  Muon Tomography (MT) based on the measurement of multiple scattering of
atmospheric cosmic ray muons traversing shipping containers is a promising
candidate for identifying threatening high-Z materials. Since
position-sensitive detectors with high spatial resolution should be
particularly suited for tracking muons in an MT application, we propose to use
compact micro-pattern gas detectors, such as Gas Electron Multipliers (GEMs),
for muon tomography. We present a detailed GEANT4 simulation of a GEM-based MT
station for various scenarios of threat material detection. Cosmic ray muon
tracks crossing the material are reconstructed with a Point-Of-Closest-Approach
algorithm to form 3D tomographic images of the target material. We investigate
acceptance, Z-discrimination capability, effects of placement of high-Z
material and shielding materials inside the cargo, and detector resolution
effects for such a MT station.
