Muon Manna? Particle shower may spotlight loose nukes

At U.S. ports and border crossings, agents are increasingly using X-ray surveillance of shipping containers and trucks to foil attempts to smuggle nuclear weapons or radioactive materials into the country. A new study indicates that radiation from the heavens may provide a way to detect such threatening cargoes without requiring potentially dangerous X-ray sources.

Konstantin N. Borozdin of Los Alamos (N.M.) National Laboratory and his coworkers have demonstrated in a laboratory experiment that they can use the relentless rain of cosmic rays to detect chunks of heavy metal. The researchers report their findings in the March 20 Nature.

The presence of such weighty metals in a vehicle could tip off authorities that dangerous nuclear contraband is onboard. Many radioactive elements–particularly the uranium and plutonium used in nuclear weapons–are among the heaviest elements.

To test their approach, the Los Alamos scientists placed a 10-kilogram cylinder of tungsten about the size of a hamburger and its bun into a cosmic-ray detector made up of two stacks of thin, aluminum chambers, each one filled with argon gas and electrified steel wires. One stack was situated above the tungsten target and another below, an arrangement that could be realized in a port or border post by placing chambers above and below a truck.

When electron-like muons, the most common type of cosmic ray, hit argon molecules in the chambers, they free up electrons that generate electric pulses in nearby steel wires. From the pulses, the team determines the trajectories of cosmic rays passing through the device and sees a characteristic bending of those paths by the tungsten.

Since cosmic rays ricochet off the nuclei of heavier elements at larger angles than off those of lighter elements, the trajectories of the cosmic rays can betray the location of heavyweight nuclei. The Los Alamos researchers report that in a minute they can acquire telling images of, say, uranium surrounded by much lighter materials.

That sounds promising, but if the contraband materials were nestled among steel or other weighty materials, the technique would probably take too much time to be practical, says physicist Simon P. Swordy of the University of Chicago.

Moreover, if faster scans are needed, there’s no way to turn up the muon rate, notes Ralph James of Brookhaven National Laboratory in Upton, N.Y. “It’s determined by extragalactic events. We’re not going to change that.”

The ultimate impact of exposure times remains to be seen, say champions of the cosmic-ray technique. George A. Greene of Brookhaven rates the approach as “an absolutely genius idea.” To speed detection, scientists working in labs may be able to reference a library of “signatures” of nuclear materials hidden within common, heavy materials, he proposes.

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