ENHANCEMENT OF THE POSITRON INTENSITY BY A TUNGSTEN SINGLE-CRYSTAL TARGET AT THE KEKB INJECTOR LINAC

A new tungsten single-crystal target has been successfully employed at the positron source of the KEKB injector linac. The crystal thickness was determined to be 10.5 mm based on the previous systematic measurements of the positron-production efficiency. The crystal axis, <111>, was precisely aligned to the direction of the 4-GeV primary electron beam. The positron yield increased by ~25 % compared to that for a conventional tungsten plate with a thickness of 14 mm. On the contrary, the heat load on the crystal target decreased by ~20%. No degradation of the positron-production efficiency was observed since the crystal target was installed in September 2006, and it has been stably operating at the KEK B-factory