Production of a High Energy, High Power Polarized Photon Beam at SLAC

Attenuation by coherent pair production in highly oriented compression annealed pyrolytic graphite has been used to polarize a 16 GeV bremsstrahlung beam. Graphite pieces of total volume 0.95 × 0.95 × 5.1 cm3 were assembled in water-cooled precision holders. Eighteen of these units were mounted in two independent coaxial assemblies. Each assembly provided precision angular orientation with respect to the beam direction, rotation of the graphite by 90° about the beam axis to rotate the plane of polarization, and removal from or insertion into the beam. One assembly, with 61 cm of graphite, served as a polarizer; the other, with 30.5 cm of graphite, was used as an analyzer. The complete device was placed between collimators in a sweeping magnet. Polarization of the attenuated beam, measured with the SLAC pair spectrometer, was 0.255 ± 0.020. This number, and the measured spectrum, are in qualitative accord with the calculations. The beam has yielded intensities greater than 4 × 108 equivalent quanta per pulse, and has been used in an experiment to measure asymmetries in pseudoscalar meson photoproduction.