A novel type of x-ray interferometer

Novel types of interferometers have been developed at the Mainz Microtron MAMI with which the complex index of refraction of thin self-supporting foils can be measured. For the vacuum ultraviolet and soft x-ray region the interferometer consists of two collinear undulators, between which a foil can be placed, and a grating spectrometer. For the hard x-ray region, up to an energy of about 40 keV, it consists of two foils in which the electron beam produces transition radiation, and a single crystal spectrometer in Bragg geometry. Taking advantage of the low emittance 855 MeV electron beam distinct intensity oscillations have been observed as a function of the distance between the undulators and foils, respectively. The complex index of refraction has been investigated at the K- and L-absorption edges of carbon and nickel.