One of the most pressing international priorities is to control the dissemination of nuclear materials that could be used in attacks by terrorists or rogue states. Nuclear materials contain unstable isotopes, which emit x-rays and gamma rays. The characteristic energies of these photons provide a fingerprint revealing which radioactive isotopes are present. Unfortunately, some isotopes that occur in benign applications emit gamma rays with energies that are very similar to those emitted by materials used in weapons, which leads to ambiguous identifications and false alarms. This problem has been worrying the United States, which is installing thousands of radiation portal monitors to detect the gamma rays emitted by nuclear materials carried by vehicles crossing the Canadian and Mexican borders. One of the worst fears of the authorities is that terrorists might smuggle highly-enriched uranium into the country to build a crude Hiroshima-style atomic bomb.