Observation of soft x rays of single-mode resonant transition radiation from a multilayer target with a submicrometer period

The soft x rays produced by resonant transition radiation from a multilayer target with a submicrometer period were measured. The target consisted of 176-nm-thick nickel as radiators and 221-nm-thick carbon as spacers alternately stacked on a SiN membrane. The target was bombarded by 15-MeV electrons, and the angular dependence of the resulting x-ray spectra was measured. The spectra had a clear single peak at energies from 2 to 4 keV depending on the observation angle. The spectral dependence on the incident angle of the electron beam was also measured, and a shift of the peak energy was observed. These experimental results showed a good agreement with theoretical estimations. These results are clear evidence of single-mode resonant transition radiation.