We report simulation results of pair production by ultra-intense lasers
irradiating a gold target using the GEANT4 Monte-Carlo code. Certain
experimental features of the positron and electron energy spectra are
reproduced, as well as trends with regard to target thickness and hot electron
temperature Te. For Te in the range 5-10 MeV, the optimal target thickness for
pair production is found to be about 3 mm. Further Monte-Carlo simulations may
aid in the optimization of laser-driven positron sources.