A device that detects gamma rays or neutrons, and determines their source location, comprises two scintillator panels separated by a shield barrier. Particles incident from one side can fully strike the first scintillator, but are blocked by the shield from reaching the second scintillator. Particles from the other side can reach only the second scintillator. Thus the detector indicates the left-right direction for the source location quickly, and then with further data localizes the source precisely by analysis of the two opposite scintillator count rates versus angle, using methods disclosed. The detector enables rapid inspections of vehicles and cargo containers for clandestine radiological and nuclear weapons, and sensitive localization of radioactive material in a walk-through portal application. Detectors with such capabilities are essential for stopping nuclear and radiological terrorism.