Experimental determination of the positron source emittance for a crystal target

Investigations on positron sources, using channeling of ultrarelativistic electrons in axially oriented crystals, are carried out in Europe and Japan. Recently, an experiment (WA 103) has been worked out in CERN, using 6 and 10 GeV incident electrons impinging on tungsten crystals along the <111> axis. Crystals 4 and 8 mm thick and compound target made of a 4 mm crystal followed by a 4 mm amorphous disk were used. The positron trajectories, in the horizontal plane, were reconstructed in a drift chamber partially immersed in a magnetic field. The positron momentum and its emission angle were, thus, determined. The momentum spectrum as well as angular distribution were obtained for each target and comparisons were made for crystal and amorphous targets of the same thickness. The experimental data has also been used to study the transverse phase space of the crystal sources. Comparison were operated with thick amorphous targets and the accepted yields, in usual matching conditions for a linear collider, have been derived for both kind of targets. (authors)