COMPARED with a thermonuclear detonation, the destructive power of even a large amount of conventional explosives wrapped in nuclear fuel or other radiological material is rather tame, notes Thomas Reed, who has designed two American nuclear weapons. But a “dirty bomb” (technically a radiological-dispersal device) is far easier to build than a nuclear weapon and would spread not just radiation but panic, adds Mr Reed, who later served as secretary of the air force. Little wonder, then, that nerves were rattled after the theft on December 2nd of a lorry carrying highly radioactive cobalt-60 from a radiation-therapy unit in Tijuana to a nuclear-waste facility near Mexico City.

Nuclear and radiological materials slipped out of regulatory control 2,331 times between 1995 and the start of this year, according to the Incident and Trafficking Database compiled by the International Atomic Energy Agency (IAEA). The materials are widely used in industry, agriculture and medicine. They are kept in many poorly guarded X-ray and cancer-treatment clinics. Such places are often not overseen with terrorism in mind. They have even been bought by crooks as front operations, says Rajiv Nayan, of India’s Institute for Defence Studies and Analyses. Raids on abandoned uranium mines in the Democratic Republic of Congo are more frequent, according to that country’s General Atomic-Energy Commission. The problem is most acute in the former Soviet Union: in Ukraine alone, roughly 2,500 organisations use radiological materials.

In Georgia a counter-trafficking unit set up by the interior ministry seven years ago has arrested two or three teams smuggling radiological material every year save 2009. The lure of profits is so strong that some ex-cons get back into the business, says Archil Pavlenishvili, leader of the unit. Interpol has said such trafficking is growing: an acute “real threat to global security”.

It all sounds scary enough. But the reality has been less so. Moreover, by many accounts the most plausible dangers appear to be declining.

G-men with Geiger counters

For a start, an “overwhelming” number of buyers turn out to be undercover cops, says Mark Hibbs of the Carnegie Endowment for International Peace, a think-tank. A sizeable network of informers helps Georgia’s interior ministry to keep a close eye on the four or five cells in the country currently trying to obtain or sell radiological material, says Mr Pavlenishvili. When one of them is lining up a potential deal, it is almost always because his or a foreign unit is preparing a sting operation, he adds. Mr Pavlenishvili’s unit has not got wind of a single profitable sale—Georgia’s underworld makes its money on other crimes such as drug-running.

Considering the growing risk and persistent lack of money to be made, it is amazing that smugglers continue to give it a shot, says Lyudmila Zaitseva, an academic working on a University of Salzburg database on nuclear and radiological trafficking. Many traffickers no doubt reckon that terror groups will pay dearly for dirty-bomb ingredients. After all, counterterrorism officials citing seized al-Qaeda documents have said as much. Yet although a terrorist-made dirty bomb of mass destruction cannot be excluded, it remains unlikely. For one thing, rooting around to obtain dangerously radioactive material is a great way to attract the attention of the authorities. A bust could doom a painstakingly assembled terror cell.

In any case, most of the stuff being peddled is fraudulent rather than dangerous, says Adrian Baciu, head of the Romanian police’s nuclear and radiological unit until 2004 (he later worked for four years in Interpol’s counter-terrorism directorate). To hype their products, sellers typically describe it as material used in a nuclear bomb, a reactor, the Space Shuttle or the like. “Excuse my language…just bullshit,” says Mr Baciu. In one sting, he arrested four men trying to sell, for several hundred thousand dollars, material from a calibration kit for radiation-detection equipment. It could be safely handled with cotton gloves. Another cell tried to pass off as dangerously radioactive a piece of ordinary iron. Trafficking in Romania, he says, has tapered to almost nothing.

Russia and other countries have greatly tightened control over radiological materials since thefts peaked in the early post- Soviet years. No highly enriched weapons-usable uranium or plutonium has been reported stolen since the 1990s, according to Ms Zaitseva. (The small amount intercepted since then appears to have been stolen earlier.) Losses of less hazardous low-enriched fissile material have fallen sharply. Employers are getting better at identifying potential risks, says Ray Landis of the Nuclear Energy Institute in Washington, DC. Employees’ travel itineraries are reviewed. An industry-wide computer system flags workers whose changing circumstances might lead to stress or need of money, perhaps to prevent a home foreclosure or to keep custody of a child.

Beyond this, intelligence agencies are hunting down traffickers with help from special “link analysis” computer programs. Also known as “network analysis” software, this crunches data from numerous sources to identify people whose travel, purchases, web searches, communications, schooling and so forth may spell trouble—perhaps an employee in radiation therapy who begins frequenting an inconveniently located bar whose owner receives phone calls from a drug-runner with growing operations.

Half a dozen Western governments “pay huge amounts of attention” to this, says an executive at a developer of the software. At least one spy agency in America, Australia, Britain, Canada, New Zealand, and an unnamed European country pays more than $1m a month to use it. The counter-trafficking units in both Georgia and Romania note that link-analysis software made by i2, owned by the giant IBM computer company, has helped to nab traffickers. Atsuko Nishigaki, the unit’s boss, says Japan’s economy ministry employs ten analysts to use a competitor’s software to identify traffickers in nuclear or radiological material.

America’s National Nuclear Security Administration has sponsored the installation of radiation-detection kit at ports in 23 countries and counting. The Megaports Initiative, as it is called, aims to have half of the world’s maritime container cargo routinely scanned by 2015. Networked systems are also being developed with detectors small enough to be worn on a police officer’s belt. The idea is to relay data on potentially dangerous radiation through a mobile-phone network to a central computer. Knowing each device’s location and the strength of the radiation it detects, the computer can “triangulate” the source’s approximate location.

Negative isotope effects

Difficult problems remain. False alarms triggered by anything from a pallet of cat litter to radiation-therapy patients and nuclear-power-plant shipments have slowed research and development on one such network at the Lawrence Livermore National Laboratory in California, says Simon Labov, a co-ordinator there. Even so, the lab’s work continues to be financed by America’s defence, energy and homeland-security departments. In October 2013 the latter’s Domestic Nuclear Detection Office asked for proposals for a similar system, dubbed Human Portable Tripwire. Other outfits that have developed technology for such schemes include Smiths Detection in Britain and, in America, Berkeley Nucleonics, General Electric, GENTAG, Passport Systems and Purdue University.

The sheer danger of making a dirty bomb is a factor too. Without the right equipment and expertise, the really nasty stuff can kill the maker of a bomb before it is ready—part of the reason, perhaps, that no spectacular dirty-bomb attack has yet been launched. Following the cobalt-60 theft in Mexico, an IAEA statement noted that, if unshielded, its gamma rays would probably kill people next to it in an hour—or perhaps just minutes.

In the event, five of the six people arrested for the theft on December 5th had avoided exposure and the sixth appeared unharmed. Mexican authorities believe that their aim was to steal the Volkswagen lorry, not the cobalt. Its movable platform and crane made it a valuable prize.