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 4and 8GeV 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 9mm-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. INTRODUCTION For future ee linear colliders and high luminosity Bfactories, it is critically important to develop a highintensity positron source. In a conventional method using an amorphous heavy metal target, the target thickness is determined to produce the largest number of lowmomentum positrons, which can be accepted in the succeeding accelerator section. The momentum acceptance is typically 5–25 MeV/c and the optimum thickness is 4–5 X0 for a 4–8 GeV electron beam. In this case, the only possibility to increase the positron intensity is to increase the incident electron intensity. However, the electron intensity is limited because of a heat load on the target. One promising method utilizing a crystal target was proposed by Chehab et al. [1] in 1989. The benefit of this method is on its high positron-production efficiency due to channeling radiation (CR) and coherent bremsstrahlung (CB), since CR and CB increase the number of low-energy photons in the radiation process. This results for the same positron yield in a thinner target compared with the conventional method. It is expected that the thin target relaxes the heat load problem, and that the spatial spread of positrons due to multiple scattering in the target is affected. There are two schemes on the target configuration for positron production utilizing a crystal target. One scheme is to use a heavy crystal target, such as a tungsten crystal, and the other is to use a light crystal target combined with amorphous tungsten. For the heavy crystal target, creation of channeling photons and positron production through ee pair production process proceed simultaneously in the heavy crystal target. On the other hand, for the latter target configuration, generation of photons by channeling proceeds in the light crystal target and the positron production proceeds in amorphous tungsten mounted just behind the crystal target. It should be experimentally investigated which target configuration shows larger amount of positron production since there have so far been only a few experimental results for the positron production on the basis of the crystal target. In this report, we summarize a series of the experiments of positron production by 4and 8-GeV electron beams hitting tungsten crystal and diamond target, respectively [2-4].