BOX 4.1
Sensors for Airport Security

Development of new and improved sensors should provide many security benefits, but perhaps none is so visible and immediate as the need for increased airport security with minimal passenger inconvenience. Until recently, security at U.S. airports was limited to metal detectors and x-ray imaging. But over the past few years, explosives detectors have been installed that use stationary ion mobility spectrometers (IMSs) or chemiluminescence sensors, both of which are capable of detecting a number of explosives, including RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), PETN (pentaerythritol tetranitrate), TNT (2,4,6-trinitrotoluene), and nitroglycerin. However, given the limited sensitivity of deployed detectors (detection limits of 1-10 picograms) and the low volatility of most explosives, these systems generally require the collecting of particles of explosive for detection. Particle collection requires tedious swabbing of luggage, and careful cleaning of the exterior of a package by a terrorist can greatly reduce the chance of detection. Another limitation of conventional technologies is that particles can be picked up from one object by another, causing a false positive.

However, new and emerging techniques could augment existing detection capabilities. A number of new technologies appear to hold promise for explosives detection, including x-ray diffraction, which detects several types of explosives; microwave/millimeter wave scanners; and nuclear quadrupole resonance (NQR) (NMAB, 2002). The use of NQR spectrometry or neutron capture for explosives detection is based on the unique physical nature of the 14N nucleus (99.6 percent natural abundance) in the nitro groups in the explosive materials. New detection-coil technologies have improved NQR considerably, and the U.S. Army is developing vehicles that use it for landmine detection. However, NQR still suffers from limitations. It has sizeable power and computational requirements, making it unsuited for a portable system. The long relaxation times of the 14N in TNT restrict the number of pulses that can be applied and thereby limit sensitivity for this explosive; there is also a reluctance to expose people to strong radio frequency fields. Neutron capture methods require a neutron source, such as a radioisotope or a particle accelerator, and present other complexities.

Methods to detect explosive vapors have many advantages: Vapor collection from people and luggage can be rapidly accomplished and is minimally invasive. Such detection needs considerable sensitivity, as is provided by mass spectroscopy, and may require new sensor advances. It will also be important to have high-sensitivity systems—unlike IMS, chemiluminescence, NQR, or neutron capture—that are portable and can be used as a handheld wand.

Additional support is needed for research to develop improved methods for detecting explosives at airports.

  • Sensors to detect chemical agents or nuclear materials in shipments (see Chapters 2 and 7);

  • Sensors to check food, water, currency, and mail for contamination;

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