Figure 2.

Schematic diagram illustrating the experimental setup for the NRVS experiment including the high-resolution monochromator. Synchrotron beam is run in pulse mode, and avalanche photodiode, APD, is disabled during the X-ray pulse (shown bottom left) in order to gate out elastically scattered 14.4 keV photons, so that only delayed photons corresponding to nuclear absorption are detected. Lateral APD detectors measure nuclear absorption, including the vibrational signal, by detecting 6.4 and 14.4 keV fluorescence emitted by excited ⁵⁷Fe nuclei, while the coherent forward-scattered 14.4 keV photons reaching the second APD provide a real-time measure of the experimental resolution function. Photo at the left is the Advanced Photon Source at Argonne National Lab. Photo courtesy of Argonne National Laboratory.