The technique involves the use of muons, which are produced when cosmic radiation decays as it hits the Earth. Los Alamos researchers have developed a system that uses muon radiography to detect uranium, plutonium or other dense materials. A suspect object, such as a cargo container, is passed through two pairs of detectors - one set above the object and one below - that record muons' paths before and after they pass through the object. Analysis of the energy and trajectory of the muons results in a three-dimensional map of the inside of the suspect object, according to a Los Alamos release.
"If we measure the muon's path and energy with two detectors going in and two coming out, we have a straight line on either side that tells us how much the target deflects the muon, and we can locate the highly dense objects, as well as distinguishing between materials," Los Alamos researcher Larry Schultz said in the release.
The detector is capable of spotting nuclear materials even through heavy shielding, such as lead, the release says.
Scientists have been able to "train the system to spot objects of interest with a rate of false positives and false negatives that is less than 3 percent," Rick Chartrand of the laboratory's Theoretical Division said in the release. "We think we can continue to improve that."
Los Alamos researchers described their efforts to develop a nuclear detector using muon radiography during a presentation in Washington at this year's meeting of the American Association for the Advancement of Science.
The Bush administration has placed a new focus on combating the smuggling of nuclear and radioactive materials, requesting funds in its proposed fiscal 2006 budget to create an office of domestic nuclear detection within the Homeland Security Department. The new office reportedly would include representatives from several governmental agencies, including the Defense, Energy and State departments, and would focus on activities such as developing new detection techniques and increasing training in their use.
Muon radiography has several advantages over detectors now deployed at U.S. borders, which use either X-rays or gamma rays, according to the laboratory. For example, gamma-ray detectors are less penetrating than those using muons, produce results that require additional interpretation and require the use of hazardous material such as cobalt.
Los Alamos scientists are now working to develop a set of muon radiography detectors large enough to scan large metal objects within 60 seconds. As the process develops, inspectors using the detectors may be able to clear a vehicle within about 20 seconds of muon exposure, the laboratory release says.
"We believe we've worked through all of the major obstacles to building a prototype system for a range of security issues," Chris Morris of the laboratory's Physics Division said in the release.