Coherent bremsstrahlung produced by relativistic positrons and electrons on a single crystal of silicon

The spectral dependence of the forward coherent bremsstrahlung produced by beams of 28-MeV positrons and electrons incident upon a 5-{μ}m thick single crystal of silicon was measured with a NaI photon spectrometer. Effects of channeling and perhaps of the nonvalidity of the first Born approximation were observed for beam directions near the $〈111〉$ axis of the crystal. A coherent peak near 0.5 MeV for both electrons and positrons was observed for a compound-interference direction, in agreement with first-order theoretical calculations.