Discovery Magazine

Photo Credit: Mike Phipps

Detection of Heavily Shielded Nuclear Contraband through Muon Radiography with Advanced Micro-pattern Detectors

Muon tomography detects and images objects composed of dense nuclear materials with high atomic number Z by measuring the multiple Coulomb scattering of atmospheric cosmic ray muons that traverse such objects. Due to the penetrating nature of multi-GeV cosmic ray muons, high-Z objects surrounded by considerable low-Z or medium-Z shielding materials can also be probed. It aims in particular at detecting and imaging well-shielded high-Z nuclear material, e.g. nuclear contraband in cargo for homeland security. This zero-dose technique exploits multiple scattering of atmospheric cosmic ray muons, which is stronger in dense, high-Z nuclear materials than in shielding materials with lower Z.

Our effort has been focused on using Gas Electron Multiplier (GEM) detectors, i.e. micropattern gas detectors (MPGD), as tracking detectors for muon tomography. MPGDs can achieve the highest spatial resolutions (50 to 150 μm) among gaseous detectors and a planar GEM detector is only on the order of 1 cm thick. This allows the construction of a more compact muon tomography station that has tracking stations with vertical dimensions smaller than 10 cm, i.e. an order of magnitude smaller than muon tomography stations employing drift tubes. We have constructed and are operating a compact muon tomography station that tracks muons using eight 30cm×30cm Triple Gas Electron Multiplier detectors. Tracking units comprising two such detectors spaced 7.6 cm apart are placed on four sides of a 27-liter cubic imaging volume. Pulse data from 2D readout strips are acquired with an RD51 Scalable Readout System. 

The station can image shielded depleted uranium using a simple point-of-closest-approach scattering reconstruction technique. With about 100k reconstructed muon tracks, the station can resolve the gap between two high-Z objects placed at least 6 mm apart in the horizontal plane. It images a 75 cm3 depleted uranium cube shielded on all sides by 2-3 cm thick medium-Z material or shielded by vertical clutter of 12 cm of steel. Shapes of metallic objects with dimensions of a few cm that are shielded by a lead box with 3.5 mm walls on all sides are reconstructed successfully in 3D with muon tomography. Using observed scattering signatures, we can distinguish these small shielded objects from each other according to their radiation lengths; this includes clearly distinguishing shielded uranium from shielded lead.