Optical constants in the hard x-ray/soft gamma-ray range

Future astrophysics missions operating in the hard X-ray/Soft Gamma ray range is slated to carry novel focusing telescopes based on the use of depth graded multilayer reflectors. Current design studies show that, at the foreseen focal lengths, it should be feasible to focus X-rays at energies as high as 300 keV. These designs use extrapolations of theoretical and experimentally determined optical constants below 100 keV. We have previously shown that determining the optical constants from traditional single layer film above 40 keV is very difficult. One needs to have substrates which are very flat and it is very important to know the exact flatness. In this paper we report on the experimental determination of optical constants up to and above 130 keV using substrates with sub arcsecond flatness. We present these results as obtained at the National Synchrotron Light Source in Brookhaven and compare these to theoretically calculated values and previous experiments.