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Non-destructive evaluation (NDE) is a relatively mature field of technology and engineering that has been developed over at least the past 100 years. Indeed, one could even argue that since visual inspection has always been, and continues to be, an important NDE method, the field is as old as humankind itself. NDE is of particular interest today as a tool for homeland security. According to the U.S. Customs Service and the Bureau of Transportation Statistics, nearly six million cargo containers enter the nation's 102 seaports every year. Twelve million trucks and 120 million personal vehicles cross our border with Mexico and Canada annually, and 60 million airline passengers arrive in the U.S. from foreign countries. Fast, reliable non-destructive inspection of cargo, vehicles, and personal baggage is a formidable technological and operational challenge. NDE denotes that an object is not destroyed by the examination and can still be used for its intended application.
In general, NDE methods can be divided into two categories. Some techniques are intended to locate surface flaws, discontinuities, or asperities, while others reveal the internal structure of an object. Both are potentially applicable to the inspection of containers and vehicles. For example, surface analysis tools may be able to determine whether a sealed container has been tampered with. Bulk internal imaging techniques may be useful in identifying contraband within sealed containers or vehicles.
Visual inspection is the usual approach to detecting surface flaws, and the most commonly used are the liquid penetrant and magnaflux methods. In the former, a visible or fluorescent dye is applied to a surface and the excess wiped off. Any fluid drawn into surface flaws will weep back to the surface, where it may be observed with optical or UV light, indicating whether a seal has been tampered with. In the magnaflux technique, one scans a magnetic field for perturbations created by anomalies in the surface, but this method works only with ferromagnetic materials. [The magnetic detectors we all walk through in airports are close cousins to the NDE magnaflux machines.]
Bulk NDE methods - those that image the internal features of materials and components - offer far greater potential for examining containers and vehicles for contraband and weapons. Bulk NDE systems can be designed for containers ranging from small parcels, personal suitcases and briefcases to large cargo containers used in global commerce. X-ray radiography is probably the most commonly used bulk NDE method for examining containers, currently used to examine carry-on luggage at airport security checkpoints. The principal difficulty is that a human operator must interpret the image and judge whether a given "shadow" is, or is not, a prohibited item. Analysis tools employing image recognition software could be developed to aid operators, but none have yet been commercialized. Radiographs along multiple axes, a tomographic technique, would also increase the probability that the image and contents of a container could be interpreted unambiguously. These devices can be expensive; their cost scales with size at a rate far greater than linear. Moreover, insufficient manufacturing capacity exists to produce the large numbers of machines that would be needed in the near future.
Neutron radiography is closely related to x-ray radiography and uses a beam of neutrons rather than x-rays to penetrate and image the contents of a container or vehicle. Neutrons are very penetrating, and neutron radiography is capable of imaging objects that are too large for x-ray machines, even up to objects as massive as railroad freight cars or truck trailers. The intensity is also sufficiently high that imaging can be performed in a few seconds. The most troublesome issue here is cost, as machines with an imaging capability of this scale are incredibly expensive. The high cost is a consequence of the "physics" used in these neutron sources, in which a proton accelerator and a high-Z target are used to generate neutrons via spallation.
Neutron activation offers another conceivable detection method. Certain materials will activate in a neutron beam, and the resulting decay spectrum could be useful in identifying contraband materials in containers and vehicles. Of special concern is the fact that some elements alloyed in engineering materials (cobalt, for example) activate readily, and one would run the risk of activating legitimate cargo and perhaps even the container or vehicle.
Microwave imaging is another possible method. If the frequency is tunable, an inspector could "scan into" the object to observe a series of sectional images This method is capable of imaging metal objects hidden behind low-Z materials, such as a gun or knife under clothing. A significant opportunity exists to develop this technology into a suitable tool for examining containers of soft items. While the public may object to using x-rays or neutrons for examining food, they would be likely to accept microwave examination. Tunable radars and image construction and analysis are the aspects of this technology most in need of development.
Ultrasonic imaging may also have applicability in limited cases. This technique senses acoustic reflections and may be suitable for inspecting solid materials. For example, it is often used to inspect the steel used to fabricate pressure vessels. A simple application to a container or vehicle may not supply useful information in the absence of other tools. But the technique may he useful in special cases where it is suspected that a solid object might be hidden in a liquid or powder.
Thermal imaging is yet another avenue for inspecting containers, vehicles and people. Any object warmer than its surrounding could be detected. The technique might prove useful for apprehending individuals hiding in containers, trailers, freight cars or vehicles. It would also probably detect any concealed device that is consuming power and therefore emitting heat.
Finally, magnetic resonance imaging (MRI) is another technique that may be useful for detecting concealed objects. MRI does not employ any form of ionizing radiation. Consequently, it may be suitable for cases where such radiation should be avoided. However, the contraband of interest must have a molecular structure that will resonate. So MRI is particularly useful for organics and might find application in scanning for drugs and explosives.
The notion that some of these techniques could be combined to enhance sensitivity should not be overlooked. For example, simultaneous radiographic and magnetic imaging would be more sensitive than either alone for certain types of contraband. Serious design work at the systems level will be required to develop an optimum approach for layered detection systems with maximum sensitivity for materials of interest.In principle, many of the NDE technologies discussed above could be employed to examine and secure the existing cargo container fleet if they are further developed and engineered into robust commercial products. This challenge should certainly be pursued. In addition, there is no question that specially designed, sensor-equipped "smart containers" are a complementary approach that holds great promise. A new generation of such containers could be very effective in determining with greater certainty and in real time whether tampering, diversion or insertion of prohibited material has occurred in transit. This is a long-term solution that should be explored in parallel.