Explosives plastic, detection

These days security is at the top of everyone s list of important concerns, especially for the people who are responsible for the safety of our transportation systems. In particular, airports need speedy and sensitive detectors for explosives. Plastic explosives are especially tricky to detect because they do not respond to metal detectors, and they can be shaped into innocent-looking objects to avoid X-ray detection. However, a team of scientists at Oak Ridge National Laboratory led by Thomas Thundat has just published a description of an inexpensive device that is extremely sensitive to two N-containing compounds found in plastic explosives. The key part of this detection device is a tiny (180-micrometer), V-shaped cantilever made of silicon. The cantilever is shown in the accompanying photo next to a human hair for size comparison. 

The investigation into the sabotage of Pan Am Flight 103, which left 269 dead, indicated that the explosive used was Semtex H, a plasticized mixture of hexahydro-l,3,5-trinitro-s-triazine and pentaerythritol tetranitrate, and that the amount used was half the quantity that the fledgling technique of Thermal Neutron Analysis (TNA) was designed to detect. Although the placement of the explosive device was fortuitous (from the terrorists point of view) and the suitcase had not been screened by TNA, this event killed the TNA prototype program. 

M. Ostafin and B. Nogaj, Detection of plastic explosives in luggage with N nuclear quadm-pole resonance spectroscopy , Appl. Magn. Reson. 19 (2000) 571—578. 

Plastic explosives contain one or more of the explosives listed above, moulded in an inert, flexible binder. Because powders do not readily hold a shape and TNT is the only common melt-castable explosive, most of the explosive powders (RDX, HMX, PETN, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)) are plasticized to make a mouldable material, for example, C-4, Semtex H, PE4, sheet explosive. A variety of plasticizers are added, but the maximum level is usually 10-15% because most plasticizers are inert and would degrade explosive output. Plastic explosives were originally developed for convenient use in military demolitions but have since been widely used in terrorist bombs. For detection techniques that rely on vapour signatures, such as canine olfaction, it is worth considering that the plasticizer is much more volatile than the explosive component. 

Another aspect of minerals and rocks laser-induced luminescence became important recently, especially in Israel. Terrorists have intensively used roadside Ijombs, while the plastic covers simulating local rocks have been used for camouflage of explosive materials. The possibilities of the luminescence method have been checked for the remote detection and identification of such camouflage materials. The luminescence properties of the colored plastics have been investigated in comparison with the corresponding properties of the dolomite rocks, which are imitated. 

Analyzed explosives included TNT, RDX, HMX, PETN, TATP, and the plastic explosives C-4, Semtex-H, and Detasheet. Limits of detection were in subnanogram to subpicogram range. 

Detection tags with high vapor pressure, particularly 2,3-dimethyl-2,3-dinitrobutane (DMNB), are commonly added to commercial explosives to facilitate the vapor detection of explosives with relatively low vapor pressure such as plastic explosives. Evans group [7] investigated the mechanism of the DMNB reduction at mercury electrodes in a dimethylformamide (DMF) medium, whereas Wang s team [8] recently... 

Imaging results with humans have shown the effectiveness of this system to detect body-worn, concealed threats fabricated with plastic materials such as C-4 and RDX bulk explosives and plastic flare guns. Wideband images from this system of a man carrying two concealed handguns - one in left pant pocket and one on abdomen tucked into belt - as well as several innocuous items including a wallet and checkbook are shown in Fig. 5. The resolution in these images is on the order of 1 cm. 

Another approach to detect anomalies on the body is the use of millimeter-wave technology, which is non-ionizing low-power radiation, enabling its use with people for detecting explosives, drugs, plastics, ceramics, wood, paper, metals, and other anomalies concealed under clothing. 

The detection of explosive materials is a very challenging one and chapter 23 of reference [5] is devoted to this issue. Figure 7 shows the concentrations of some different explosives and their different molecular weights. Some explosives have vapour pressures in the ppm range, such as dinitrotoluene (DNT), but others have vapour pressures in the ppt level, such as plastic explosives like RDX. The problem is that the molecular weight of many explosives is high being over 150 amu or Daltons and these materials can barely be described as volatile ... 

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