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20 September 2007 Stacked depth graded multilayer for hard X-rays measured up to 130 keV
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Depth graded multilayer designs for hard x-ray telescopes in the 10 keV to 70-80 keV energy range have had either W or Pt as the heavy element. These materials have been chosen because of reasonable optical constants, the possibility to grow smooth interfaces with the spacer material, and the stability over time. On the flip side both W and Pt have an absorption edge -- 69.5 keV (W) and 78.4 keV (Pt) -- which is very close to the two 44Ti lines at 67.9 keV and 78.4 keV that are produced in the envelope of a super nova explosion. Other materials have better optical constants and no absorption edges in this energy range, for example Ni0.93V0.07, but are not used because of high interface roughness. By using a WC/SiC multilayer for the bottom and a Ni0.93V0.07/SiC multilayer for the thicker top layers of a depth graded multilayer we have made a reflector that doesn't have a clear absorption edge. This reflector has been measured at energies between 8 keV and 130 keV. At a graze angle of 0.11 degree there is still nearly the same reflectivity below the W absorption edge as for a traditional W based coating, and above the W absorption edge there is still 48% reflection at 80 keV.
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C. P. Jensen, F. E. Christensen, S. Romaine, R. Bruni, and Z. Zhong "Stacked depth graded multilayer for hard X-rays measured up to 130 keV", Proc. SPIE 6688, Optics for EUV, X-Ray, and Gamma-Ray Astronomy III, 66880T (20 September 2007);

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