Phonon Density of States Measured by Inelastic Nuclear Resonant Scattering

The phonon density of states was measured by observing the nuclear resonant fluorescence of ${}^{57}$Fe versus the energy of incident x rays from a synchrotron radiation beam. An energy resolution of 6 meV was achieved by use of high-resolution crystal optics for the incident beam. Extremely low background levels were obtained via time discrimination of the nuclear fluorescent radiation.