Abstract
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 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 $\ensuremath{\sim}25%$ compared to that for a conventional tungsten plate with a thickness of 14 mm. On the contrary, the steady-state heat load on the crystal target decreased by $\ensuremath{\sim}20%$. After a two-month operation, no degradation of the positron-production efficiency was observed, and the crystal target has been stably operating at the KEK B factory.
Highlights
In order to achieve high luminosities in high-energy eeÿ collider experiments, positron sources must be reinforced, especially towards the generation of B factories and eeÿ linear colliders.In a conventional positron source, positrons are produced by high-energy electrons hitting a heavy-metal target, and low-momentum positrons are captured and accelerated in the succeeding positron capture section
For practical applications of the crystal target at the KEKB injector linac, we systematically studied the FIRST APPLICATION OF A TUNGSTEN SINGLE
The results show that the positron-production efficiencies of the first bunch are 0:25 0:01 (0:26 0:01) and 0:2 0:01 (0:2 0:01) on average for the tungsten crystal target and the previously used tungsten target, respectively
Summary
In order to achieve high luminosities in high-energy eeÿ collider experiments, positron sources must be reinforced, especially towards the generation of B factories and eeÿ linear colliders. The maximum positron-production efficiencies are obtained with an optimized target thickness of 4 –5 X0 (radiation length) and with a typical momentum acceptance of 5–25 MeV=c for 4 –8 GeV incident electrons. In this case, a possible scheme to increase the positron intensity is to increase the incident electron intensity and energy. The results show that the positron yield was enhanced by 40% compared with that from a disoriented target with 3-GeV electrons [11] Encouraged by these stimulating results, we proceeded to systematic studies on the positron-production efficiencies with tungsten crystals having various thicknesses using 4- and 8-GeV electron beams [12,13]. Performance of the tungsten crystal target are described in detail
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