Experimental study of positron production from silicon and diamond crystals by 8-GeV channeling electrons

We measured positron-production efficiency from an axially-oriented single crystal target by 8-GeV channeling electrons at the KEK electron/positron injector linac. The targets tested are silicon crystals with the thicknesses of 9.9, 29.9 and 48.15 mm, a diamond crystal with the thickness of 4.57 mm, and their combinations with amorphous tungsten plates. The crystal axis 〈110〉 has been aligned to the electron beam by a precise goniometer. The positron-production efficiency in the momentum range of 10-30 MeV/c was systematically investigated for each target by measuring the positron yield generated in the forward direction with a magnetic analyzer. The result shows that the maximum positron-production efficiency from the 29.9-mm-thick silicon and the 4.57-mm-thick diamond combined with amorphous tungsten are 6.1% and 2.9%, respectively, larger than that from amorphous tungsten at the momentum of 20 MeV/c. In such a light crystal target combined with amorphous tungsten, the total target thickness, which gives the maximum positron yield, is clearly reduced in comparison with that of the amorphous tungsten alone. The result is also compared with our previous experiments for tungsten crystal targets.