Positron Production at JLab Simulated Using Geant4

The results of a Geant4 Monte-Carlo study of the production of slow positrons using a 140 MeV electron beam which might be available at Jefferson Lab are presented. Positrons are produced by pair production for the gamma-rays produced by bremsstrahlung on the target which is also the stopping medium for the positrons. Positrons which diffuse to the surface of the stopping medium are assumed to be ejected due to a negative work function. Here the target and moderator are combined into one piece. For an osmium target/moderator 3 cm long with transverse dimensions of 1 cm by 1 mm, we obtain a slow positron yield of about 8.5·1010/(s·mA) If these positrons were remoderated and re-emitted with a 23% probability we would obtain 2·1010/(s·mA) in a micro-beam.