  We have simulated the performances of a GEM with a large density of
multiplication holes. In particular in this note we fully simulate the GEM
detector used in the PixiE experiment. The detector will be employed in the
measurement of the polarization of X-rays produced by astrophysical sources and
its optimization is critical. In particular we have studied the gain of the GEM
and the diffusion of the charge inside the detector for different
configurations of electric fields and for different gas mixtures. This study
has been performed to finalize the design of the PixiE Imager Detector.
