  New kind of positron sources for future linear colliders, where the converter
is a tungsten crystal oriented on the <111> axis, has been studied at CERN in
the WA103 experiment. In such sources the photons which create the $e^+ e^-$
pairs result from channeling radiation and coherent bremsstrahlung. In this
experiment electron beams of 6 and 10 GeV were sent on different kinds of
targets: a 4 mm thick crystal, a 8 mm thick crystal and a compound taraget (4
mm crystal + 4 mm amorphous disk). An amorphous tungsten target 20 mm thick was
also used for comparison with the 8 mm crystal. Tracks of outgoing charged
particles were detected and analyzed by a drift chamber in a magnetic field.
The energy and angle spectra of the positrons were obtained for energies up to
150 MeV and angles up to 30 degrees. The measured positron distribution in
momentum space (longitudinal versus transverse) is also presented, giving a
full momentum space description of the source. Results on outgoing photons are
also presented. A significant enhancement of both photon and positron
production is clearly observed. At 10 GeV incident energy, the positron
enhancement factor is 4 for the 4 mm crystal, about 2 for the 8 mm crystal.
Besides, the simulation code for the crystal processes is validated by a quite
good agreement between the simulated and experimental spectra, both for
positrons and photons.
