Large-Area Time & Position Sensitive RPC for Muon Scattering Tomography

Over the past decades, Resistive Plate Chambers (RPCs) have become widely used in high-energy physics experiments, notably for muon triggering and tracking across large areas. RPCs are particularly well suited for muon detection over extensive areas, as they combine relatively low construction cost with high efficiency and precise spatial and temporal resolution. Nevertheless, instrumenting a sensitive area of tens or hundreds of square meters with a strip pitch in the millimeter range can represent a financial challenge primarily driven by the electronics. To address this issue, a novel readout encoding scheme was designed and tested, enabling a significant extension of the detector area without a proportional increase in electronic channels. The method relies on a Signal Merging Printed Circuit Board (SMPCB), which combines signals from multiple readout strips in parallel before feeding them to preamplifiers. By connecting several SMPCBs in a daisy-chain configuration, only 48 electronic channels were needed to read out 888 strips on a 130 × 90 cm² timing Resistive Plate Chamber (tRPC). Comprehensive tests using 30 × 30 cm² prototypes and the 130 × 90 cm² detector demonstrated the effectiveness of the proposed signal multiplexing approach during cosmic-ray measurements. Sub-1 mm 2D spatial resolution and sub-100 ps time resolution were achieved in high-efficiency tRPCs with areas exceeding 1 m², while the number of electronic channels was reduced by an order of magnitude.
FLUKA Monte Carlo simulations were also performed to evaluate the use of tRPCs with high spatial and temporal resolution for scanning very large volumes through the Muon Scattering Tomography (MST) technique. Extensive simulations were conducted using the sea-level muon flux as the source term, transported through both a medium-sized volume (1 m³) and a truck-sized geometry (150 m³). Two methods were developed to obtain the muon spectrum at the Earth’s surface: one based on propagating Primary Cosmic Rays (PCRs) through the FLUKA atmospheric model, and the other employing a muon generator implemented for these tests. An acquisition time of 1 minute was found sufficient to detect a 10 × 10 × 10 cm³ tungsten block inside a large shipping container. The simulation results for the medium-sized volume were also compared with experimental data from an MST scan of blocks made of different materials, performed using a 2 m² muon tracker. The combination of millimeter spatial resolution and sub-100 ps time resolution for low-energy muon rejection proved adequate for this muon imaging method.