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Coherent transition radiation, generated by the passage of mm-long bunches of electrons through a thin metal foil, has been observed in the far infrared. The intensity was compared to the smaller amount of Cherenkov radiation produced when SF₆ gas was introduced into the electron path. Wake-field radiation was ruled out by varying the geometry of the metal chamber near the electron beam. The intensity, the polarization, and the spectral and angular distribution of the transition radiation were measured and its coherent nature was identified.
Happek et al. (1991) studied this question.