Screening for Terrorism – Scientific American

A passenger walks through a millimeter wave portal in a New Jersey commuter train station.

For a few weeks in July, a commuter rail station in New Jersey enjoyed the same screening protection as that surrounding the soldiers and civilians in Baghdad’s Green Zone. Using millimeter waves–wavelengths of light shorter than infrared but longer than x-rays–a walk-through portal produced images of passengers before they boarded the trains. Like an x-ray, the technology creates a revealing picture that can highlight items, such as plastic guns, that typical transit security sensors fail to detect. And, essential for transit hubs such as New Jersey’s Exchange Place Station, which serves at least 200,000 passengers a day, it does so quickly.

Be they trains, planes or automobiles, the world’s transportation network needs better protection. In the U.S. alone, Government Accountability Office investigators snuck bomb components through more than 20 airports and the recent roll-up of a terror plot in the U.K. highlighted the inability of current technologies to detect items such as liquid explosives or their precursors. But a slew of new devices could help fill this security gap, including millimeter-wave cameras.

Developed by companies ranging from industry heavyweights General Electric (GE) and L-3 Communications to smaller firms such as QinetiQ, millimeter wave detectors work in one of two ways: active or passive. Active devices bombard people with millimeter waves to reveal what may be hidden inside clothing, whereas passive devices rely on collecting the ambient waves in the environment. “Millimeter wave covers a broad range of frequencies–from 30 to 300 GHz. At some, the sky illuminates the object, the image being collected in much the same way as an optical camera,” explains John Salkeld, QinetiQ’s director of optronics. “At others, you can pick up emission from the human body.”

Whether active or passive, millimeter waves’ real attraction lies in what it is not: overly revealing. But revealing is exactly what security experts desire for passenger and luggage screening. And for that, x-ray remains the best probing wave. Already, x-ray machines form the core of checked baggage security, peering inside suitcases much as doctors peer inside bodies using CAT scans. Carry-on baggage screening also enhances x-ray imagery by overlaying color-specific highlights that identify the type of material.

millimeter wave image
Millimeter wave provides an image that can reveal concealed items.

Carry-on luggage is a cluttered affair, however, and adding so-called backscatter x-ray machines–those that pick up the x-rays scattered by materials, rather than just those that pass through or are absorbed–can help clarify images. These devices can detect items otherwise obscured in baggage (such as the water bottle glowing to the right of the normal colored x-ray image above). Such x-rays have been offered as a solution for passenger screening as well, though radiation and privacy concerns have limited their application in the U.S.

Besides seeing what passengers bring on board, security officials also want to sniff them. The most common “smelling” devices are trace detection portals, known as puffers, which work by loosening particles on a passenger’s clothing with blasts of air and then analyzing them for traces of explosives or other suspicious chemicals. But the machines have proved susceptible to malfunction, prompting a halt in their installation at airports, and some experts question their effectiveness. “Airplane security stops the sloppy and the stupid,” argues Bruce Schneier, chief technology officer of Counterpane Internet Security. Puffer makers, though, are quick to defend their technology. “It doesn’t take much to be sloppy,” counters Jay Hill, chief technology officer for GE Security. “We’re talking about picogram concentrations.”

Regardless of whether terrorists are sloppy or stupid, they do have a wide array of explosive tools at their disposal: from highly volatile chemical bombs manufactured from relatively common ingredients, such as the hexamethylene triperoxide diamine (HMTD) suspected in the foiled London terror plot, to the military grade plastic explosive Semtex. Because of the problems with existing puffers, some security experts are looking at alternatives, such as quadrupole resonance, terahertz detectors and neutron bombardment machines.

Color imaging
Color coding reveals materials in a traditional x-ray image but backscatter pierces the clutter to highlight a hidden bottle.

Quadrupole resonance machines, in development for shoe scanners and other applications, hit an object with radio waves of varying frequencies, as do terahertz detectors. This injects energy into the material in question, which it releases when the pulse stops. By analyzing the frequency that comes back, scanners can precisely identify the material, whether solid, liquid or gas. Neutron bombardment works much the same way, shooting neutrons into an object and analyzing the gamma rays that return. Such technologies are already in use by bomb squads but might find a useful place in transportation security. “We would integrate it as a secondary system,” says Sean Moore, vice president of sales and marketing at neutron-based machine-maker HiEnergy Technologies. “You always have those bags that need to be rescreened.”

Determining how many bags–or passengers–should qualify for more screening is the difficult balance of any security procedure: screen too many, and the utility of the transportation is compromised; screen too few, and its security is. “They have done the analysis on what is a minimum threat quantity that would be the minimum amount necessary if placed strategically to cause a catastrophic event,” explains Peter Kant, vice president for global government affairs for detection device maker Rapiscan Systems. “All the machines are tested towards that threat quantity.” In other words, if the machine is too lenient it will not make it into use, but if it is too stringent it will not find application either.

And critics charge that much of transportation security development is targeted at past threats, such as shoe bombs, not at future, untried tactics. It may also be providing a perverse incentive. “If you look at the history of aviation security, starting with the Cuban hijackings in the ’60’s, each security measure put in place may have deterred things for a while but ended up raising the bar,” offers R. John Hansman, director of M.I.T.’s International Center for Air Transportation. “It would be much more effective to have more random screenings with more types of devices. From a deterrence standpoint, an attacker does not know what screen will be used.”

Advanced terror detection technologies remain expensive and sometimes difficult to use, challenges that will have to be overcome before they can find widespread deployment, but they are useful in liberating the most important part of any transportation security scheme: security officers themselves. “You might want to be getting more technology out there to free up the people to do other things like behavioral pattern recognition, interfacing with passengers,” notes Craig Coy, president and chief operating officer of the Homeland Security Group at L-3 Communications. Technology may be able to help here as well, via biometric devices that read a person’s physical signals for signs of agitation or other warnings.

“In this world, our technology development cycle has to be faster than the bad guy’s learning cycle,” adds Randy Null, chief technology officer at the Transportation Security Administration. “[But] we clearly would like to find people before we have to find items.” Technology can detect bombs or other terrorist devices but it is the people behind the machines–and behind the intelligence rendering this last line of defense redundant–that truly protect.

By David Biello

Source: Scientific American.