Exo-interferometric Phase Determination of X rays

The time-dependent phase change of x rays after transmission through a sample has been determined experimentally. We used an x-ray interferometer with a reference sample containing sharp nuclear resonances to prepare a phase-controlled x-ray beam. A sample with similar resonances was placed in the beam but outside of the interferometer; hence our use of the term ``exo-interferometric.'' We show that the phase change of the x rays due to the sample can be uniquely determined from the time-dependent transmitted intensities. The energy response of the sample was reconstructed from the phase and the intensity data with a resolution of 23neV.