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SYSTEMS 9 REVIEW OF EXISTING AND POTENTIAL STANDOFF EXPLOSIVE DETECTION TECHNIQUES Chris Murphy described an ongoing NRC study aimed at standoff detection of explosives (e.g., those contained in a belt worn by a suicide bomber). In this case, standoff is defined as detection from at least 20 meters with a response time of 5â10 seconds. Promising technologies include x-ray backscatter, IR imaging, and terahertz imaging, but there does not appear to be a clear winner for all scenarios. A systems-level approach is needed, with fusion of data from multiple sensors, discrimination, and identification to yield a high probability of detection with a low false alarm rate. PANEL 2 DISCUSSION Discussion in this segment began with concerns being expressed about the feasibility of molecular-scale sensing devices. For example, would bacteria in the background environment interfere with the detection of biological agents? Kwok's answer was that one is not dealing with a single 1 µm x 1 µm CMOS chip; rather, if the sensor is structured in a crossed-wire configuration with target-specific antibody sites at the nodes, one has the equivalent of an array of 1,000 such chips, and only one triggered cross point is needed for detection. Diffusion is fast across this area and one can design 255 different signatures to detect an equivalent number of agents. Another issue was raised: the significant problem with detecting small things in large volumesâfor example, screening 5 liters of liquid to find one organism. Selectivity is yet another key issue, indeed a bottleneck: The transduction element must only respond to the target ligand. Kwok's response was that DARPA's goal is not to develop new chem/bio sensor systems per se but to push nanotechnology forward using the performance of a dog's nose as a metric. He wants to âbuild a different way to look at the problem.â It was also asked whether having more nodes in sensors is necessarily better. With a large number of nodes, there could be a data processing problem that would degrade the overall signal. An important question for sensor integration is, How many nodes are enough? Rich Fletcher was asked about the limitations imposed on the covertness of RF tags by the need for antennas, and whether negative refractive index materials (and photonic band gap materials in general) might be useful in addressing this problem. What power levels are required for these materials as a function of distance? Fletcher did not answer directly but noted that while good work on negative refractive index materials is being done in academia, the work has not yet been transitioned to the commercial world. With regard to antennas, while they may be long, they can be made so thin that they are invisible to the eye. One can also use higher- frequency systems to reduce antenna size, but the higher frequencies don't penetrate well in enclosed spaces. It was also asked why if capacitive RFID tags are so promising, Motorola had dropped its program in this area. Fletcher believes that this was a case of throwing out the baby with the bath water: Motorola determined it was losing money in the tagging area and terminated all of its programs in that area.
SYSTEMS 10
