Compact Positron Source Optimization

High brightness positron tools are limited to a few labs in the world due to the scale of the facilities required to generate >10⁷ slow positrons/second. The development of a compact slow positron source at SLAC National Accelerator Laboratory relies upon the optimization of every component. In conventional linac based positron sources, an electron beam is incident upon a high Z target which produces positrons with a large spread of energies and angles, which is followed by a moderator which converts a tiny fraction of those positrons into a slow positron beam of about 3 eV in energy and thermal energy spread. Typical total efficiencies for these systems are of the order 10^-7 positrons per incident electron. Moderators are known to have efficiencies highly dependent on the energy of the incident fast positrons, so the overall increase in fast positron count provided by using beams of higher electron energies is mitigated by the increase in mean fast positron energy. Using Geant4 based modeling, we optimize both target geometry and the use of a decelerating linac to tailor the fast positron spectrum and increase slow positron capture by over an order of magnitude. Applications to Positron Annihilation Lifetime Spectroscopy or Positron Diffraction are discussed.