Secretary of State Condoleezza Rice is pushing North Korea's neighbors to use more radiation detectors, to keep Kim & Co. from smuggling nuclear materials into or out of the country.

Of course, that assumes that the sensors can actually handle the job. In many cases, they can't.

The problem is, most detectors today rely on solid crystals of sodium iodide, which light up when they're hit with gamma rays. And those sodium sensors "cannot distinguish between a plutonium bomb and the radioactive potassium-40 found in bananas," New Scientist notes. "More importantly, they fail to detect the most dangerous nuclear material of all: highly enriched uranium (HEU)."

Unlike plutonium, which emits... high-energy gamma rays that are almost impossible to shield, HEU emits only low-energy gamma rays... If the uranium is shielded by just a thin layer of lead, or even wood, the detectors miss it.

There are better sensors in the works, Homeland Security Watch's Christian Beckner says. They use "germanium instead of sodium iodide, or [rely on] active detection technologies, where you bombard the container with high-energy particles that create new signatures which are easier to detect." Unfortunately, they cost a ton -- seven times more than the current sensors. And they "haven't been proven yet in the field." Not here. And certainly not in China or South Korea, where they would do the most good to contain the current crisis.

But not all is lost. Two U.S. government programs -- the Container Security Initiative (CSI) and the Megaports Initiative -- are helping foreign inspectors get advanced x-ray machines which are better at seeing through shielding. The machines can pick up fishy-looking shapes, if not radiation signatures.

CSI also puts American customs officers at "foreign ports to liaison with officials there and get them to inspect [potentially risky] containers," Beckner says. It's not a perfect program, relying "too much on the personal relationship between the American officers and the foreign port officials; the former don't have the authority to force foreign ports to comply, and Senate investigators have found that in many cases these foreign ports aren't fully responsive."

But, overall, Beckner thinks "it's been a relatively effective program." And for right now, it's really all we've got.