New Multilayer Design for ASTRO-H/Hard X-ray Telescope and Missions Beyond

Japan's sixth X-ray satellite missions, ASTRO-H, features two hard X-ray telescopes with depth-graded, multilayer reflectors. Our laboratory has advanced the technology for multilayer reflectors through development of a balloon-borne experiment, InFOCuS. While its base model for the multilayer design (with the minimum incident angle of 0.11 degrees and 8m focal length) is adequate for the ASTRO-H mission, we opt to improve it further so that better performance can be derived with the minimum incident angle of 0.07 degrees and 12m focal length. The improved design has resulted in the reduction of layers from 28 to 10-21 layers per reflector while gaining higher reflectivity by 3-5% at 40-70 keV. A multilayer optics will be used in ATHENA. ATHENA's principal design requires its energy bandpass of 0.1-20 keV with the minimum incident angle of 0.31 degrees and 12m focal length. With the narrower bandpass and the larger incident angles, it necessitates following changes in the multilayer design: adjustment of top Pt layer thickness and a total number of top layers to boost reflectivity (especially in the lower energy band), and refinement of Pt/C-layer spacing and depth-graded period below the third layer to bunch up the Bragg peaks tightly in the required hard X-ray band. In this presentation, we discuss the key parameters for ATHENA's multilayers to achieve a larger effective area at 10-20 keV.