Experiments with Coherent γ Fields: Gamma Echo and Related Phenomena

The role of coherence played in several phenomena in the gamma energy region has not been well recognised until recently. The Mössbauer source nuclei are noncorrelated, but the absorber response has a well-defined phase relation to the instantaneous source radiation within the relaxation time of the nuclear system. By rapidly changing this phase relation with mechanical displacement, new types of coherent transients in gamma regime have been observed. The gamma echo, replicating and amplifying the enchained decay through a thick sample, is perhaps the best known example of these phenomena. In addition, a simple gamma-ray interferometer based on rapid transit through the resonance has been demonstrated. Fast and intense gamma pulses have been obtained with stepwise phase modulation. These and the gamma-NMR double resonance experiments are reviewed in this paper.