Coherent bremsstrahlung and channeling radiation from electrons of one to three MeV in silicon and gold

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Abstract
We report on the observation of sharp peaks in the x-ray spectrum from 1-3-MeV electrons striking thin single crystals of silicon and gold. These peaks were observed in the range 1-25 keV. The peaks are of two different origins, both direct results of the periodic nature of the target crystals. The first kind of radiation is caused by the interference of incoming and scattered electron wave functions. Because of the periodicity of the target material there is a coherence effect for certain bremsstrahlung wave vectors. This coherent bremsstrahlung, though well known at very high electron energies, has never been adequately studied at electron energies below several hundred MeV. We show detailed agreement between theoretical predictions and observations in silicon. The second kind of radiation is caused by electrons channeled along major crystal axes. The electrons enter certain quantized orbits as they channel and may emit photons as a consequence of transitions between the various orbits. We present observations of channeling radiation for various crystal axes in silicon. We also observed both phenomena in gold, the first such observations for any metallic target.