Design of an atmospheric muon tomographer for material identification based on CORSIKA+GEANT4 simulations

In recent years, muon tomography has turned into a powerful and innovative technique for non-invasive imaging of large and small structures with applications in different areas like geology, archaeology, security, etc. We present the design and simulation of a transportable and easy to construct detector based on plastic scintillator and Silicon photomultipliers current technology. From a flux of cosmic rays reaching the atmosphere we simulated atmospheric muons at ground using CORSIKA. The detector and the object to analyze are simulated with GEANT4, where the previously obtained muon flux is transported. We use two methods for muon tomography to differentiate objects made of different materials: absorption and scattering. The statistical differences for several object sizes and materials are quantified. Using a threshold of 3 $σ$ in the first method, we conclude that materials made of lead can be differentiated from objects made of other materials. The observation time needed to differentiate an object made of lead from one of aluminum was 4.9 and 9.9 days using the first and second method, respectively. In general, the absorption method gives the best results.

💡 Research Summary

This paper presents the design and simulation-based performance evaluation of a portable atmospheric muon tomographer for non-invasive material identification. Muon tomography utilizes highly penetrating cosmic ray muons, which are naturally abundant and can reveal internal density and composition variations in objects without physical intrusion.

The core of the study is the design of a cost-effective and transportable detector system. The basic sensing unit consists of a BC408 plastic scintillator tile coupled to a Silicon Photomultiplier (SiPM). An array of 8x8 such units forms a detection plane. The complete tomographer comprises two “sub-detectors,” each made of two parallel detection planes separated by 20 cm, placed before and after the object under investigation. This configuration allows for tracking the incoming and outgoing muon trajectories, achieving an angular resolution of about 1 degree.

The performance was rigorously assessed through a two-stage simulation pipeline. First, the CORSIKA software was used to simulate extensive air showers initiated by cosmic rays, generating a realistic flux of muons at ground level (average energy ~4 GeV) for the geographical conditions of Lima, Peru. Second, this muon flux was fed into a detailed GEANT4 simulation that modeled the passage of muons through the designed detector geometry and through target blocks of various materials (aluminum, iron, lead, concrete, water, air) and thicknesses (50 to 250 cm).

The research compared two fundamental muon tomography techniques: the absorption method and the scattering method. The absorption method measures the attenuation of the muon flux after traversing the object, which correlates with density and thickness. The scattering method measures the angular deflection of muon trajectories caused by multiple Coulomb scattering, which is sensitive to the atomic number of the material.

Key results from the simulations show that both methods can differentiate materials, but with varying efficiency. Materials with high atomic number and density, like lead, showed the most significant muon attenuation and scattering. A statistical analysis using a 3-sigma threshold confirmed that lead could be clearly distinguished from other materials like aluminum and iron. A direct comparison revealed that the absorption method is more efficient for material discrimination under the studied conditions. For instance, to differentiate a lead block from an aluminum one, the required observation time was calculated to be 4.9 days using the absorption method, compared to 9.9 days for the scattering method. However, distinguishing between materials with similar densities, such as aluminum and concrete, proved challenging with both approaches.

In conclusion, the study demonstrates the feasibility of a compact, scintillator-SiPM based muon tomographer. It validates that the absorption method provides a faster means to identify high-Z materials like lead, suggesting potential applications in security (e.g., detecting shielded nuclear materials) and civil engineering. The authors note that while the simulation assumes simple, homogeneous blocks, future work must address image reconstruction algorithms for complex, heterogeneous real-world objects.