Channeling and coherent bremsstrahlung effects for relativistic positrons and electrons

Channeling of positrons in single crystals of silicon has been observed in transmission and scattering measurements for incident energies from 16 to 23 MeV. In addition, the spectral dependence upon crystal orientation of the forward coherent bremsstrahlung produced by beams of 23 MeV positrons and electrons incident upon a 5 μm thick single crystal of silicon was measured with a NaI photon spectrometer. Effects of channeling and perhaps of the non-validity of the first Born approximation were observed for beam directions near the <111> axis of the crystal, and coherent peaks near 0.5 MeV were observed for a compound interference direction, in agreement with first-order theoretical calculations