Channeling radiation from 2 to 20 GeV/c electrons and positrons (II).: Axial case

Radiation spectra for GeV electrons and positrons have been measured in a broad range of incidence angles in Si and Ge crystals. An enhancement up to a factor of 50 is found for axially channeled particles. In nearly all cases, the very low-energy part of the photon spectra is strongly reduced either due to equalization in the doughnut or due to the strong multiple scattering from the atomic rows. For large incident angles to the axis, the reduction is due to the so-called Landau-Pomeranchuk effect. For proper channeled positrons, stable oscillations give rise to a peak in the photon spectra. In the transition region from axial to planar channeling, the photon spectra change from being structureless to showing pronounced peaks from the first harmonics. The experimental results are compared to both classical and quantal calculations.