A radiation detection system that measures radiation (e.g., signature energy-level gamma radiation and neutrons) is employed aboard a ship or other transport vehicle along with conventional cargo to monitor for the presence of a fissile material, as would be found in a nuclear weapon, or for the presence of other sources of radiation. The detection system can be used over the course of the cargo transport, thereby enabling finely tuned monitoring for fissile material across distances of many meters extending through surrounding cargo containers. Because the system can be utilized during a ship's transport, a positive detection of fissile material can be made and acted upon while the ship is still at sea far from the destination port, where detonation of a nuclear weapon could have catastrophic consequences.