Soft-x-ray interferometer for measuring the refractive index of materials

We have designed and built a soft-x-ray interferometer to test the possibility of a direct measurement of the refractive index (i.e., the real part of the complex index) of materials in the soft-x-ray range. The interferometer is based on the Fresnel bimirror setup. It works near the zero path difference and requires only little spatial coherence. Plane mirrors at grazing incidence are the only optical elements. Interference fringes have been recorded at 4.8 nm, near the K edge of carbon. An index value could be obtained by measuring the fringe pattern shift between two such records, one without and one with a sample in one optical path. An estimation of the noise-limited accuracy in such an index determination shows that a few parts in 10^−6 can be anticipated.