LAST week鈥檚 devastating bomb attack in London, and the similar attack last year in Madrid, might have been thwarted had the two cities鈥� transport systems been fitted with a machine that for a while last year stood in a train station in New Carrollton, Pennsylvania. The machine, a scanner that looks similar to the metal detectors in airport checkpoints, detects molecules of explosive. But it takes 12 seconds to scan each person, and the trial was ended early before the busy Memorial Day weekend, due to fears the delays would cause chaos.
The story of the New Carrollton scanner sums up why it is so difficult to prevent bomb attacks on busy transport networks. The technology exists to remotely detect explosives concealed within packages, bags and under clothes. But it is not yet cheap, reliable and efficient enough to scan the millions of passengers that travel around cities every day. It will be soon, but then the problem will be deciding how to act on the information.
Carrollton鈥檚 machine, made by US firm GE Ion Track based in Wilmington, Massachusetts, and used at a few US airports, tests the air around commuters using an ion mobility spectrometer, which identifies molecules based on how fast they move in an electric field. But current models take too long. 鈥淲e need one we can put at the bottom of an escalator, and identify someone in real time at walking speed,鈥� says Bonner Denton, a chemist at the University of Arizona in Tucson who has designed such a machine.
Advertisement
He has adapted an ion mobility spectrometer to make it much more sensitive. Using a capacitive transimpedance amplifier, a device normally used to boost signals received by infrared telescopes, Denton鈥檚 new sensor can remotely detect a few attograms of explosives carried by a would-be bomber, making it 1000 times more sensitive than other scanners. 鈥淲e hope to get it to the zeptogram level,鈥� he says, which means it would be able to detect a billionth of a billionth of a gram of explosive. Denton is also working on miniaturising the device so that it could be used as a handheld scanner.
A scanner like that could even identify someone who has handled explosives. 鈥淚t is just not true that you can鈥檛 stop someone getting on a train with a bomb,鈥� says Denton. 鈥淭here just hasn鈥檛 been enough interest until now.鈥� Since the London atrocity, he says, US government labs have made contact to ask when he can have a prototype ready. His detector should be available in two years.
鈥淭erahertz and microwave beams can reveal the dense, characteristically shaped packages that might be bombs鈥�
Other technologies in development do more than sniff out traces of explosives. When bombarded with various energy beams, materials absorb and reflect different frequencies of radiation, producing a unique signature. Explosives show up under X-rays and gamma rays, but both are potentially harmful, making them unsuitable for mass-screening commuters. Terahertz rays and microwaves, however, are innocuous. Under a terahertz beam, skin appears black, metal appears white, and clothing is invisible. Such beams, if used for imaging passengers, can reveal the unusually dense, characteristically shaped packages that might be bombs. Terahertz signals can also reveal the chemical signature of the explosive itself.
鈥淲e鈥檙e working on a small covert spectroscopic camera,鈥� says remote scanning expert Dan Van Der Weide of the University of Wisconsin, Madison. This device will identify an anomalous parcel from 20 metres away. 鈥淚f it had been available it could have detected the London bombers,鈥� he says.
A similar technology, created in the UK and being developed in the US and Australia, is nuclear quadrupole resonance (NQR). It uses radio waves to excite certain atoms such as nitrogen, which is ubiquitous in explosives. The wavelength spectrum of the energy re-emitted by the excited atoms can then be used to identify the molecule.
More far-fetched options include the idea that it may one day be possible to detect physiological changes in people who have handled explosives. Others think it might be possible to detect a bomb by looking for changes in negative ions in the air around explosives. But meanwhile, some claim that more conventional techniques such as sniffer dogs are adequate.
Given the pros and cons of each technology, a report by the US National Research Council last year concluded that several techniques should be used together. For instance, NQR might detect that someone has entered a station carrying explosives. Video surveillance could then be used to narrow down the list of suspects, who would be checked by security guards carrying hand-held scanners. Or an ion mobility spectrometer at the base of an escalator could sound an alarm, with confirmation that a suspect bag does indeed contain explosives coming from a terahertz scanner and a sniffer dog.
But there would be problems making these technologies work in a modern mass-transit system, says Brian Jenkins, a transport expert with the Rand Corporation in California. 鈥淭he issue isn鈥檛 just technology. The issue is, can you keep moving volumes of people efficiently, and can you afford it?鈥� There would be an enormous staffing requirement, with a team at every station ready to respond to an alarm, at a time when many transit systems are cutting staff.
鈥淪o you detect someone. Who picks him out, stops him and searches him?鈥� asks Jenkins. 鈥淭hen you have to keep traffic moving 鈥� and keep watching in case the first guy was only a decoy. And do it at every entrance to every station.鈥� Even if the cost is only half what security screening now costs at airports, he estimates it would add a prohibitive $2 or $3 to every fare. There are also privacy concerns with terahertz scanners, as they can see through clothing (快猫短视频, 10 July 2004, p 20), although the NRC reports that these can be eliminated in an automated system that simply emits alarms when it recognises suspicious patterns.
鈥淲e should develop this technology. But we may have to be creative about how we deploy it鈥�
鈥淲e should develop this technology,鈥� Jenkins says. 鈥淏ut we may have to be very creative about how we deploy it, and perhaps use it only during a particular threat, or after the first bomb goes off, to detect the next ones.鈥�
