How do we detect nuclear materials at the border?

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June 8 2005 7:13 PM

Is Cat Litter Really Radioactive?

And how do we detect nuclear materials at the border?

Last Friday, the Department of Homeland Security announced that the nation's busiest seaports will have enough radiation detectors to screen every cargo container that comes through. But congressional critics say expensive "Radiation Portal Monitors" can't tell the difference between highly enriched uranium and cat litter. Is cat litter really radioactive?

Yes, the clay in cat litter does give off radiation in very small quantities. There is naturally occurring radiation all around us; the radiation in cat litter comes from trace amounts of uranium, thorium, and potassium-40. Many other consumer products are also radioactive. Among these are some ceramic tiles (which can contain uranium), glossy magazines (which are sometimes coated with material that's high in uranium and thorium), and Brazil nuts (which have a fair amount of radium).

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

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The quantity of radiation in cat litter—and all of the other consumer products listed above—is small enough that it poses no risk for either humans or their pets. So, why can't screening devices distinguish between common sources of radiation and the material used to make a nuclear bomb? For one, the most common ingredients in bombs, uranium-235 and plutonium-239, don't give off very much radiation at all. To suss out these materials, a detector must be set to a very high sensitivity. If the sensitivity of the screening device is high enough, then naturally occurring radioactive materials can set off a false alarm. (A small percentage of the uranium found in nature, for example, happens to be in the form of the U-235 isotope.)

A radioactive material gives off alpha, beta, and gamma emissions. Gamma rays are both the easiest to detect and the most dangerous, since they can travel through most materials. (Alpha emissions can't get through the surface of our skin or even a piece of paper.)

The Radiation Portal Monitors the government uses work by converting invisible gamma rays into visible light using a heavy crystal cylinder. If the detector records enough photons, a cargo container will be flagged as radioactive. Compared with a Geiger counter, this technique is very efficient: Radiation can be detected at long distances and at relatively high speeds.

Though these drive-through detectors are good for quick scans at busy seaports and border crossings, they don't convey much specific information. Other types of detectors, like those that use a semiconducting material, are slower but provide more data about the gamma rays they pick up. A high-resolution spectrograph of an emission can tell you something about where it came from—like whether you're looking at a bomb or a Brazil nut.

Screeners can use X-rays to evaluate radioactive containers more carefully. (Of course, this extra step costs time and money.) Fissile materials tend to be very dense, heavy elements—the kind that would show up in an X-ray scan. Lead radiation shields around concealed radioactive material would pop out on an X-ray.

A new generation of radiation monitors that's now being tested uses gamma-ray detection along with something called "muon imaging," which uses naturally occurring, highly charged particles to examine cargo containers. If the technology works, muons would be faster and easier to use than X-rays.

Explainer thanks C-K Chris Wang of the Georgia Institute of Technology, and several Slate readers for asking the question.

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