GEM Detectors for Muon Tomography of Nuclear Contraband

The construction of a Muon Tomography station is presented. Muon Tomography (MT), based on scattering of cosmic ray muons, is an improvement to actual portal monitors at borders, since the current techniques use regular radiation detection that are not very sensitive to nuclear contraband (U, Pu), if these materials are well shielded to absorb emanating radiation. We use a low mass, high spatial resolution ($\sim $50 $\mu $m) Gas Electron Multiplier (GEM) detectors for MT to overcome the intrinsic limitations. The prototype MT station employs 6 tracking stations based on 33cm x 33cm triple-GEM detectors with 2D readout. The detectors are arranged into tracking superlayers at the top and bottom of the probed volume. Due to the excellent spatial resolution of GEM, it is sufficient to use few cm gap between tracking stations. We present details of the production and assembly of the GEM-based tracking stations in collaboration with CERN-GDD lab and RD51 experiment as well as the design of the corresponding front-end electronics and readout system. Discussion about GEM detectors in two sides of the probed volume for a complete muon tracking, and large-area (1m x 1m) GEM-based MT station prototype to be tested under realistic conditions, are made.