HARD PHOTON YIELDS FROM (70-240) GEV ELECTRONS INCIDENT NEAR AXIAL DIRECTIONS ON SI, GE AND W SINGLE-CRYSTALS WITH A LARGE THICKNESS VARIATION

The dramatic peak found in photon spectra from 150 GeV channelled electrons has for the first time been investigated for 70, 150, and 240 GeV electrons incident on crystals with thickness from 100μ (Si). Very pronounced variations for the high energy part of the photon spectra are found. In Si the photon peak is not found for lower energies and thin crystals. For well-aligned electrons in Si the yield is more than 160 times the Bethe-Heitler one. In general the peak in the photon spectra disaapears for incident angles larger than half the critical angle for channeling. Dramatic radiative energy losses are found along axial directions - in 0.6 mm 〈110〉 Ge a 150 GeV electron loses more than 60% of its total energy. © 1990.