Experimental study of positron production from a W single crystal by the KEK 8-GeV electron linac beam

We have measured the positron production efficiency from tungsten single-crystal targets using an 8 GeV electron beam. A single-bunch beam with a bunch width of 10 ps, a repetition rate of 2 Hz, and an intensity of 0.2 nC/bunch was incident on a target mounted on a precision goniometer. Positrons produced in the forward direction were detected by a magnetic spectrometer in the 10-20 MeV/c momentum range. Systematic data on the target-thickness dependence and the momentum distribution of the produced positrons were obtained for crystal targets. The results show that, when the crystal axis <111> is aligned to the electron beam direction, the positron yield increases compared to the amorphous case by factors of 6.5, 3.4 and 2.3 at 10 MeV/c for 2.2, 5.3 and 9. 0 mm thick crystals, respectively. We observed that the positron yield from the 9.0 mm thick crystal is larger than the maximum yield attainable with 18-20 mm thick amorphous targets at 8 GeV. (C) 2002 Elsevier Science B.V. All rights reserved.