Abstract: Apparatus and methods for effectively detecting and locating explosive substances within remote targets, including improvised explosive devices (IEDs). The detection apparatus includes a neutron beam generator, a gamma ray detector, data collection modules and sensors, and a detection processing module. The neutron beam generator includes a fast neutron source, a neutron moderator to slow some or all of the fast neutrons to thermal energies, a partially enclosing neutron shield, and a rotatable neutron shield surrounding the generated neutrons. The neutron shield has an aperture to form a neutron beam directed at a remote target. If the remote target contains explosive substances, gamma rays radiate isotropically from the remote target when it is bombarded by the neutrons. A portion of these gamma rays are intercepted and detected by the gamma ray detector, which is spaced apart from the neutron source.