carried out yet. In the hard x-ray region above 10keV, the use of ordinary mirrors with single layer of metal as reflecting surface is no longer realisitic. For such high energies, multilayers with depth-graded periodic length, so called ``supermirrors'', are known to be promising. Also, due to poor photon statistics in this energy region, photon collecting efficiency has to be maximized. From this reason, tightly nested thin foil optics is adequate.
The performances of telescope are integrated into photon collecting power and image quality. The peak reflectivity of a multilayer, which has large impact on photon collecting power, strongly depends on the interfacial roughness which results from the surface roughness of substrates, material combinations, the substrate temperature and deposition methods. The thin foil mirrors are impossible to fabricate via direct machining. Therefore, replication method is adopted. In this method, once we deposit a multilayer onto a smooth glass master, and transfer the multilayer onto a substrate via epoxy adhesive. And we have to optimize processes not to cause deformation of foil mirrors, otherwise image quality, another key of telescope performance, is degraded.

We formed aluminum foils into conical shape and optimized parameters of epoxy adhesive(e.g. thickness, dilution rate, curing condition, etc.) and we established the epoxy replication method. Using the epoxy replication method, we made various mirrors; single layer of gold, single layer of platinum, platinum-carbon multilayer and platinum-carbon multilayer supermirror. Fabricated mirrors were then characterized by a uniform parallel beam of optical light, a laser scan profilometer and X-ray pencil beam facility. The angular resolution is obtained as about 2 arcmin, and the interfacial roughness is reduced to 3-4[A]. We showed, for the first time, that films other than gold can be replicated into large area foil mirrors with acceptable imaging quality. In addition, we showed that interfacial roughness we obtained is significantly better than depositing multilayers onto pre-replicated foil mirrors. The reduction of interfacial roughness, thus improvement of reflectivity will increase effective area of telescope by a factor of about 1.5. This type of technology is also applicable to X-ray microscopes, X-ray optics for synchrotron radiation facilities.