Although x-ray vision is a staple of science fiction stories, technologies that peer through big and bulky objects remain rather crude and costly. At airports, for instance, scanning through something as small as a suitcase requires x-rays strong enough to render photographic film grey and fuzzy. One can then begin to imagine the woes of those whose job is to stop nuclear materials from entering the United States, stashed away in cars or shipping containers. The amount of radiation required to see through such masses of metal would be a health hazard to drivers and border guards alike.
But Dr. Christopher Morris and Dr. Rick Chartrand, researchers at Los Alamos National Laboratories in New Mexico, may have found a way to quickly screen cars and containers at border crossings. And it does not involve irradiating the occupants. Morris and Chartrand rigged up detectors of sub-atomic particles called muons to track their billiard-ball-like collisions with atoms in the object they wish to probe. Motorists drive in between two detectors and come to a rest for 60 seconds. The researchers say they should be able to detect grapefruit-sized spheres of uranium hidden anywhere in the car, based on its effect on the cosmic ray muons.
Morris and Chartrand propose making the United States’ borders with Canada and Mexico proliferator-proof by deploying muon detectors to sense the presence of what they call “nuclear threat objects.” The total tab would be around US$1 billion, according to Morris, since muon detectors cost about US$1-2 million apiece. They presented their findings at the annual meeting of the American Association for the Advancement of Science (AAAS) last month.
Muons constantly shower down from space onto the surface of the Earth at nearly the speed of light. One muon lands on an area the size of a fingernail each minute. But given their speed and low mass, muons pass harmlessly through matter-except when they hit the nucleus of atoms. The larger the nucleus the muon careens into, the larger the sideways kick the muon gets from it.
By placing muon detectors above and below the object under examination, the researchers track the paths of muons as they zip through the object, and work the size of the kicks muons received in their collisions with atoms as they passed through the car or container. Since muons scatter very strongly off uranium and plutonium, the heaviest atoms found in nature, a minute of exposure between the detectors is sufficient to pinpoint their presence.
Their detector has showed promise in detecting 20-kilogram uranium sphere hidden in the boot of a car all but three per cent of the time. Chartrand, a mathematician, applied so-called machine learning techniques to train computers to automatically read the muon detector data. This, he says, is in order to circumvent current problems with airport luggage screening, which depend on the attendant’s attention and acuity to spot threat objects.
