The Vancouver Sun’s Ian MacLeod writes that most experts believe there is a low probability that terrorists could muster the technical sophistication and complex planning needed to pull off a strike on U.S. soil, yet Osama bin Laden has made it clear that al-Qaeda wants to nuke the United States. Even a small chance of that happening is one of the great worries of U.S. leaders and many security officials, magnified by the political strife in nuclear-armed Pakistan, where al-Qaeda’s core command has taken sanctuary.

Preventing terrorists from detonating a nuclear weapon in a major U.S. city is therefore a key national security priority, with more than $3 billion spent since 2002 on nuclear monitors alone at Canadian and Mexican land border crossings and U.S. seaports.

U.S. DHS secretary Janet Napolitano hailed the completion of the Canadian component, two months ahead of schedule, as a “major security milestone”: “This technology enhances our capability to guard against terrorism and criminal threats while expediting border crossings for lawful trade and travel,” she said in a statement.

MacLeod notes that the effectiveness of the U.S. radiation monitors is debatable. The non-intrusive monitors are erected alongside the car lane approaches to customs’ booth inspections, with larger monitors for transport trucks stationed in cargo inspection areas. Each detects certain types of energy within a limited area but not the exact radioactive source.

For that, a suspect vehicle is sent for a secondary inspection that includes a scan with a hand-held detection device to identify the source and whether it constitutes a threat. Benign emissions from lingering medical isotopes in people’s bodies, scrap metal, natural sources of radiation and even Kitty Litter trigger frequent false alarms.

Reducing them and the accompanying border-crossing delays with the current polyvinyl toluene (PVT) monitors would mean re-calibrating their detection threshold, or sensitivity. Uranium-235, which in a concentrated, highly enriched form becomes weapons-grade uranium, is already weak radioactively, however, and reducing the monitors’ threshold would make detection more difficult.

What is more, PVT monitors can only detect unshielded or lightly shielded sources, which seems unrealistic, considering the sophisticated smuggling tactics determined nuclear terrorists would likely employ.

The United States is instead debating the cost-effectiveness of replacing PVT technology with “advanced spectroscopic portals” or ASP, a new type of portal monitor designed to both detect radiation and identify the source.

The U.S. Government Accounting Office (GAO) reports that ASP monitors use more sophisticated software, and have a more extensive library of radiation signatures that may provide more consistent and rapid screening and may increase the likelihood of correct identification. They are also almost three times more expensive than PVT monitors.

In the meantime, the United States says the PVT monitors are now scanning 100 percent of all vehicle traffic entering from Canada and Mexico, plus all mail and courier packages from Mexico and a further 98 percent of all arriving seaborne container cargo.