Chemical and Explosives Detection

By Mark Coffey

Chemical weapons and explosives present immediate threats to public health and safety. Their detection is therefore of high importance. While several detection methods are available, it is expected that future research could be very useful. In addition, terrorist activities are sometimes partly funded by illegal drug trafficking. In this regard, the detection of illegal drugs and the ensuing intervention can also play an important role in the war on terrorism by undercutting a significant financial source.

The danger to persons and infrastructure from conventional high explosives such as RDX, TNT and C4 is obvious. In addition, recent years have shown the concrete effects of nerve agents such as sarin gas. Prompt and accurate detection and identification of crippling or lethal concentrations of such chemicals or agents is required. Current detection methods may be either active or passive. Active methods include neutron activation, wherein an enhanced output neutron flux is monitored.

Standard laboratory instruments for chemical detection and analysis include gas and liquid chromatography and various spectroscopies, including mass spectroscopy. These instruments can provide definitive multi-species identification. For counter terrorism applications, detection methods which can be made field portable are of high interest. Portable detectors can now be based upon ion mobility spectrometry and surface acoustic waves. Among the deployment questions to be addressed for a given environment is that of whether multiple types of sensors are required. This question arises as a function of a tolerable level of false positives, a tolerable level of confidence of identification, and chemical range of identification.

Among the promising bulk detection methods is nuclear quadrupole resonance (NQR). NQR uses an externally applied radio frequency magnetic field pulse at a characteristic frequency to generate a coherent signal that can be detected with a tuned antenna and a very sensitive receiver. The NQR resonant frequency is specific to individual compounds, resulting in a very low incidence of false alarms. Due to the compound specificity, NQR has potential for detecting a variety of drugs and explosives. NQR can not detect liquid explosives and struggles with TNT at this point, although much progress has been made. It is particularly well suited for RDX—the primary component of the highest threat aviation security explosive.

One aspect of NQR that could benefit from further investigation is dealing with shielding. A metal casing around an explosive alters the tuning of a detector coil. While automatic tuning of the coil will indicate the presence of an rf-shielded volume, further detection capability is highly desirable. NQR has also shown promise in detecting explosives in land mines. One DARPA- sponsored demonstration of detecting C4 explosive in antipersonnel and antitank land mines showed both high probability of detection and low incidence of false alarms. Many system-level issues such as power, size and weight have been addressed aggressively through the DARPA program, as well as the detection of TNT.

To be the most useful, NQR or other methods should be combined with sensitive detection techniques. Here, physics can also be brought to bear in the arena of nonlinear signal amplification, including the technique of stochastic resonance (SR). With SR, small periodic perturbations can be greatly amplified by large noise fluctuations. In all main types of SR, the signal-to-noise ratio is exponentially dependent upon the ratio of an energy barrier height or threshold to the input noise intensity.

The topic of airport security screening begs the larger question of just what combination of technologies is necessary to ensure detection at some specified high level and illustrates the system engineering issues. For instance, some detection methods will have higher probabilities of success than others, but will also totally miss certain substances. It seems that a serious, across-the-board scientific and engineering analysis of this problem, including cost-benefit aspects, has not been performed. In addition to probability of detection and the false alarm rate, another dimension in the study would be throughput, or processing rate. The deterrence effect on contemplated terrorist activity should not be underestimated. Historical data to support this assertion is available from an early 1970s FAA Aviation Security Program. This psychological aspect may mean that lower probabilities of detection are practical.

In sum, investigations by the physics community in the area of chemical and explosives detection can range from proofs of concept to implementation and studies of issues of system deployment. Many environments pose serious challenges to the detection of chemical weapons and explosives, with problems of shielding and noise sources. These aspects also warrant further research in areas such as signal detection and amplification.