Interest from governments in screening cargo for nuclear material increased dramatically after the 11 September 2001 terrorist attacks. At present, the US Department of Homeland Security (DHS) uses passive-radiation detectors to scan all cargo containers entering the country by land and sea; similar systems are used in the United Kingdom and elsewhere. But passive detectors are prone to producing false positives caused by benign, but slightly radioactive materials such as cat litter and stoneware. More problematically, a terrorist can shield material such as uranium so that a passive system cannot detect it.
In the past few decades, active-screening technologies have become available. These devices produce images of cargo containers using high-energy γ-radiation, similar to medical X-rays. Technology currently used in the United States and United Kingdom generates this radiation through a process known as 'bremsstrahlung', in which decelerating charged particles emit γ-rays at a range of energies. The γ-rays travel through cargo and strike a detector on the other side, creating an image.
These images can alert inspectors to dense material hidden inside lighter cargo — a chunk of uranium inside a shipment of wheat, for example. But they cannot distinguish metals and other dense materials from one another, so shielding nuclear material is not that difficult. “If you take a nuclear weapon and you throw it into scrap metal or into some random cargo, it won’t look like anything,” says physicist Areg Danagoulian of the Massachusetts Institute of Technology (MIT) in Cambridge, who led the latest research.