  For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at
CERN's Antiproton Decelerator (AD) facility we have constructed a source of
slow positrons, which uses a low-energy electron linear accelerator (linac).
The driver linac produces electrons of 9 MeV kinetic energy that create
positrons from bremsstrahlung-induced pair production. Staying below 10 MeV
ensures no persistent radioactive activation in the target zone and that the
radiation level outside the biological shield is safe for public access. An
annealed tungsten-mesh assembly placed directly behind the target acts as a
positron moderator. The system produces $5\times10^7$ slow positrons per
second, a performance demonstrating that a low-energy electron linac is a
superior choice over positron-emitting radioactive sources for high positron
flux.
