  Spatial resolution is a significant factor in the GEM performance in view of
X-rays radiography and UV, visible light imaging. Monte-Carlo method is used to
investigate the spatial resolution determined by the transverse diffusion in
the device. The simulation results indicate that the electrical parameters,
such as the GEM voltages and the electric field at the drift and induction
regions, only have minor effects on the spatial resolution. The geometrical
parameters and the working gases chosen, on the other hand, are the main
parameters that determine the spatial resolution. The spatial resolution is
determined more on the drift and diffusion processes than on the avalanche
process. Especially for the different working gases, the square root function
of the ratio of the electron diffusion coefficient and the mobility has a
significant effect on the spatial resolution.
