The Muon Portal: a detector for the inspection of container by the muon tomography

    The Muon Portal is a recently project which aims at the construction of a real size tracking detector (18 square meters) for detection of cosmic muons. This apparatus has been designed to inspect the travelling cargo containers using the muon tomography technique: by measuring the deflection of muons when traversing high-Z materials, it is possible to reconstruct a 3D image of the volume to be inspected and detect the presence of fissile (U, Pu) samples inside containers, in a reasonable amount of time, compatible with the requirement of a fast inspection technique. The detection setup is based on 8 position-sensitive X-Y planes, four placed below and four above the volume to be inspected, with good tracking capabilities for charged particles. Compared with similar apparatus designed for the same application by other research groups, the detector of the Muon Portal Project shows some peculiarities inherited from several research fields, making it an innovative detection setup on the whole. The detection planes are segmented into strips of extruded plastic scintillators with WLS fibres to transport the light produced in the scintillator material to the photo-sensors (SiPMs) at one of the fibre ends, in order to optimize the amount of collected photons, still maintaining at a reasonable level the cost and the size of the detection setup. To reduce the overall number of channels (in the order of 10000), the electronics was designed to combine the signals from different fibres in order to reconstruct the muon hits on each plane and unambiguously identify the interested strip inside each module. Detailed GEANT simulations have been made to investigate the response of the apparatus, including the effect of the mechanical supports, the basic structure of the container (roof and doors) and the albedo due to the soil. The tomographic images are reconstructed developing tracking algorithms and suitable imaging software tools. Due to the large acceptance of the detector for cosmic rays, coupled to the good angular reconstruction of the muon tracks, it is also planned to employ such detector for cosmic ray studies, complementing its detection capabilities with a set of trigger detectors located at some distance from it, in order to measure multiple muon events associated to extensive air showers.