Measurement of axial effects on the pair creation by 30-150 GeV photons and on the radiation emitted by 150 GeV electrons and positrons in Ge crystals

Crystal-assisted processes around the 〈110〉 axis of a Ge crystal cooled to 100 K have been investigated at CERN with 150 GeV e^- and e⁺ incident beams and 30-150 GeV incident photons. A large enhancement of the radiation emitted by incident e⁺ and e^- is observed. Electrons are found to radiate more than positrons. When the crystal is tilted around 〈110〉 clear channeling effects are still observed. However the radiation enhancement persists at angles much larger than the channeling critical angle. The total pair creation rate by photons aligned along 〈110〉 has also been measured: in contrast with what is observed for a random orientation, the rate increases sharply with the photon energy. A scan through 〈110〉 shows that the pair production does not reach a maximum for axial incidence but at a tilt angle which decreases when the photon energy increases. These measurements indicate that crystal assisted pair production is not governed by the channeling of the created electrons.