Concerns about the illicit movement of radionuclides across national borders have heightened the degree of protection that the US and western European countries have instituted. The challenge is not only to detect hidden radioactive materials, but also to distinguish them from legitimate radionuclides such as radiopharmaceuticals that are often transported across borders and shipped throughout a country. Every day, more than 300 000 vehicles, roughly 2500 aircraft, and nearly 600 ships pass through US ports of entry. With more than 600 US border sites to protect, screening imported radioactive material requires a careful balance of high throughput and high search efficiency. Unfortunately, the two requirements are at odds: Rapid screening implies less counting time available to detectors (see the article by Jay Davis and Don Prosnitz in Physics Today, Physics Today 0031-9228 56 4 2003 39 https://doi.org/10.1063/1.1580048 April 2003, page 39 ).
What complicates the efficiency of this screening...
