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 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.

Until the Rensselaer/Rochester work, standoff THz sensing of explosives was difficult due to fundamental limitations in the technology caused by ambient water vapor in the air, which disturbs THz radiation. Clough, Liu and Zhang added a new twist: their technology uses fluorescence and sound waves from a plasma laser to boost the effective distance of THz spectroscopy from a few feet to many meters. Clough says what’s exciting is that the laser plasma is now being used both as a THz emitter and detector. “This means the device uses sound waves to remotely “see” or “hear” the THz signals,” he says.

It works this way. By focusing two laser beams into the air, small bursts of plasma are created, which in turn creates THz pulses. Another pair of lasers is aimed near the target to create a second plasma for detecting THz pulses after they have interacted with the suspect material. The detection plasma produces acoustic and fluorescence waves as it ionizes the air.

Clough discovered that by using a sensitive microphone to “listen” to the plasma, he could detect THz wave information embedded in these sound waves. This audio information can then be converted into digital data and instantly checked against a library of known THz “fingerprints” to determine the chemical composition of the unknown material.

Clough said that although the technology is extremely new and remains relatively unexplored, “with the fast-paced evolution of laser technology, and the mature acoustic and optical detection technology already available, these sensing methods have potential for commercialization.”

Stand back

Some recent explosive detection techniques focus on chemical identification, but require close proximity to the object in question.

Desorption electrospray ionization, for instance, is an ambient ionization technique that can be used in mass spectrometry for chemical analysis. DESI, developed a few years ago by Purdue chemistry professor Graham Cooks, and now available commercially from Prosolia in Indianapolis, is used mostly for close-quarter, direct contact detection of trace levels of explosives from ranges on the order of 10 mm, with no surface preparation necessary. DESI works by directing charged droplets produced from a pneumatically-assisted electrospray onto a surface to be analyzed at atmospheric conditions. Aside from explosive agents, DESI is also effective at rapidly detecting chemical warfare simulants from a variety of insulating or conducting surfaces, from human skin to leather car seats.

Other detection methods use imaging technologies to screen for explosive threats. German researchers at the Fraunhofer Institute for Applied Solid State Physics are currently developing an optical standoff explosive detection technique capable of revealing surface trace contaminations. This method uses a compact mid-infrared laser that can be tuned rapidly over the wavelength range of interest. The German scheme, called IRLDEX, or Infrared Laser-based Imaging Detection of Explosives, is capable of detecting at a distance of 5 to 25 meters the slightest trace of explosives on suspect surfaces, such as might be left behind by fingerprints.

Scientists at NASA’s Jet Propulsion Laboratory reported in the IEEE Transactions on Terahertz Science and Technology (Sept. 2011) they have a standoff device for screening humans based on THz imaging radar. The device is capable of rapid through-clothing imaging of concealed objects, including explosives, weapons, and contraband. In 2010, the team successfully extended the imaging radar’s standoff capabilities from four meters to 25 meters, while reducing imaging times from two minutes to less than one second.

Princeton Lightwave of Cranbury, New Jersey was recently awarded a patent on a novel shortwave infrared imaging technology that achieves standoff explosive detection distances of 25 to 50 meters, and possibly more. This method (called the Solid-State Hyperspectral Imager for Realtime Standoff Explosives Detection using Shortwave Infrared Imaging) is based on measuring the reflected short-wave infrared radiation on an imaged scene and comparing it to the reflected response of known explosive materials.

Princeton Lightwave technical program manager Bora Onat says testing demonstrates detection probabilities of 90 percent, with a corresponding false alarm rate of 10 percent. The technology is compact, lightweight, low-power, and eye-safe since it does not require high energy laser beams to ionize target particles. It is designed to provide inconspicuous operation at checkpoints or for mounted area or crowd surveillance.

Onat says the system is factory calibrated and does not require extensive setup procedures in the field, and can be used in a variety of settings, including border patrol, airport security, IED detection, and military checkpoints. Unlike THz systems, there is no privacy issue, since the technology does not see body parts under clothing.

ChemImage in Pittsburgh is currently developing a stationary and mobile system using realtime hyperspectral imaging—a novel sensor concept for optical standoff detection of explosives that combines Raman hyperspectral imaging and laser induced breakdown spectroscopy. Raman spectroscopy is a tool for analyzing the properties of molecules. By incorporating specific algorithms into the system’s software, it can detect unique chemical markers found in materials commonly used to produce explosives and alert users of possible threats. According to a company spokesperson, ChemImage’s HSI system is capable of detecting explosives from up to 1,000 meters.

A key feature of the HSI system is the use of wide-field imaging, which allows realtime scanning of larger areas than those provided by traditional point detection systems. ChemImage says it is currently developing a multi-sensor vehicle screening system so military personnel can screen vehicle-born explosive threats.

Bad air day

One of the challenges with extending standoff detection distance is the impact of various atmospheric and environmental interferences, as well as screening threats that are in motion. Lasers seem to be particularly adept at overcoming many of these obstacles.

In March 2012, Australian researchers announced the use of deep Raman spectroscopy for standoff detection of concealed chemical agents from a distance of 15 meters under normal conditions. Raman spectroscopy relies on Raman scattering of monochromatic (single color) light, usually from a laser, to achieve rapid, non-destructive chemical analysis of solids, powders, liquids, and gases. The Australians reported they were able to non-invasively identify various explosive precursors hidden in opaque plastic containers within five seconds of data acquisition.

Laser research at Pranalytica in Santa Monica, California has resulted in even greater steps backward. A technique reported in 2010 claims detection and identification of trace quantities of explosives can now be achieved at standoff distances up to 1,000 meters. This method uses a rather bulky tunable CO2 laser that scans the absorption fingerprint of the target explosives. The technique involves illuminating (heating) the target object with laser radiation, then remotely monitoring the increased black-body (thermal) radiation from the sample.

CEO Kumar Patel says current company research is focusing on a smaller, shoulder-held technology using a new class of semiconductor lasers called quantum cascade lasers, that will yield detection standoff of 25 to 50 meters, a more practical distance from a first responder user standpoint. The current prototype is housed in what resembles a 5-inch Newtonian telescope aimed at the target by resting the device on the shoulder and peering through a small finder scope.

“This technology may play an important role in screening personnel and their belongings at short distances, such as in airports, and also for detecting and identifying explosives material residue on persons,” Patel says.

 

Douglas Page writes about science, technology and medicine from Pine Mountain, California. He can be reached at douglaspage@earthlink.net.

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