X-ray Supermirrors: Novel multilayer structures for broad band hard X-ray applications

Abstract

With the increased availability of hard X-ray (≥ 20 keV) synchrotron sources and a concurrent wish for hard X-ray telescopes. the need for hard X-ray optics has become considerable. For some applications crystals and periodic multilayers are desirable; for others the bandwidth of these may be too narrow. A wider bandwidth can be obtained by using total reflection from heavy elements. This approach. however, is impractical for harder X-rays since it is limited by the inverse proportionality between the critical angle of total reflection and the energy. One solution to this broad-band reflection problem may be the supermirror: a multilayer structure where the bi-layer thickness is gradually changed from top to bottom. In such a structure radiation with different wavelengths will have the criteria for constructive interference fulfilled at different depths in the multilayer. If the bi-layer thicknesses are changed sufficiently slowly. simultaneous diffraction of a continuum of X-ray energies can be obtained. The multilayered supermirror was originally designed for neutron applications1 such as increasing the throughput of guide-tubes. For neutrons, where absorption is extremely low for most materials. reflectivities up to95 can be obtained in a continuous band up to 3 times the critical angle or energy2. For X-rays. the absorption is considerably larger and this strongly influences the performance of the X-ray supermirror.