Existing and Potential Standoff Explosives Detection Techniques (2004) / Chapter Skim
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5 Existing Detection Techniques and Potential Applications to Standoff Detection
5 Existing Detection Techniques and Potential Applications to Standoff Detection
Pages 71-96
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From page 71... ...
Trace detection relies on vapors emitted from the explosive or on explosive particles that are deposited on nearby surfaces. Explosive detection is a very challenging task, and combinations of the various techniques offer increased sensitivities and selectivities.
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From page 72... ...
The amount of standoff distance that is required will depend on the size of the explosive device, but standoff distances are usually defined as 10 m or more. In Figure 5.1, technologies that have the potential to achieve standoff detection are distinguished by cross-hatching.
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From page 73... ...
Trace Detection Electronic / Chemical Optical Biosensors GC + GC1 + Ion Mobility GC + LIDAR Transmission Nonlinear Dogs, Bees, Rats Surface Mass Spectrometry DIAL & Optics Acoustic Specto - DIRL Reflection Wave metry Antigens GC + Electron Spectroscopy Antibodies Capture Detector CARS Thermo Photoacoustic GC + Chemical Redox Spectroscopy Optical Luminescence Phase Robotic detector Field Ion Conjugation Insects Spectro - Cavity Ringdown metry Spectroscopy Coherent Distributed Control microresonators MEMS Standoff Detection Potential 2 Electronic Nose Remote Detection Direction Distributed 1 GC, Gas Chromatography, is Network typically used to increase the R&D Direction Selectivity of nitrogen containing molecules 2 Any of the "sniffers " qualify as electronic noses. FIGURE 5.1 Chart showing many of the existing technologies for (a)
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From page 74... ...
research directions that may be fruitful in advancing or expanding the applicability of the technique. BULK DETECTION Imaging is a primary technique for standoff detection scenarios.
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From page 75... ...
made of heavier elements. As in the case of transmission imaging, the detectors for backscattering could be located closer to the target than the transmitter to enhance image resolution and decrease losses caused by absorption in air and the angular
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From page 76... ...
There is good potential for X-ray imaging at standoff distance of approximately 15 m. Research in the areas of high photon flux X-ray sources, pulsed X-ray sources, smaller focal spots for scanned beams, and focused X-ray beams7 can contribute to the successful development of standoff X 5American Science and Engineering (AS&E)
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From page 77... ...
In summary, X-ray imaging has good potential for standoff detection for distances up to approximately 15 m. Its advantages are excellent image resolution along with limited discrimination between explosives and background items.
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From page 78... ...
target area.8 The IR detection scheme can easily detect image patterns in this time frame. Real-time motion videos using multiple views (possibly filtered into various spectral windows)
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From page 79... ...
To optimize standoff detection, other important areas for development include cooled detector arrays and advanced image processing techniques. In summary, IR imaging is a very important technique for standoff detection.
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A very interesting potential, compact, lowcost source is the quantum cascade laser. These tiny semiconductor lasers have now been operated down to frequencies as low as 1.5 THz.13 Another interesting compact source14 is based on nonlinear mixing between closely spaced diode laser sources and Raman shifted laser lines in the infrared to form coherent beams in the terahertz range.
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From page 81... ...
These large dimensions place constraints on the concealment and portability of the detection apparatus useful for standoff detection at distances greater than 10 m. This fundamental resolution limit indicates that one should choose the shortest wavelength possible in order to resolve objects at standoff distances in the terahertz regime.
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From page 82... ...
K Chen, Bell Laboratories, Lucent Technologies, Murray Hill, NJ, personal communication.
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From page 83... ...
At frequencies below 50 GHz in the microwave regime, the resolution at standoff distances is fundamentally limited, according to Equation 1. FIGURE 5.7 Visible and terahertz images of a man carrying a knife hidden in a newspaper and a gun in his pants.
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From page 84... ...
Neutrons, Gamma Rays, Magnetic Resonance, and Magnetic Fields Neutron and -ray explosive detection both suffer from a combination of health hazards and limitations in sensitivity for standoff detection. Neutrons penetrate typical bulk explosives.
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From page 85... ...
Ships at docks and trucks on bridges that could be carrying tons of explosives in cargo containers are important scenarios for explosives detection. Gamma-ray and neutron beam explosive detection instruments can potentially screen these large cargo containers at ports of entry, although throughput rates may prohibit the screening of all cargo in some cases.
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From page 86... ...
TRACE DETECTION Trace detection at standoff distances is a particularly challenging task. Saturated vapor pressures for many of the common explosives are very low (approximately 10 parts per billion [ppb]
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From page 87... ...
Concentrators are another common sampling technique for trace explosive detection. For example, a vacuum system near a portal can be used to accumulate a large air sample from which explosive molecules can be concentrated by filters.
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From page 88... ...
For example, common fumes and odors in the environment might be troublesome interferents in many explosive detection scenarios. In principle it is possible to form a remote explosive sensor using a large area coated with a luminescent material designed so that the luminescence is quenched by small quantities of explosive molecules.
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From page 89... ...
34Burschini, C Commercial Systems for the Direct Detection of Explosives (for Explosive Ordnance Disposal Tasks)
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From page 90... ...
For example, photoacoustic spectroscopy37 using infrared active vibrational transitions has the sensitivity to detect 10 ppt with an averaging time of the order of 10 seconds. Similarly, surface enhanced Raman scattering38 and cavity ring-down spectroscopy39 (decreasing the Q of an optical cavity by the vibrational absorption of a molecule)
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From page 91... ...
Backscatter can result from particulates in the air. At standoff distances in the 10- to 30-m range, the very low molecular concentrations characteristic of explosive molecules result in sensitivity limits for these laser ranging techniques.
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From page 92... ...
For example, if two exciting laser beams are focused on a volume of gas containing explosive molecules, the light scattered from a third laser that is tuned to have a frequency shifted from the two exciting lasers by the vibrational frequency of the molecule, will produce a fourth beam whose intensity varies as the square of the exciting beams (see Figure 5.9)
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From page 93... ...
A surface monolayer of molecules can be detected by CARS techniques in the laboratory; it remains to be seen if this can be done in the field. Other nonlinear optical techniques that could be exploited include optical phase conjugation and excitation of index gratings using crossed laser beams.
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From page 94... ...
In summary, nonlinear optical techniques have the potential for increased signal-to-noise ratios relative to linear techniques. Optical phase conjugation should be studied for optimizing the signal returned from LIDAR excitation.
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From page 95... ...
Hyperspectral Detectors Imaging is a powerful tool for standoff detection. However, the information presented in the image varies widely with the spectral region imaged.
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From page 96... ...
Many composition detection techniques only identify an anomalous nitrogen content, and do not provide conclusive evidence of the presence of an explosive device. A combined image, showing wires or a dielectric shape in the form of an explosive device and an anomalously large nitrogen concentration, would enhance the decision-making task.
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