Novel digital K-edge imaging system with transition radiation from an 855 MeV electron beam

A novel K-edge imaging method has been developed at the Mainz Microtron MAMI aiming at a very efficient use of the transition radiation (TR) flux generated by the external 855 MeV electron beam in a foil stack. A fan-like quasi-monochromatic hard X-ray beam is produced from the ±1 mrad wide TR cone with a highly oriented pyrolytic graphite (HOPG) crystal. The absorption of the object in front of a 30×10 mm² pn-CCD photon detector is measured at every pixel by a broad band energy scan around the K-absorption edge. This is accomplished by a synchronous variation of the lateral crystal position and the electron beam direction which defines also the direction of the TR code. The system has been checked with a phantom consisting of a 2.5 μm thick molybdenum sample embedded in a 136 or 272 μm thick copper bulk foil. A numerical analysis of the energy spectrum for every pixel demonstrates that data as far as ±0.75 keV away from the K edge of molybdenum at 20 keV still improve the signal-to-noise ratio. Prospects are discussed to investigate the human lungs with xenon as a contrast agent at the available total primary photon flux of 2·10 ¹⁰/(s·0.1% BW) only