Standoff explosive detection takes step backward

On August 8, portions of Dallas-Fort Worth International Airport’s Terminal A were evacuated when the airport received bomb threats made toward inbound flights. On July 17, dozens of police, security guards, and federal agents searched Comerica Park...


On August 8, portions of Dallas-Fort Worth International Airport’s Terminal A were evacuated when the airport received bomb threats made toward inbound flights.

On July 17, dozens of police, security guards, and federal agents searched Comerica Park for a possible bomb while 40,000 baseball fans watched a Detroit Tigers—Los Angeles Angels ball game.

Bomb squads have rarely been busier. In July, at least 11 bomb threats were called in to Wal-Mart stores in Missouri and Kansas. Bomb scares crippled the University of Pittsburgh early 2012, when over 60 were reported on campus. One Maryland school district experienced more than 150 bomb threats in one school year.

Bomb squads are now deployed to major concert and sporting events. They inspect buildings and bridges, and provide oversight on several terrorism committees. They’re also the ones that must approach the bomb; the closer they get to the threat, the greater the potential danger.

One giant step

Standoff explosive detection improvements, therefore, have never been more welcome. A variety of technologies are emerging to move explosive detection backwards, away from the threat, some up to 50 meters or more.

Terahertz spectroscopy, for instance, has the unique ability to identify hidden explosives and hazardous materials, but until recently a key limitation was that detection had to be done at close range, possibly jeopardizing operators by positioning them close to the threat source.

THz technology has taken a giant step backward in several laboratories. Benjamin Clough and Jingle Liu, former students at Rensselaer Polytechnic institute, along with their advisor Prof. Xi-Cheng Zhang, have moved the standoff distance back at least 30 meters, a huge step in the direction of safe, remote THz sensing. Clough has since taken a position with a government entity. Zhang is now at the Institute of Optics, University of Rochester, where the work continues.

That 30-meter standoff distance may end up being longer. The team has currently demonstrated standoff terahertz wave capabilities utilizing fluorescence and acoustic techniques at 10 meters (10 meters was limited by the available lab space).

THz sensors are desirable to law enforcement, homeland security, and military personnel because THz rays can penetrate packaging or clothing and identify the unique chemical fingerprints of hidden materials. THz radiation is used to create a fingerprint of a given explosive material, a signature that is obtained by sending a short pulse of radiation to interact with a suspect material.

“This allows us to see which frequencies have been absorbed by the material due to the material’s vibrational and rotational atomic-level movement incited by the THz frequencies,” Zhang says. “Once we have an understanding of what a certain material signature looks like, we are able to compose a reference library that can be compared against future encounters with that material, providing a direct way to identify it.”

Another advantage of THz, or T-rays, is, unlike X-rays and microwaves, T-rays pose no known health threat to humans. “Many first responders and soldiers are faced with dangerous situations that involve close proximity to potential explosive threats,” Clough says. He believes this technology will help responders determine the nature of a potential explosive threat earlier, from a safe distance.

A related type of THz technology can already be seen in airports. Some full body scanners use the technology since it can see through clothing, a feature that generates personal privacy concerns among passengers scanned before boarding. THz technology was also used to examine panels on the space shuttle fleet to locate defects.

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