Hard-photon emission from 150-GeV electrons incident on Si and Ge single crystals near axial directions

The emission of high-energy photons from 150-GeV electrons traversing single crystals near axial directions is studied experimentally for Ge and, for the first time, also for Si. Enhancements relative to random up to 2 orders of magnitude are observed. For incident angles much less than the critical channeling angle ψ₁ a pronounced peak appears in the photon spectra near {~}85% of the electron energy for both the Si and the Ge crystals. The peak disappears for incident angles larger than {~}0.3ψ₁. The experimental findings are compared to theoretical results.