Stopping Nuclear Smugglers

Photo: A red lamp that warns when X-ray is on

Around 80 per cent of freight containers coming into the US are already screened by a radiation portal monitor, which detects any gamma rays and neutrons that escape from the container. But these scanners have significant limitations. Firstly, they wrongly flag around 2 per cent of all containers as suspect - they cannot distinguish between a plutonium bomb and the radioactive potassium-40 found in for example bananas. More importantly, they fail to detect the most dangerous nuclear material of all: highly enriched uranium (HEU). That is because unlike plutonium HEU emits only low-energy gamma rays. If uranium is shielded by just a thin layer of lead, or even wood, the detectors miss it.

Now, physicist William Bertozzi at the Massachusetts Institute of Technology is working on a technology known as nuclear resonance fluorescence imaging (NRFI). In NRFI, an X-ray beam excites all the nuclei of all atoms in the container. A gamma-ray detector then identifies the atoms according to the unique energies and intensities of the photons emitted as the nuclei return to their original state without undergoing fission.

Bertozzi decided to apply NRFI to cargo containers when he realised that an X-ray beam of high enough energy to penetrate a lead shield would also span the range of energies needed to excite each of the atoms in the periodic table. The technique can identify both radioactive and non-radioactive isotopes in a container.

By carefully positioning the detectors and the beam, Bertozzi can illuminate each 20-centimetre cube of the space within the container individually, so putting the resulting images together will give a 3D image of the location of all the radioactive and non-radioactive isotopes in the cargo.; Source: New Scientist