Instrument simulation for the analysis of cosmic ray electron with the Fermi LAT

The Fermi LAT collaboration has built up a detailed Monte Carlo simulation to characterize the instrument response and tune its performance. The simulation code is built around the widely used GEANT4 toolkit and was carefully validated against beam test and flight data. This poster shows how the full LAT simulation is used to develop the event selection for the Cosmic-Ray Electron (\emph{CRE}) analysis so as to optimize the instrument performance. In particular, we will show in detail the determination of the geometry factor and the residual hadron contamination. The very accurate MC simulation proved to be fundamental to control the systematic uncertainties on the CRE spectrum measured by the Fermi LAT.