Bent Si reflector with DLC deposition developed for X-ray imaging polarimetry

We report a development of a bent crystal for use of X-ray polarimeter. We deposited Diamond-Like Carbon (DLC) on the back side of reflection surface of a Si(400) crystal sheet, and bent the crystal sheet by the residual stress between the DLC and Si. We confirmed that the curvature can be controlled with the thickness of DLC. An angular reflectivity of the crystal was measured with the line emission at 8 keV (Cu-Ka). The angular width is broadened to 2 degree, centered at the Bragg angle expected for the (400) plane of Si(100). The angular width is equivalent to the energy width of 0.5 keV. We found that as the curvature radius of Si becomes small, the integrated reflectivity becomes larger. The modulation factor was measured to be about 0.8 for 8 keV. For the Fe-K lines (similar to 7 keV), which are very important for X-ray astronomy, higher modulation factor close to 1 is expected. The large sensitivity in wide energy band with the high modulation factor indicates that the bent crystal can be a new tool for the X-ray imaging polarimeter. A preliminary design of the polarimetric optics composed by the Si(100) crystal and a small-size imaging detector (e. g. X-ray CCD camera) is proposed in our presentation. Since any kind of crystals can be bent with our method, higher performance will be obtained with changing the crystal.