Nuclear forward scattering of synchrotron radiation

The coherent nuclear reemission in the forward direction has been measured following excitation of the Mössbauer transitions in polycrystalline ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ foils by synchrotron radiation. The excited resonances are sufficiently narrow in energy that the time dependence of their decay can be straightforwardly observed, and in particular the details of the collective response to single-photon excitation can be studied. The collective nuclear scattering conditions were varied directly over a large range by using foil thicknesses between 0.5 and 28.5 μm. The envelope of the time evolution showed significant changes with increasing foil thickness. For thin foils (up to 3 μm) the envelope was exponential in the observed time window, with a decay time decreasing with increasing foil thickness. For thicker foils an oscillatory modulation of the envelope and a significant shift in the phase of the beat pattern were observed. A simple optical model can explain all the observed phenomena.