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The re-emission of gamma -quanta in the forward direction by an ensemble of excited nuclei after abruptly changing the energy of the nuclear excitation is studied. A sublevel of the 14.4 keV excited state with a definite spin projection of 57Fe nuclei in a magnetic 57FeBO3 crystal is selectively populated. The abrupt change of the energy of the excited nuclear state is achieved by fast (

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The propagation of nuclear polaritons through a scattering system composed of two spatially separated targets was studied. The stainless-steel targets were highly enriched in resonant 57 Fe nuclei and had different thicknesses, and, in addition, the resonance width of one target could be increased by cooling it to low temperatures. A nuclear polariton state was created in both targets almost instantly by a short flash of synchrotron radiation. The propagation of the polariton could be perturbed by a time-dependent Doppler shift of the resonance energy in one of the targets. The effect of inversion of the target sequence on the polariton propagation was in the focus of our investigations. It was shown both theoretically and experimentally that in general a two-target scattering system is not commutative.