Intense positron sources are being widely investigated for the next-generation linear colliders and B factories. A new method utilizing an axially oriented crystal as a positron-production target is one of the bright schemes, since it provides a powerful photon source through channeling and coherent bremsstrahlung processes when high-energy electrons penetrate the target. A series of positron-production experiments with tungsten crystals hit by 4- and 8-GeV single-bunch electron beams were carried out at the KEKB 8-GeV injector linac. Three tungsten crystals with different thicknesses (2.2, 5.3, and 9.0 mm) and those combined with amorphous tungsten plates were tested on a precise goniometer. The positron-production yields were measured with a magnetic spectrometer in the positron momentum ${(P}_{e^{+}})$ range from 5 to $20\mathrm{MeV}/c.\mathrm{}$ The angle of the $〈111〉$ crystal axis with respect to the electron-beam direction was controlled by measuring the relative intensities of the produced positrons as a function of the rotational angle of the goniometer. The results show that the enhancements of the positron yield from crystal targets compared to amorphous targets of the same thickness at $P_{e^{+}}=20\mathrm{}\mathrm{MeV}/c$ are from 1.5 to 3.7 and from 1.8 to 5.1, depending upon the target thickness for 4- and 8-GeV electrons, respectively.

• Received 28 May 2001

DOI:https://doi.org/10.1103/PhysRevE.67.016502