If a terrorist tried to sneak a "dirty" bomb into the United States, would anyone notice?

Possibly. Radiation detectors rushed into service since 9/11 might sound the alarm at seaports, border checkpoints, and mail-handling facilities.

Then again, the sensors have been set off by everything from loads of kitty litter to bananas. And a smart terrorist could hide a basketball-size chunk of highly enriched uranium by using lead shielding less than an inch thick.

That's why the US is set to begin deploying a new generation of radiation detectors intended to be America's "last line of defense" against weapons of mass destruction. By early spring, the Department of Homeland Security (DHS) will pick technologies from among 10 companies, whose newest generation of nuclear detectors was tested in the Nevada desert this summer. Their devices will begin field-testing at a few ports of entry by next June, with a full-production decision expected by 2007.

Some experts are breathing a sigh of relief. "We're now on the cusp of seeing the next generation of [nuclear and radiological] detectors," says Benn Tannenbaum, a physicist and expert on sensor technology at the Center for Science, Technology & Security Policy at the American Association for the Advancement of Science in Washington.

But others say the US is not moving fast enough to install a multilayered defense against one of its biggest security threats. While billions of dollars have been spent on biological countermeasures, nuclear detection efforts have lagged.

"Little steps are being taken that may be in the right direction," says Richard Wagner Jr., a senior staffer at Los Alamos National Laboratory, who served in the Pentagon during the Reagan administration. "It's the rate of progress I'm concerned about."

That pace may be picking up as disturbing evidence accumulates.

About a year ago, the National Intelligence Council warned that "undetected smuggling has occurred, and we are concerned about the total amount of [nuclear and radiological] material that could have been diverted or stolen in the past 13 years" around the world.

The International Atomic Energy Agency (IAEA), which has documented 650 cases of trafficking since 1993, echoed that report.

About $300 million has been spentby the Department of Homeland Security since 1994 to deploy 470 radiation-detection systems at America's border crossings and ports, according to a Government Accountability Office report in June.

But their shortcomings have become obvious.

In March, DHS officials told Congress port detectors were working and had registered at least 10,000 radiation hits.

But questions about the value of those hits arose in a June congressional hearing, when the security manager for the Port Authority of New York and New Jersey reported 150 "false positives" per day.

That amounted to a false alarm - and possibly a time- consuming search - for about 1 in every 40 shipping containers. The resulting delays, in turn, often caused detection sensitivity to be turned down, crippling a sensor's ability to detect weapons material, the Port Authority security manager and other experts say.

Next-generation sensors will generally be far smaller, often mobile, and smarter - networked with other sensors and able to detect the difference between radiation emitted from a nuclear bomb and a load of bananas.

Overseeing the effort is a brand new office within the Department of Homeland Security devoted to one goal: detecting terrorist nuclear material before it can get into the country.

Established by presidential directive in April, its first assignment is to create a network of US nuclear detectors as part of a larger "global architecture" of detectors to be deployed overseas.

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