X-ray phase contrast imaging at MAMI

Experiments have been performed to explore the potential of the low emittance 855 MeV electron beam of the Mainz Microtron MAMI for imaging with coherent X-rays. Transition radiation from a micro-focused electron beam traversing a foil stack served as X-ray source with good transverse coherence. Refraction contrast radiographs of low absorbing materials, in particular polymer strings with diameters between 30 and 450 pin, were taken with a polychromatic transition radiatiori X-ray source with a spectral distribution in the energy range between 8 and about 40 keV. The electron beam spot size had standard deviation sigma(h) = (8.6 +/- 0. 1) pin in the horizontal and sigma(v) = (7.5 +/- 0. 1) ILm in the vertical direction. X-ray films were used as detectors. The source-to-detector distance amounted to 11.4 in. The objects were placed in a distance of up to 6 in from the X-ray film. Holograms of strings were taken with a beam spot size sigma(v) = (0.50 +/- 0.05) mu m in vertical direction, and a monochromatic X-ray beam of 6 keV energy. A good longitudinal coherence has been obtained by the (111) reflection of a flat silicon single crystal in Bragg geometry. It has been demonstrated that a direct exposure CCD chip with a pixel size of 13 x 13 mu m(2) provides a highly efficient on-line detector. Contrast images can easily be generated with a complete elimination of all parasitic background. The on-line capability allows a minimization of the beam spot, size by observing the smallest visible interference fringe spacings or the number of visible fringes. It has been demonstrated that X-ray films are also very useful detectors. The main advantage in comparison with the direct exposure CCD chip is the resolution. For the Structurix D3 (Agfa) X-ray film the standard deviation of the resolution was measured to be sigma(f) = (1.2 +/- 0.4) mu m, which is about a factor of 6 better than for the direct exposure CCD chip. With the small effective X-ray spot size in vertical direction of sigma(v) = (1.2 +/- 0.3) mu m and a geometrical magnification of up to 7.4 high-quality holograms of tiny transparent strings were taken in which the holographic information is contained in up to 18 interference fringes.