Simulation of one-fold (above), two-fold and three-fold coincidences (below) after a 10 min inspection of a container filled with a 0.2 g.cm⁻³ iron cargo. Neutron generator total emission is 3 × 10⁷ s⁻¹ and a HEU block, bare or in cadmium and lead shielding, is in the center of container. For comparison, simulations without nuclear materials (shield alone or bare iron matrix) are also reported.