The Hunt for the Dirty Bomb

The Washington Post has a fascinating article on the 2003 Holiday Orange alert. One of the greatest worries for authorities was a dirty bomb.

The terrorism crisis began late on Dec. 19, when analysts assembled what they described as extremely specific intelligence, including electronic intercepts of al Qaeda operatives' telephone calls or e-mails. One fear was that al Qaeda would hijack and crash an overseas flight into a U.S. city or the ocean. Another was that terrorists would shoot down an airliner with a shoulder-fired missile. U.S. officials also became concerned that a large, open-air New Year's Eve celebration might be targeted. While the perimeters of football stadiums can generally be secured, outdoor celebrations are much more vulnerable, they said. One of the U.S. officials' main fears was of a dirty bomb, in which a conventional bomb is detonated and spews radioactive material and radiation across a small area.

Detection teams were dispatched to all the likely venues. And at Las Vegas, the detectors went apeshit.

On Dec. 29 in Las Vegas, the searchers got their first and only radiation "spike," at a rented storage facility near downtown. The finding sent a jolt of tension through the nation's security apparatus; the White House was notified. The experts rechecked the reading with a more precise machine that told them that inside the cinderblock storage unit was radium, a radioactive material used in medical equipment and on watch dials. As rare snow fell on the city that early morning, FBI agents secured the industrial neighborhood around the site, and a small army of agents and scientists converged on the business. Soon the renter of the storage closet in question, a homeless man, happened on the odd scene and asked the officers not to cut his padlock. He supplied the key. The scientists sent in a robot to snag a duffel bag in which the man had been storing a cigar-size radium pellet -- which is used to treat uterine cancer -- since he found the shiny stainless-steel object three years before. Not knowing what the object was, he had wrapped it in his nighttime pillow.

Many modern detectors are of a class called gamma ray detectors, containing substances which give off an electrical signal when impinged by gamma rays. Together with the associated electronics, they can be tuned to listen for various substances, an ability which allowed the searchers in the story above to know they were dealing with radium and not, say cobalt. Although the sensitivity and portability of the devices is always being improved, by supercooling and other methods, the signature of a dirty bomb can also be reduced by means of shielding, such as encasement in lead. The published distance to detection cited from 1989 studies is 200 feet. While it may be much better today, the ranges are still limited, which means that detectors are best deployed at choke points, such as at baggage scanners, security portals or highway checkpoints.

The necessity for shielding means a terrorist dirty bomb may be too heavy to move large distances on a man's back. Vehicles can deal with the weight but they can be herded through choke points. Shipping provides an ideal opportunity to move a large radiological weapon because a seagoing vessel can heft enough shielding to attenuate the radiation signature of the terrorist weapon. There are few chokepoints at sea until the vessel makes harbor and then the wolf will be in the sheepfold.

One is struck by the similarity between the radiation detection strategems now in place and the acoustic detectors used in antisubmarine warfare during the Cold War. Like the passive acoustic detectors of that era, radiation detectors can be deployed in unattended arrays and linked by a network to a central processing computer to permit the localization and prosecution of targets. Vans with detectors and even personnel on foot with handheld equipment stand in for the destroyers and helos in the ASW model. The problem is essentially the same: passively detecting a mobile and invisible target against background noise over an entire planet.

One certainty is that any American detection technology breakthroughs will be held a close secret. During the Cold War, the US discovery of ways to process low frequency submarine generated noise led to two decades of overwhelming advantage over Soviet Subs -- the Happy Time. Soviets were tracked at hundreds of miles range, under certain conditions, without even being aware of it, but it was years before the US Navy could fuse sensors, platform and weaponry to assure tactical success. The War on Terror has rightly been compared to epic struggle between America and Soviet tyranny. Settle in for the long watch.