Cosmic-ray muons can be used to detect nuclear materials being smuggled into the country. These images were obtained by measuring muon deflections within a volume containing a steel C-clamp flanked by two steel support rails. The C-clamp rests on a sheet of plastic inserted into channel slots in the rails, which are structural members of an experimental rack. The sequential images—at different planes below, through, and above the C-clamp—show the points in space where the muons are deflected, that is, where the steel is located. When combined, these "slices" yield a three-dimensional reconstruction of the imaged volume. Steel was used in this proof-of-principle experiment as a surrogate for dense nuclear materials.

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