Ratio of three-fold coincidences in the fission-neutron time window when the tagged neutron beam aims at the suspicious item (above: HEU shielded with Cd and Pb; middle: bare iron block; below: bare lead block) to three-fold coincidences when the beam aims at another region of the 0.2 g.cm⁻³ iron cargo. This ratio is given as a function of the (y,z) coordinates of the suspicious item in the container (left: 3D representation; right: 2D projection). The origin of the coordinate system is the center of the container: y = −110 cm corresponds to the first wall of the container (close to the neutron generator shown as a red point on the 2D projection maps) and y = +110 cm corresponds to the second wall.