Measurement of the speed of X-rays

X-ray pulses from the Advanced Photon Source at Argonne National Laboratory were used to measure the speed of X-rays in the energy range between 21 and 60 keV. An LiNbO(3)-based 0.58 GHz surface acoustic wave device served as a temporal analyzer in the stroboscopic time-resolved diffraction measurements. By synchronizing the surface acoustic wave excitation and periodic X-ray illumination of the LiNbO(3) crystal, the temporal modifications in the LiNbO(3) diffraction profiles could be followed and the time points of X-ray arrivals at the analyzer position for different analyzer to storage ring distances determined. The speed of the X-rays was determined as the ratio of measured spacings and corresponding delay time intervals. Within the experimental error bars, the obtained X-ray velocities converged to the tabulated constant for the speed of light in a vacuum.