Muon scattering tomography with resistive plate chambers

Muon Scattering Tomography is a scanning technique which uses cosmic muons as probes to gather information on the content of the inspected volume. Because the scattering angle depends on the Z² of the materials in the volume, it is possible to obtain a 3D image of the volume content by carefully tracking the muon paths. This has very interesting potential applications in several fields, from engineering to homeland security, where MST could be used to inspect shipping containers and trucks in search of special nuclear materials (SNM). For such applications to be feasible it is necessary to have large area detectors (10-100 square meters) while maintaining the efficiency and angular resolution necessary to discriminate between low-Z and high-Z materials. Resistive Plate Chambers (RPCs) are very good candidates. RPCs are widely adopted in high energy physics experiments thanks to their excellent performance in terms of time resolution, charged particle detection efficiency and low cost per unit area. In collaboration with the UK Atomic Weapon Establishment we have built and tested a prototype based on 12 RPCs (50 cm × 50 cm). To obtain the required spatial resolution we adopted a novel approach for the RPC readout, coupling each detector with 300 fine pitch strips (1.5 mm) and using multiplexing analog readout chips to reduce the amount of readout channels. The prototype performs very well: all the chambers have efficiency above 99% and purity above 95%. The signal-to-noise for the electronic readout ranges from 25 to 90. The spatial resolution for the layers is better than 1 mm and we show that this is sufficient to successfully image a block of lead of 10 cm × 10 cm × 15 cm. We are now in the process of upgrading the electronics with new ASICs which feature built-in ADCs. The first test