Experimental studies of positron sources using multi-GeV channelled electrons at CERN(a) and KEK(b)

Enhancements of photon production by multi-GeV electrons moving along the axes of aligned crystals are used to generate high yields of positrons in the same crystals or in amorphous disks put after the crystals. The experiment WA103 used tungsten crystals oriented along their < 111 > axis, alone or followed by tungsten amorphous disks. The incident electron energy was chosen between 5 and 40 GeV and mainly used at 6 and 10 GeV. Photon as positron production enhanced by channelling and coherent bremsstrahlung have been measured with photon and positron detectors. The positron detector consisted in a drift chamber immersed in a magnetic field and presenting rather large momentum and horizontal angle acceptances of 150 MeV/c and 30 degrees, respectively. The KEK experiment on the KEKB injector used 8 GeV incident electron beam and different kinds of crystals. Tungsten crystals and more recently Silicon and Diamond crystals have been used. The measurements concerned the positrons. The results gathered on both experiments are presented and commented.