Experimental study of positron production from monocrystalline targets at the KEKB injector linac

We report a series of positron-production experiments from an axially-oriented single-crystal target by 4- And 8- GeV channeling electrons at the KEKB injector linac. The targets tested are tungsten crystals with different thicknesses (14.2 mm at maximum), and diamond crystals with two different thicknesses (7.25 mm at maximum). The axis <111> for the tungsten crystals and that <110> for the diamond crystals have been aligned to the electron beam by a precise goniometer. The positron-production efficiencies in the momentum range of less than 30 MeV/c were systematically investigated for each target by measuring the positron yield generated in the forward direction with a magnetic analyzer. The results show that the maximum positron-production efficiencies from the 9- mm-thick tungsten crystal and from the 7.25-mm-thick diamond combined with amorphous tungsten are 17% and 12%, respectively, larger than that from amorphous tungsten at the momentum of 20 MeV/c. In a series of the experiments using the crystal targets, the target thickness giving the maximum positron yield is clearly reduced in comparison with that of the amorphous tungsten alone due to the crystal effect.