Exploiting undulator radiation for X-ray interferometry

Undulator radiation from a low emittance electron beam exhibits symmetric side lobes, coherent from any one electron, and having substantial fluxes. A configuration is proposed that uses these lobes as coherent sources for X-ray interferometry. The undulator beam, after optical collimation using an “undulator-specific X-ray lens”, passes through a monochromator that excludes the central peak but passes two symmetrically opposite lobes. The relative phase of these beams can be altered by placing variable thickness material in one of their paths. After refocusing both beams using another undulator-specific lens, the beams impinge on a detector. Interference fringes are anticipated which shift as the relative phase of the two amplitudes is altered. Since the fringe spacing is still too small for other than photographic detection, the combined beams then pass through a “double crystal fringe expander”. One possible further apparatus to prove the interferometerʹs ability to measure an absolute X-ray diffraction phase is sketched.