On NPR today, they did a story about the equipment being used to check shipments for radioactive materials and they said that it couldn’t tell the difference between a WMD and the natural background radiation of “kitty litter.” So is all kitty litter radioactive? Or just certain types? Why is it radioactive? And does this, in fact, explain feline behaviour? 
Clay cat litters are naturally slightly radioactive, as are some glossy magazines, Brazil Nuts, and some ceramic tiles.
As I understand it, most things are radioactive. You have a certain amount of carbon-14 in your body (as well as other radioactive isotopes) and therefore, you are slightly radioactive. For the same reason, the cat is slightly radioactive. I don’t know off the top of my head what the level of normal background radiation is or if kitty litter is higher than that or if they just picked kitty litter because it was funnier than, say, a sucking chest wound.
FWIW,
Rob
I think they’d be concerned about a sucking chest wound being smuggled into the country in a container as well.
Kitty litter often contains clay, and many clays exhibit measurable natural radioactivity. Ceramics are often culprits for causing false positives as well. It’s not going to be enough to be anywhere near dangerous, but a huge batch of kitty litter or ceramic objects might be hard to tell from leakage from a shielded WMD.
Is kittly litter perhaps made in part from cinder? When you burn, say, coal, the carbon all turns into gaseous carbon dioxide, but there will be some trace material which doesn’t go up the flue. This residue will then have higher concentrations of heavy elements (since most of the light elements got combined into gasses), and since most radioactive elements are heavy, it’ll also have a higher concentrations of those. I know that power plants sell the cinders for use in concrete (hence the term “cinder blocks”); might it also be used for litter?
And there are a lot of methods used to look for nuclear weapons (and the materials to make them). Some check for the radioactivity directly. Some look (using a variety of techniques) for the dense materials which would be used to shield the direct radioactivity. Some look for spectral lines from specific dangerous materials, whether from the electrons, or in the hard X-ray or gamma ray spectrum from the nucleus. And sometimes they open up the box and inspect the contents in more detail.
Most clumping kitty litter is largely bentonite clay. A guy named Lowe, who first came up with commercial kitty litters, used another form of clay, fuller’s earth. People DID use ashes, sand, earth, etc, before they commonly purchased kitty litter:
Although, we might note that bentonite forms from the weathering of volcanic ash. And “fuller’s earth” is a rather non-specific term, which may include it.
(We need a longer edit time out.)