GEANT4 simulation of the testbeam set-up for the ALFA detector

We present the results of a simulation in GEANT4 of a testbeam set-up for the ALFA luminosity detector for ATLAS. In particular, we compare the expected detector resolution with the experimental measurements obtained with a low-energy electron testbeam. The different electromagnetic processes contributing to the energy evolution of the resolution are studied in detail. Finally, the extrapolation of the detector performance under LHC conditions indicates that an ultimate space-point resolution of about 20 mum is possible, fully satisfying the physics requests for luminosity calibration.