The Oklahoma City bombing finally may have convinced legislators and the public that terrorism in the U.S. can be homegrown. The blast, the deadliest terrorist attack ever on U.S. soil, has riveted the nation in a way quite different from other acts of terrorism, such as the destruction of Pan Am flight 103 over Scotland in 1988 and the bombing of New York City's World Trade Center in 1993. Tragic and terrifying as they were, these earlier attacks were somehow foreign, replete with Middle Eastern connections. But the people now being charged with the Oklahoma blast apparently have no foreign agenda and could be anyone's neighbors. Terrorism truly has become a domestic issue.
Debris from the Alfred P. Murrah Federal Building after the Oklahoma City blast and a makeshift memorial to the victims. (AP/Wide World photos)
Like the mother who transformed the sorrow of losing her son in Pan Am 103 into sculptures, individuals have unique ways of dealing with tragedy. But collectively, victims and nonvictims alike look to the government for protection, for assurance that such dastardly acts will not happen again.
Technological tools are available to deter terrorist acts. But their use involves high costs and thus may meet strong resistance. Economic costs will be incurred to deploy surveillance equipment or to reformulate products to discourage their use by criminal elements. But more difficult to quantify is the cost to an open society of intrusions on individual privacy and curtailment of people's movements. Policymakers must sort out whether the benefits of preventive technology justify the economic and social costs.
The government is reviewing countermeasures to terrorism. At the urging of President Clinton after the Oklahoma City blast, bills giving law enforcement agencies more money and legal powers to combat terrorism are moving through both houses of Congress. Testifying last month before the House Judiciary Committee hearing on the Comprehensive Antiterrorism Act of 1995 (H.R. 1710), Abraham D. Sofaer, formerly a legal adviser to the Department of State, noted: "The most important contribution of the pending [antiterrorism] legislation is its focus on the prevention ... of terrorist acts." Sofaer also told the House panel that "the most effective measures for preventing acts of terror are usually technological."
Technology has been a major part of the government's response to terrorism. For instance, the Pan Am disaster led to the Aviation Security Improvement Act of 1990. The act required the Federal Aviation Administration (FAA) to help accelerate R&D in new technologies to protect civil aviation from terrorists. In response to the mandate, FAA has been working with industry to develop technologies ranging from bomb detection to reinforcement of aircraft bodies and baggage containers.
Counterterrorism R&D also is being carried out by the Federal Bureau of Investigation. As the lead agency responsible for combating terrorism in the U.S., the FBI has been collaborating with government laboratories to identify and develop new approaches to bomb detection and residue analysis. Mindful that its contribution to this article might compromise current investigations of the Oklahoma City bombing, the agency declined C&EN's requests for interviews.
Remnant (right) of the Boeing 747 that carried Pan Am flight 103; Dark Elegy (left) a traveling exhibit (shown here in Mendham, N.J.) by Suse Lowenstein, whose son was killed on Pan Am flight 103, freezes in time the grief of women who lost loved ones on that flight. (Photos by S. Lowenstein and AP/Wide World)
Technologies, however, are worthless unless they are used. The disruptions at Los Angeles Airport in late June because of a threat from the serial bomber "Unabom" have raised criticisms against FAA for being slow to adopt new technologies. It puzzles the public that bomb detection equipment produced by U.S. companies is being widely used in airports overseas but is still under testing in the U.S. And whereas countries like the U.K. already are aiming for multilevel baggage screening in all major airports by 1996, FAA says it needs at least 18 months more to decide whether an explosives detection system it has certified should indeed be deployed.
The Oklahoma City bomb, believed to have been made from ammonium nitrate fertilizer and fuel oil, highlighted the destructive power of ordinary materials (C&EN, May 1, page 8). The dual nature of such common items that are freely traded creates a dilemma: risk their use by criminal elements or remove them from the market. Some observers think there's a middle ground, that technology exists to neutralize ammonium nitrate fertilizer as an explosive without jeopardizing its agricultural properties (C&EN, May 29, page 6). The antiterrorism bill passed by the Senate, the Comprehensive Terrorism Prevention Act of 1995 (S. 735), has provisions directing the Treasury Department, through the Bureau of Alcohol, Tobacco & Firearms (ATF), to examine whether additives can desensitize fertilizers (C&EN, June 12, page 13). But the U.K. experience with terrorists in Northern Ireland and recent testing of supposedly desensitized ammonium nitrate in the U.S. indicate that none of the proposed additives is effective.
Taggants - markers that help trace explosives - represent another U.S. technology that's already being used elsewhere but only now being revisited here. Switzerland has been exploiting taggants since the 1980s as a countermeasure to terrorism. ATF had been actively involved in developing this technology, but its work stopped once Congress, after lobbying by special interest groups, failed to pass laws requiring tagging of explosives. Both the Senate and House antiterrorism bills call for ATF to review the matter. Oklahoma City has revived this debate once again.
The debate has focused the spotlight on a Minneapolis-based company, Microtrace Inc. The company is the only U.S. producer of postblast identification taggants, microscopic color-coded particles that help track the source of explosives after a blast. Its product, called Microtaggant, was invented by Richard G. Livesay, Microtrace's founder and director of technology. Livesay developed the product while he was a chemist at 3M, St. Paul, Minn.
A Microtaggant particle is a chemically stable material consisting of several layers (typically eight or nine for those used with explosives) of a highly cross-linked melamine polymer. Each layer has a different color, and the color sequence can be translated to a numeric series according to the color codes used with electrical resistors (0 = black, 1 = brown, 2 = red, and so on). Ferromagnetic and/or fluorescent layers are incorporated for detection and retrieval.
The particles used for explosives are about the size of ground pepper. But different sizes can be made, down to 44 µm in mesh size. The material is mixed thoroughly with the explosive so it is part of the product. When a tagged explosive is detonated, the thermal effects of the blast will destroy most of the particles. But the population of taggants in a given quantity of explosive is high enough that some particles survive. These can be found with a magnet or with ultraviolet light, and the color sequence can be read visually with a pocket microscope of at least 100x magnification.
"Basically, it's a fingerprint," says William J. Kerns, Microtrace vice president for sales and marketing. Explosives can be traced to a manufacturer, and, through use of mandatory distribution records, they can be tracked to a purchaser or a point of theft. Although Microtaggant was developed specifically for postblast tracing of explosives, various U.S. companies incorporate it in products likely to be counterfeited or covered by warranty, Kerns says. And except in Switzerland, the company has stayed out of the explosives marking business.
The Swiss government passed a law in 1980 requiring explosives to contain a marker substance that allows reliable determination of origin after detonation. According to the Swiss Scientific Research Service (SRS), in Zurich, the law was prompted by the worldwide spread of terrorism in the 1970s, and its primary purpose is to assist the police in investigations of criminal offenses involving explosions.
SRS says Microtaggant is one of three markers currently authorized for use in Switzerland. A product called HF 6, manufactured by the Swiss company SW Blasting, in Bülach, is similar in structure and properties to Microtaggant. The third authorized product is Explotracer, produced by another Swiss company, Plast Labor, in Bulle. It is a polyethylene matrix blended with rare-earth elements. Also blended in are fluorescent pigments and iron powder, for detection and retrieval. The code is based on the melting point of the matrix and the composition of the embedded elements. Thus, the code can be deciphered only after physical and X-ray analysis of a particle in a laboratory.
Microtrace General Counsel Charles W. Faulkner tells C&EN the company has given up on the U.S. explosives marking market because of the long history of intense lobbying against tagging of explosives. In 1979 and 1993, the National Rifle Association and the Institute of Makers of Explosives (IME), among others, lobbied against bills requiring use of taggants with explosives.
The Office of Technology Assessment (OTA) analyzed Congress' 1979 proposal to require taggants in commercial explosives. According to its report, "Taggants in Explosives," released in 1980, taggants would be useful law enforcement tools. However, OTA stressed the need for further development and studies, especially with regard to safety.
At last month's House hearing, J. Christopher Ronay, president of IME, expressed doubts about the safety of mixing taggants with explosives during manufacture. "The manufacture of explosives is a very sensitive process that must be done with 100% safety. ... Any change in the explosives formula, and particularly the addition of any new substance, can destabilize the mixture, setting off reactions that create a severe safety hazard," he said.
Doubts about safety are bolstered by studies suggesting taggants are incompatible with explosives. For example, OTA's 1980 report mentions a chemical reaction that occurs at high temperature when a high concentration of taggants is added to smokeless powder and booster material. And Ronay's testimony cites studies suggesting that taggants destabilize TNT (trinitrotoluene) as well as a lawsuit filed against 3M by gunpowder manufacturer Goex, claiming that a 1979 explosion at a Goex plant in East Camden, Ark., was caused by Microtaggant. At the time, the taggants were still part of the 3M product line.
Undesirable reactions can occur when taggants are used in high concentrations, says Microtrace's Faulkner. But the recommended level of taggants in explosives is very low, ranging from 10 to 500 ppm. He says Microtrace has not seen the data suggesting destabilization of TNT, and the company is concerned that some testing may have been carried out under conditions for which the product was not designed. And blaming the Goex explosion on taggants has no basis, Faulkner asserts. The Goex lawsuit was dismissed after it became clear there was no evidence that taggants could have contributed to the explosion.
Moreover, Faulkner tells C&EN: "At the time [of the OTA study], there was no track record because the products had never been used. But now, we have 10 to 12 years of safe and effective use in the Swiss explosives industry, where we've had no safety problems and no safety concerns raised."
IME believes the Swiss experience cannot be used as a basis for a similar requirement in the U.S. It's like mixing apples and oranges, according to Ronay, who cites the smallness and simplicity of the Swiss explosives industry (about 6.6 million lb produced in 1994, few product lines) compared with that of the U.S. (4.1 billion lb consumed annually, multiple product lines).
The safety issue could be a smoke screen. Says Kerns: "We think the concern may be more [one] of liability. Taggants will identify manufacturers. If an explosive is used improperly, the manufacturer may be held accountable."
The small, privately held company seems uncomfortable with the unsolicited attention it has been getting. "We have chosen not to go to Washington and get involved [in the debate about taggants], because we think the product stands on its own merit," says Faulkner. "But we don't want to create any kind of safety hazard, and so we have strongly supported continued testing by independent testing organizations."
"Our point," adds Kerns, "is that this tool is proven. Someone has to decide whether to use it. If people need it, we can work with industry and federal agencies to meet their needs."
Unlike identification taggants, taggants for detecting plastic explosives have a better chance of being adopted. Plastic explosives are difficult to detect because they have low vapor pressures and can be shaped in a variety of ways. With detection taggants, which have higher vapor pressures, plastic explosives are more likely to be picked up by bomb detectors.
The antiterrorism bill passed by the Senate calls for full implementation of the Convention on the Marking of Plastic Explosives for the Purpose of Detection. The treaty, which was ratified by the Senate on Nov. 20, 1993, requires incorporation of any of four detection agents into plastic explosives: ethylene glycol dinitrate; 2,3-dimethyl-2,3-dinitrobutane; para-mononitrotoluene; or ortho-mononitrotoluene.
The Swiss experience with identification taggants may be a strong argument for those who want to adopt that technology without further ado. The case for using additives to desensitize ammonium nitrate fertilizer, however, is not as straightforward, despite the practice in some European countries.
In Northern Ireland and the Republic of Ireland, because of terrorist activities, fertilizer containing more than 79% ammonium nitrate is considered an explosive and subject to controls. Thus, ammonium nitrate fertilizer marketed in those areas is a mixture of ammonium nitrate (78.5%) and calcium carbonate (21.5%) in the form of dolomite or limestone. The product is called calcium ammonium nitrate (CAN).
Irish Fertilizer Industries, the sole producer of CAN in Ireland, says the use of CAN outside Ireland is not required by European laws. However, other European countries - for example, Belgium, Denmark, Germany, and the Netherlands - also forbid marketing of plain ammonium nitrate.
Whether CAN deters terrorists is doubtful. It seems CAN has been used in terrorist bombings in the U.K. "as is" or after some processing. The extra operations required could be discouraging, but for those with the wherewithal and patience, the explosive power is still there.
The Oklahoma City blast prompted a second look at a patent awarded 30 years ago to Samuel J. Porter, a hazardous chemicals consultant from Woodbridge, Va. (C&EN, May 29, page 6). The patent claims addition of 5 to 10% by weight di- or monoammonium phosphate will prevent ammonium nitrate fertilizer from exploding.
Four victims of the Oklahoma City bombing have filed a lawsuit against ICI Explosives. The Dallas-based company is a major producer of ammonium nitrate industrial explosives. But it also produces fertilizer-grade ammonium nitrate at its operations in Joplin, Mo., and ships it to agricultural cooperatives for distribution to farmers in Missouri, Kansas, Oklahoma, Arkansas, Iowa, and Texas. The lawsuit claims ICI Explosives should have incorporated additives such as those mentioned in Porter's patent to render the ammonium nitrate fertilizer it produces less explosive (C&EN, May 22, page 11).
E. I. (Joe) Brawner, ICI Explosives president, has told employees: "Our contact with investigating authorities has not resulted in any indication that ammonium nitrate fertilizer produced by ICI was involved in the criminal act in Oklahoma City." Nevertheless, last month, the company carried out experiments to test the claims in the Porter patent. The company prepared "desensitized" ammonium nitrate according to the patent instructions. Then it called on an independent consulting firm that investigates engineering or scientific failures to test the material. The consulting firm, Failure Analysis Associates, Menlo Park, Calif., tested 10-gal charges at a site near Phoenix.
The results, which were recorded on videotape, show neither diammonium phosphate, monoammonium phosphate, nor calcium carbonate eliminates the explosiveness of ammonium nitrate. ICI Explosives suggests the 1968 patent "covers explosions on such a small scale as to make[ it] irrelevant to the terrorist issue." The importance of the scale of testing is reinforced by Peter G. Urben, editor of "Bretherick's Handbook of Reactive Chemical Hazards." He tells C&EN: "Because terrorists now use 1,000-lb charges or larger - and the larger [the charges] are, the more likely they will explode, especially if confined - very large detonation tests will be required to suggest inhibition."
Besides, a simple countermeasure exists, points out Kay R. Brower, professor of chemistry at the New Mexico Institute of Mining & Technology, Socorro. The ammonium phosphate can be removed easily from an aqueous solution by reaction with calcium nitrate, to give even more ammonium nitrate:
(NH4)2HPO4 + Ca(NO3)2 -> 2 NH4NO3 + CaHPO4The calcium salt precipitates, leaving dissolved ammonium nitrate, which can be recovered by evaporation.
But worse, the heat released from detonation of a mixture of ammonium nitrate and ammonium phosphate is up to 60% greater than that from ammonium nitrate alone, Urben says. He explains that ammonium phosphate acts as a fire retardant for cellulosics by forming a crust that prevents oxygen from reaching the fuel. But in an explosion or deflagration, oxygen already is intimately mixed with the fuel, and the ammonium in ammonium phosphate serves as additional fuel.
Ammonium sulfate, another proposed desensitizer, presents a similar scenario. In a recent article in Chemical Health & Safety [2(3), 22 (1995)], retired organic chemist Leslie Bretherick recalls how, in 1921, a 4,500-metric-ton stockpile of ammonium nitrate containing 45% ammonium sulfate exploded at a chemical factory in Oppau, Germany. The accident killed 600 people, injured 1,500 more, and left 7,000 homeless. The heat released by the reaction of a 2:1 mixture of the salts is also greater than that from ammonium nitrate alone.
The Bureau of Mines has carried out limited studies on urea as an additive. Research physicist J. Edmund Hay, at the bureau's Pittsburgh research center, says preliminary data indicate that sufficient dilution with urea (40% by weight or more) reduces the sensitivity of ammonium nitrate to detonation. How long this research can continue is up in the air, however, because the bureau is being targeted by Congress for abolition.
Another potential additive is potassium nitrate, says retired chemist Carl Boyars, formerly a senior scientist at the Naval Surface Warfare Center, Silver Spring, Md. Boyars explains that the likelihood of ammonium nitrate's exploding when shocked by a booster explosive is higher each time the ambient temperature cycles through 32 °C. At this temperature, the compound undergoes a phase change. Because the porosities of the phases are substantially different, the porosity of the ammonium nitrate granules increases with each cycle, making the material more sensitive to detonation. If potassium nitrate is added to ammonium nitrate to form a solid solution, the phase change does not occur, and the material is less sensitive to detonation. Boyars says military explosive compositions incorporating ammonium nitrate cocrystallized with 10% potassium nitrate were less sensitive to detonation than similar compositions with plain ammonium nitrate. He is not aware of any follow-up work on this approach to desensitizing ammonium nitrate.
Perhaps no additive can desensitize ammonium nitrate fertilizer. ICI Explosives' Brawner says the company is willing to help find a taggant that can be used to trace the source of ammonium nitrate fertilizer used in terrorist attacks.
Proving the effectiveness of taggants or of fertilizer desensitization may be much easier than developing a detector that could have stopped the destruction of Pan Am flight 103 over Lockerbie, Scotland. The bomb that killed 259 people on board and 11 on the ground is believed to have weighed less than 1 lb, concealed in a boom-box-type radio. That such a small amount of explosive could have brought down a Boeing 747 raised troubling questions, not only about security lapses at several airports where the suitcase containing the bomb was supposed to have been transferred but also about the ability of airports to detect the amount and type of explosive used.
The Aviation Security Improvement Act of 1990 set a goal for FAA to put explosives detection equipment in airports by November 1993. Despite FAA's best efforts, decisions to deploy explosives detection systems (EDSs) may not be made until 1997. The problem is not money nor bureaucratic foot-dragging. "It's a question of scientific laws," says FAA's James H. Farrell, manager of the technology integration division in civil aviation security. He tells C&EN: "[The act] set up stringent criteria that [EDSs] must meet before [FAA] can mandate wide-scale deployment. ... A lot of technological challenges are involved in developing equipment that meets the standards."
Neither FAA nor equipment manufacturers will discuss specific criteria for certification. But three issues obviously are critical: detection probability, false alarm rate, and throughput rate. The chance of detecting real threats is complicated by the variety of explosives. Some, such as plastic explosives, pack tremendous power and can wreak havoc in small quantities. Detection systems must be sensitive enough to find small amounts hidden in complex matrices. They also must be specific, able to discriminate between threatening and benign materials, and not generate too many false alarms. And because of the number of baggage pieces that must be screened - over a billion per year, according to OTA - they must do the job quickly.
It took until December 1994 for FAA, working with industry, to certify an EDS that meets the criteria for inspecting checked baggage. The system, CTX 5000, is produced by InVision Technologies, Foster City, Calif. Developed with FAA support, CTX 5000 is based on computed tomography (CT), the numerical reconstruction of a cross-sectional image from X-ray projections at various angles around an object. The CT image formed is a mapping of X-ray attenuation properties of each volume element within the cross-section. X-ray attenuation refers to the absorption and scatter of X-rays by an object. It can be correlated with the object's density and composition.
The system is automated, using red and green lights to indicate the presence or absence of a threat. But it does not completely eliminate human involvement. When the system decides a threat may exist, the operator is alerted. The operator then uses the instrument's threat resolution features to assess whether the threat is real.
CTX 5000 has the distinction of being the first and, at present, only FAA-certified EDS. Certification means it has passed FAA testing governed by protocols developed by the National Academy of Sciences. FAA's Farrell describes the testing procedure as measuring by the numbers: "We assemble the threat articles that test the machine against all kinds of explosives we require it to be able to detect," he says. "We test every machine the same way, against the same threat articles laid out the same way." Testing is carried out at a world-class multi-million-dollar security laboratory in the FAA's technical center in Atlantic City, N.J. The lab is one of only a few in the world where explosives detectors are tested with live, not simulated, explosives.
InVision Technologies' CTX 5000, the only explosives detection
system certified so far by the Federal Aviation Administration (FAA),
produces both projection X-ray and computed tomographic (CT) images. Data
from each CT slice are rapidly acquired and processed by a computer. If the
data indicate a threat, the location of the suspicious item is highlighted on
After a device is certified, the next step is operational demonstration. FAA will be carrying out three demonstrations of CTX 5000. The first will begin this fall with United Airlines in San Francisco. Two others will start next spring, one with Delta Airlines in Atlanta, in time for the 1996 summer Olympic Games, and another planned for Manila, in the Philippines. The field tests will run about a year, says Farrell, during which time FAA will collect the operational and cost information it needs to decide on deployment.
But even while the U.S. is still testing CTX 5000, the equipment is being used overseas. CTX 5000 systems are operating in Brussels and Tel Aviv. Scanners are being installed in Gatwick and Manchester Airports in the U.K. And early this month, one unit was shipped to Japan.
Until new EDSs are deployed, the only explosives detectors available in U.S. airports are X-ray scanners. X-ray imaging depends on the amount of an object and its ability to absorb X-rays. Standard scanners have poor specificity. They cannot distinguish between a thin slice of a strong X-ray absorber and a thick piece of a weak absorber, for example. But the X-ray image gives a good picture of dense items with distinctive shapes, such as guns or blasting caps. At present, most machines in U.S. airports are not automated. Although better technology apparently is making X-ray scanners more useful for explosives detection, they still must rely on humans to look for patterns that indicate a threat.
The wide use of X-ray scanners in U.S. airports attests to the public's acceptance of, and perhaps reliance on, this security measure. What many people may not know is that U.S. airports inspect only carry-on baggage for domestic flights; checked-in baggage for domestic flights is not screened, sources say. FAA's Farrell declines to comment on specific security requirements, saying only that "Security measures have been implemented in overseas and U.S. airports with regard to screening of baggage and cargo. These are geared toward the levels of threat and risk at the particular locations we apply them, domestic or overseas. The highest risk and threat have been more in overseas locations, generally speaking, and those measures are geared in that direction."
The potential windfall following certification is an alluring incentive to develop FAA-certifiable devices, and companies, as well as government laboratories, are looking at a dizzying array of bomb detection technologies.
One promising technology for baggage screening is nuclear quadrupole resonance. NQR is a magnetic resonance technique based on the quadrupole moments of the nonspherical charge distribution around the nuclei of solids. In an electric field gradient, different parts of a nucleus encounter different electric fields, causing the quadrupole to precess about the axis of the gradient. That motion is associated with a nuclear magnetic moment.
When an alternating magnetic field is applied in phase with the precession, the nuclei flip their orientation with respect to the electric field gradient. As the nuclei return to their preferred orientations, they produce a unique signal defined by two parameters: the specific quadrupolar nucleus and its chemical environment. A compound with quadrupolar nuclei, therefore, can have multiple unique NQR frequencies. For explosives detection, the important quadrupolar nucleus is nitrogen-14. The trick is to find the transition frequencies of this nucleus in all possible explosives.
Transition frequencies are within the radio-frequency field. Thus, unlike magnetic resonance imaging used in hospitals, NQR operates without magnets, which could be unwieldy and costly. Compared with the CTX 5000, which will cost close to $1 million to buy and install, an EDS based on NQR will be relatively inexpensive.
Employees at Quantum Magnetics test prototype nuclear quadrupole resonance scanners for explosives detection.
Detection works very much like finding a station on the radio, says Lowell J. Burnett, chief technical officer of Quantum Magnetics, San Diego. A device can be "tuned" to scan frequencies specific to explosives, and if an explosive is present, it picks up a return signal. Because the transition frequency is defined by chemical environment, detection can be avoided only by altering the structure of a material, which would also change its explosive and other properties. For the same reason, quadrupolar nuclei in nonexplosive materials do not interfere.
With FAA support and under a licensing agreement with the Naval Research Laboratory, Washington, D.C., Quantum Magnetics has developed a portable NQR device called the Contraband Detector. The device can be used to detect explosives or narcotics. A prototype tested last year at FAA's technical center in Atlantic City reportedly performed six times better in detecting military explosives than a state-of-the-art X-ray system. The company is building stand-alone prototypes for screening airline baggage and cargo.
FAA's Farrell is impressed with the technique's specificity, saying, "It looks like it will have a very low nuisance alarm rate." But it may be a while before NQR devices appear in airports. "It will take time to tune it for the full range of compounds necessary to meet [FAA's] performance requirements," Farrell says.
Quantum Magnetics is using strategic partnerships to maximize the potential of NQR technology. A move likely to hasten commercialization of its detector for airport security is an alliance with EG&G Astrophysics, a supplier of X-ray systems based in Wellesley, Mass. The agreement allows EG&G to integrate Quantum Magnetics' detector subassemblies with its X-ray scanners. Integrating two technologies based on different physical principles vastly improves the detection capability, says Burnett, who believes this dual technology can get FAA certification within a year.
The privately held company is seeking to improve its detector by using high-temperature superconductors developed by IBM Corp. Last year, the two companies signed an agreement that gives Quantum Magnetics exclusive commercial use of IBM's superconducting know-how in magnetic-sensing systems. The company is now testing laboratory prototypes of superconduction-based sensors.
Science Applications International Corp. (SAIC), based in San Diego, is using thermal neutron analysis (TNA) and pulsed fast-neutron analysis (PFNA) for explosives detection. In TNA, the material to be inspected is bathed with thermal, or slow, neutrons. When neutrons interact with individual nuclei, some neutrons are absorbed, and gamma-rays are emitted. The energy and intensity of the gamma-rays given off are characteristic of the absorbing nuclei.
Contents of cargo container are displayed in three dimensions by
Science Applications International Corp.'s inspection system based on pulsed
fast-neutron analysis. Light colored areas indicate location of a
The emitted gamma-rays are collected and measured by an array of detectors. The presence of explosives can be ruled out if the gamma-rays due to nitrogen-14 are below a certain level. But if nitrogen levels are high, the imaging and the neural network analysis in the TNA will determine whether the local nitrogen concentration is high enough to be caused by an explosive material.
With support from FAA, TNA units were used to check more than 1 million bags at airports in New York City, Miami, London, and Washington, D.C., between 1989 and 1994. In the early 1990s, both OTA and the National Research Council (NRC) identified TNA technology as one of the most promising for airport security. Results of testing and evaluation of the system at Kennedy International Airport became the basis of FAA's protocol for operational testing of explosive detection devices or systems.
"TNA established a standard and showed what could be done," says SAIC Vice President Patrick M. Shea. But now the company is not seeking FAA certification for business reasons. "It would cost us more to meet the standards than it would be worth at this point," says Shea, implying that a U.S. market for airport baggage screening does not yet exist. Instead, the company is looking at other applications, such as detection of environmental contaminants and contraband drugs.
SAIC is also leaning toward use of fast-neutron analysis (FNA) to improve the specificity of its detectors. Because thermal neutrons are slow, their ability to penetrate is limited, as is the range of reactions that can occur with organic materials, including explosives. Thus, TNA can be used only for luggage, not for cargo, and some elements that can be very helpful in detecting explosives, like oxygen, cannot react. With more energetic neutrons, bigger objects can be screened and more reactions giving off characteristic gamma-rays can occur. Because more data are generated, specificity is better and false alarms are fewer.
When fast neutrons are delivered in very narrow (nanosecond-wide) pulses, the technique, PFNA, becomes very similar to CT. A neutron beam scanning an item at precisely timed intervals is doing the same thing that an X-ray beam does as it looks at different cross-sections of an object in CT. Whereas X-ray CT looks only at X-ray absorption and scattering, PFNA sees the reactions of all the elements in explosives: carbon, nitrogen, oxygen, and hydrogen. The outcome is an elemental profile at regular intervals that makes it easy to pinpoint a suspect object. With huge cargo containers, this ability is important. As one SAIC scientist puts it: "If you have an 8-foot cargo container, you want to know if the suspicious material is in the first foot or the last one. Pulsing tells exactly where it is."
Searching for bombs also can be based on methods that examine the vapor emanating from explosives. Because some explosives have very low vapor pressures, vapor detectors must be exquisitely sensitive.
The canine nose is an extremely sensitive molecular sniffer, able to detect vapors at concentrations three to five orders of magnitude lower than those discernable by humans. Dogs and their handlers have been and continue to be vital members of law enforcement agencies' bomb detection squads. The U.S. Secret Service has been training dogs to detect bombs since 1975 at a facility in Beltsville, Md. And as part of an antiterrorism assistance program of the State Department, ATF has been training dogs for foreign governments since 1991. For the first time this year, one ATF dog has been trained for use in the U.S.
"The dogs are extremely sensitive," says Richard A. Strobel, chief of the explosives section of ATF's forensic laboratory in Rockville, Md. "They cover a very wide range of explosives ... anything from black powder to [the plastic explosive] Semtex."
Strobel says ATF dogs are trained on specific explosive ingredients instead of a specific product: "Instead of training on dynamite, we train on nitroglycerine. This way, we don't have to worry about what formulation is being used." Machines usually are fixed, portal-type detectors, whereas mobile dogs can clear a large space such as an auditorium, for example, or inspect a building floor-to-floor.
High-tech sniffers, on the other hand, probably can be relied upon to perform more consistently than dogs. One company that's focusing on vapor detectors is Thermedics Detection, based in Chelmsford, Mass. With support from FAA and the Departments of State and Transportation, it has developed an explosives detector dubbed Egis, based on high-speed gas chromatography (GC).
Inspector at Zurich Airport uses Thermedics Detection's Egis explosives detector to screen air cargo.
Speed coupled with high resolution is the unique feature of Egis, says Thermedics Detection Vice President James E. Buckley. For example, separation of a mixture of explosives ranging from very volatile to practically nonvolatile, which can take up to 45 minutes in a forensic laboratory, takes Egis only eight to 10 seconds for the same resolution, he explains. That speed and resolution are combined with a level of automation that requires from an operator only one push of a button.
A sampling unit and an analysis unit make up Egis. The sampler is a handheld, battery-powered vacuum aspirator resembling a flashlight. It picks up vapor samples like a vacuum cleaner picks up dirt. After sample collection, the unit is attached to the analyzer, where separation and detection take place. Computers evaluate the data, and depending on the result, a display panel flashes a green or a red light.
Egis recognizes the nitro groups characteristic of explosives. The analysis unit houses two high-speed GC columns in series with a low-temperature (400 °C) and a high-temperature (800 °C) pyrolyzer. The sample goes through the first column, and the separated components go to the first pyrolyzer. At 400 °C, the nitro groups in N-nitrosamines, nitramines, and nitrite esters are converted quantitatively to nitric oxide (NO). The nitric oxide passes through the second column to a chemiluminescence detector. Compounds that do not decompose at the lower temperature go to the second GC column and through the second pyrolyzer. At 800 °C, nitro groups in aliphatic and aromatic nitro compounds are transformed to NO, which subsequently is detected.
The chemiluminescence detector in Egis relies on the reaction between NO and ozone (O3), which takes place in an evacuated chamber at reduced pressure. The reaction produces electronically excited nitrogen dioxide (NO2*):
NO + O3 -> NO2* + O2The excited species emits infrared light when it returns to the ground state:
NO2* -> NO2 + IR light (600 to 1,800 nm)The light produced is captured by a photomultiplier tube; the output is proportional to the amount of NO in the reaction chamber.
The company, however, is not seeking FAA certification for Egis as an EDS. That doesn't mean that Egis can't be integrated into an EDS with multiple parts. "In many cases, Egis is used as an explosives detector in combination with X-ray," says Buckley. "The combination of image-based and chemistry-based detection gives a good synergy that results in a very low false alarm rate and good detection capability."
Right now, Egis is not used in any U.S. airport. But it is used to screen baggage in airports in at least six European countries and in Japan; in Frankfurt Airport alone, 11 units are operational. In Israel, it is also used to detect explosives at border crossings with Jordan and Egypt.
Other detectors can be used with GC. For example, vapor detectors from Sentex Systems, Ridgefield, N.J., use electron capture detection, based on the high affinity of nitro groups in explosives for electrons.
Ion-mobility spectrometry (IMS) is the technology behind Ionscan, a vapor detector made by Barringer Technologies, New Providence, N.J. In IMS, ions of a sample are formed in a reactor through ion-molecule reactions. The ions are released into a separation region that is under the influence of an electric field. They move through this region at a rate proportional to their mass and against the flow of a gas. Heavy ions, such as those of explosive compounds, move slowly. When the ions reach the collector, they register a current peak. The magnitude of the peak and the time it registers are analyzed by a computer to determine the vapor being detected.
Mass spectrometric methods are also being harnessed for vapor detection. At Oak Ridge National Laboratory (ORNL), scientists led by Scott A. McLuckey, head of the analytical spectroscopy section in the chemical and analytical sciences division, have been developing a detector based on quadrupole ion-trap mass spectrometry. The work, which is being funded by the Department of Energy, began as part of research efforts to find ways to protect U.S. nuclear weapons facilities from sabotage, says McLuckey.
The technology is very specific - that is, has low false alarm rates - McLuckey explains, because an analyte must pass three tests before it is identified as an explosive. The analyte must be able to form a negative ion, the parent ion must have a mass-to-charge (m/z) ratio corresponding to that of an explosive, and fragments from the parent ion must have m/z ratios corresponding to those of an explosive.
In this technology, analyte molecules are first converted to negative ions through a technique called atmospheric sampling glow-discharge ionization. Explosives readily capture electrons to form anions, but most organic compounds in nature do not. Thus, analyzing in the negative-ion mode eliminates interferences from the start, increasing sensitivity. The negative ions then are injected into the quadrupole ion trap.
Negative ions accumulate in the ion trap in a mass-selective fashion, by allowing in only parent ions with m/z ratios of interest. If molecular ion peaks corresponding to explosives are produced during this first mass spectral analysis, a second analysis is carried out for identification. This time, a signal is applied to the ion trap to excite the molecular ion. The ion dissociates into fragments, and those masses are analyzed by the spectrometer. A positive identification is made only if the m/z ratios of the fragments correspond to those from explosives.
ORNL is working with Teledyne Electronic Technologies, Mountain View, Calif., to commercialize the technology. The company says it will be at least a year before commercial systems become available.
These commercial or precommercial systems are but a sampling of what's out there. Scientists continue to explore novel ways to detect the weapons of terrorists. Just recently, for example, researchers at Los Alamos National Lab came up with the idea of using the deflagration characteristics of explosives as the basis for a detector (C&EN, May 29, page 36). And at the Naval Research Lab, with support from FAA, researchers are developing detectors with antibody-based sensors.
Whether Congress will demand the use of some of these technologies remains to be seen. Meanwhile, Oklahoma City has shown, yet again and close to home, what can happen when a terrorist act is not stopped. The number of people killed there has yielded at least 167 more reasons why such acts should be prevented.
As Sofaer said before the House Judiciary Committee last month, "The primary task of our government ... is prevention ... [because] the damage the perpetrators inflict far surpasses the damage we can thereafter inflict upon them through the legal system."
InVision Technologies' CTX 5000, the only explosives detection system certified so far by the Federal Aviation Administration (FAA), produces both projection X-ray and computed tomographic (CT) images. Data from each CT slice are rapidly acquired and processed by a computer. If the data indicate a threat, the location of the suspicious item is highlighted on both images. Inset: CTX 5000 in operation at Manchester Airport, U.K.
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