[Technology Report]
Homeland Security's Techno War On Terror
Technologies combine to detect radiological, biological, and chemical weapons of mass destruction.
Dangerous times call for high-tech answers. Today's detection and neutralization systems help guard our borders against people, cargo, and vehicles that may be carrying explosives, arms, and contraband that could include pathogenic and even nuclear material. All these perils make for a lucrative industry, as analysts say the homeland security market could total tens of billions of dollars. Just look at the technologies in the mix: X-ray, ultrasonic, neutron-bombardment, gravity gradient, optical, infrared (IR), and terahertz (THz) imaging, as well as RFID technology.
X-Ray Marks The Spot A number of recent advances have fortified conventional security methods. Widely used in airports, government buildings, courthouses, and correctional facilities, fixed-location and limited-mobility X-ray systems are now giving way to portable, lower-cost, back-scatter X-ray imagers that can be set up anywhere. Back-scatter X-ray imaging reveals details missed by traditional transmission X-ray imaging.
For example, American Science and Engineering developed the Gemini dual-energy X-ray imaging technique, which allows for the simultaneous detection of previously undetectable organic and inorganic items under inspection (Fig. 1).
Gemini's Z-Backscatter system features enhanced detection of organic-materials—like sheet and bulk explosives, narcotics, and plastic weapons—frequently missed with conventional transmission X-ray systems. The system colorizes the objects it scans by determining the material type (the atomic number) of any object being scanned, generating a clear uncluttered display.
X-ray photons scatter differently when they encounter varied types of materials. Take Compton scattering, which is material-dependent. Materials with lower atomic numbers scatter more strongly than materials with higher numbers. Elements with higher atomic numbers are more likely to absorb X-rays, either before or after being scattered.
X-ray imaging can be augmented with neutron-based systems to detect "dirty bombs." Experts at the Institute Ruder Boskovic, Zagreb, Croatia, are proposing such a system. Here, X-rays can be used to conduct a fast initial check to identify a "suspect" region and then provide the region's coordinates to the neutron-based system for final confirmation. The system would use a 14-MeV neutron beam defined by the detection of the associated alpha particles. Tests have shown that such a system can accurately distinguish between depleted uranium, lead, and iron.
Active neutron activation, a technique developed at the Lawrence Livermore National Laboratory (LLNL), is considered a major breakthrough in cargo inspection. The method's lower energy levels (3.7 MeV versus 14 MeV) eliminate the nitrogen interference due to oxygen activation found in conventional inspection techniques (Fig. 2).
Another promising LLNL technique is pulsed fast-neutron analysis for element-and metal-specific imaging. It consists of a neutron source, gamma detectors, signal-processing systems, and an operator display (Fig. 3). And, LLNL is investigating other techniques for cargo inspection, besides neutron-based systems.
One interesting concept is based on gravity gradiometry, a passive technique that shielding can't defeat. It positively detects fissile material in most simulations, and it's recommended as a complement to other techniques such as photon or neutron detection. LLNL researchers modeled such a system to resolve a 15-cm3 plutonium pit surrounded by lead shielding.
Neutron-based systems aren't the sole purview of the U.S. The European Union is developing the EURITRACK (EUropean Illicit TRafficking Countermeasures Kit) tagged-neutron project to inspect cargo containers. It's part of the EU's 6th Framework Program.
The Tagged Neutron Inspection System (TNIS) features a transportable deuterium-tritium neutron generator, position-sensitive alpha detectors, fast-neutron and gamma-ray detectors, and data-acquisition and analysis circuitry (Fig. 4). System components will be integrated this year at Croatia's Zagreb Ruder Boskovic Institute.The entire system will be demonstrated next year in France.
Reprints
