New technique may provide better port screening for nuclear materials

Nature : Cargo containers that enter a country’s ports are inspected for a variety of illegal goods, including nuclear materials. However, it is relatively easy to shield nuclear materials from detectors that are passively looking for emitted radiation. An active option for penetrating shielding uses gamma rays to create images of the materials inside containers. The screening technique exploits the fact that high-Z elements absorb high-energy gamma rays more readily than low-Z elements do. Using a broad-spectrum source to generate the gamma rays would require levels of radiation that are dangerous to the people in the area, including port employees, stowaways, and human trafficking victims hidden inside cargo containers. Now Areg Danagoulian of MIT and his colleagues have developed a variant of this technique that reduces the danger by using just two gamma-ray wavelengths, 4.4 MeV and 15.1 MeV. The two wavelengths have been shown to clearly distinguish between types of metals based on their elemental weights. This means that uranium or plutonium, which are both much heavier than most nonnuclear metals, would appear as distinct from lead or iron. So far, Danagoulian’s team has demonstrated the technique’s capabilities only with nonnuclear materials.