I thought it was a joke. The title of this news story:
Port to get nuclear detectors that won't be set off by cat litterNuclear detectors that shouldn't be triggered by cat litter and other harmless materials will be installed this fall at major seaports and border crossings, the government said Friday.
[...]
The estimated 670 detectors currently in place at ports and borders have long frustrated Homeland Security officials because of false positives triggered by medical supplies, cat litter, banana truckloads and other innocuous materials with low levels of naturally occurring radiation.
"We don't want to send the red flag up every time someone moves a shipment in of perfectly respectable granite," Chertoff told reporters in announcing $1.1 billion in contracts to three companies to help develop and deploy the systems. Officials said the new detectors are expected to reduce about 831,000 false positives each year to 15,000.
Two years ago, the government awarded a contract to San Diego-based Science Applications International Corp. to manufacture cargo-screening monitors that detect radioactive material. The government bought and installed about 400 of the scanners at many of the nation's border crossings and ports. Cost: $220 million.[...]
What's the problem? Well, for starters, the monitors can't distinguish between a nuclear bomb and radiation that occurs naturally in a variety of materials, including ceramic tiles, quarry tile, cat litter, fertilizer, and bananas, according to the congressional Government Accountability Office and officials of U.S. Customs and Border Protection (an agency of the Department of Homeland Security), which purchased the equipment. As a result, the detectors reportedly go off frequently. Some Border Patrol officials turn the machine's sensitivity monitors down to avoid "nuisance" alarms, which risks letting dangerous material through.
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).
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.
For fun, I measured the radionuclide activities in one sample of cat litter. The results were as follows: 4 picocuries per gram (pCi/g) for members of the uranium series, 3 pCi/g for members of the thorium series, and 8 pCi/g of potassium-40.At these concentrations the exposure rate at six inches above a box of cat litter would be approximately 0.1 micro roentgen per hour (uR/hr) above background. Since cats don't spend a lot of time in the litter box (you never see them taking along something to read when they are heading that way), the radiation exposure is probably minimal.
I've seen varying estimates of the amount of cat litter sold in the US, but for the sake of the calculation, lets assume 4 billion pounds per year. If true, this means that approximately 50,000 pounds of uranium and 120,000 pounds of thorium are purchased in the form of cat litter each year by the American consumer.