Theory of coherent phenomena and fundamentals in nuclear resonant scattering

We discuss the general theory of coherent phenomena in nuclear resonant interaction of γ-quanta with crystals. The coherence is realized in collective excitation of the ensemble of nuclei (nuclear exciton) with the conservation of phase memory and in the transformation of a γ-quantum into a quasi-particle of Bloch type in a crystal. The collective character of excitations causes a change in the resonant nuclear parameters and in the lifetime of the excited state. This manifests itself in a speed-up of the decay in the forward direction in a thin crystal and, on the contrary, a strong reduction of elastic scattering in a thick crystal. The reconstruction of the wavefunction of an individual γ-quantum in scattering under Laue or Bragg conditions leads to the suppression effect of inelastic incoherent channels. This effect is discussed in detail. The analysis is based on a derived general system of equations describing the resonant diffraction of γ-quanta in a crystal with an arbitrary relation between the coherent and incoherent channels. This system is used to deduce the equations describing the time-dependent nuclear resonant scattering of synchrotron radiation. We discuss the most instructive experiments with revealing coherent phenomena.