Apparatus and Method for Rapid Detection of Explosives Residue From the Deflagration Signature Thereof

The present invention relates generally to an apparatus for detecting explosives and, more particularly, to an apparatus for the rapid detection and identification of explosive residue from its optical deflagration signature.

Explosives are a core component of nuclear, biological, chemical and conventional weapons, as well as of terrorist devices such as car, luggage, and letter bombs. Current methods for detecting the presence of explosives, include vapor detection, bulk detection, and tagging. However, these methods have significant difficulties dependent upon the nature of the signature that is detected. See, Fetterolf et al., Portable Instrumentation: New Weapons in the War Against Drugs and Terrorism," Proc. SPIE 2092 (1993) 40, Yinon and Zitrin, in Modern Methods and Applications in Analysis of Explosions, (Wiley, New York, 1993) Chap. 6; and references therein. Vapor detection is achieved using trained animals, gas chromatography, ion mobility mass spectrometry, and bioluminescence, as examples. All of these techniques suffer from the inherently low vapor pressures of most explosives. Bulk detection of explosives may be performed using x-ray imaging, which cannot detect the explosives themselves, but rather detects metallic device components. Another method for bulk detection involves using energetic x-rays to activate nitrogen atoms in the explosives, thereby generating positrons which are detected. This technique requires an x-ray generator and a minimum of several hundred grams of explosives. Bulk detection is also accomplished using thermal neutron activation, which requires a source of neutrons and a .gamma.-radiation detector. Thus, bulk detection is not sensitive to trace quantities of explosives and requires large, expensive instrumentation. Tagging requires that all explosives be tagged with, for example, an easily detected vapor. However, since tagging is not mandatory in the United States, this procedure is clearly not reliable. It turns out that none of the instruments described can perform accurate, real-time (<6 sec) detection and analysis of trace explosives in situ except for trained dogs.

It is known that surfaces in contact with explosives (for example, during storage, handling, or device fabrication) will readily become contaminated with explosive particulates, as a result of their inherent stickiness. This phenomenon is illustrated in studies that show large persistence of explosives on hands, even after several washings (J. D. Twibell et al., "Transfer of Nitroglycerine to Hands During Contact with Commercial Explosives," J. Forensic Science 27 (1982) 783; J. D. Twibell et al., "The persistence of Military Explosives on Hands," J. Forensic Science 29 (1984) 284). Furthermore, cross contamination, in which a secondary surface is contaminated by contact with a contaminated primary surface, can also readily occur. For example, a measurable amount of ammonium nitrate (AN) residue has been found on the lease documents for a rental trucks, and significant amounts of the explosives PETN (pentaerythritol tetranitrate) and/or AN have been found on clothing and inside vehicles of suspects in two well-publicized bombings. Therefore, explosive residue will likely persist in large amounts on the explosive packaging and environs, as well as on the individuals involved in building the explosive device, which provides an avenue for detection of the presence of explosives.

Accordingly, it is an object of the present invention to provide a detection and monitoring system capable of quantitatively detecting submicrogram and larger quantities of explosive materials.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. Key features of useful explosives detectors include high sensitivity and accuracy, substantial specificity, simplicity of operation, reliability, real-time measurement and analysis capability, low cost, and portability.

To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus for determining the presence of particulate from explosive materials hereof may include an enclosure; means, located within the enclosure, for heating the particulate to a temperature above the deflagration temperature thereof; and solar blind photodetector means, located within the enclosure, and disposed such that the optical signature (signal strength, time dependence and perhaps wavelength characteristics) emitted from the particulate as a result of particulate deflagration is detected.

In another aspect of the present invention, in accordance with its objects and purposes, the method for determining the presence of particulate from explosive materials hereof may include the steps of: heating the particulate to a temperature above its deflagration temperature, and detecting the optical signature emitted from the deflagration of the particulate.

Benefits and advantages of the invention include broad applicability for a wide range of explosives, high specificity and sensitivity for real-time explosive detection.

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate two embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 is a schematic representation of the apparatus of the present invention.

FIG. 2 is a schematic representation of the apparatus used to demonstrate the method of the invention.

FIGS. 3a and b illustrate the ultraviolet emission signature resulting form the deflagration of a particle of nitrocellulose gunpowder as a function of time, FIG. 3a showing the signal from an ultraviolet photodetector, while FIG. 3b shows a similar signal with an infrared and visible radiation filter in front of the photodetector.

Briefly, the present invention includes an apparatus and method for detecting and monitoring explosive residue by utilizing its characteristic deflagration (burn) signature. Although combustion kinetics can be quite complex, two important properties governing the combustion rate of materials are the mixing of fuel and oxidant, and the reaction kinetics. If a combustion process requires atmospheric oxygen to burn or the kinetics are complex, as is the case for most common materials, then the burn rate is typically slow. Explosives possess high oxygen content, resulting from a pre-mix of fuel and oxidant, and, in general, simple, rapid combustion kinetics. These fundamental differences between explosives and nonexplosives can be utilized to differentiate between the two types of materials. The deflagration energy release per explosive molecule ranges from approximately 11 eV for TNT to approximately 21 eV for PETN. Because of the rapid release of energy, a fraction of the energy released upon deflagration is in the form of ultraviolet, visible, and/or infrared light that can be measured and analyzed in order to detect the presence and/or type of explosive. Since the reaction kinetics of explosives are different than nonexplosives, a fingerprint of the explosive may be derived using characteristic optical emissions. Furthermore, since explosives burn more rapidly and therefore hotter than nonexplosives, the continuum (black body) optical radiation spectrum should be extended to shorter wavelengths.

Ignition temperatures for common explosives range from approximately 170.degree. to 300.degree. C. To initiate deflagration of residue particulates, then, particulates must be heated to at least their ignition temperature, which can be achieved, for example, by collecting the particulates and directing them onto a hot surface or by heating the air surrounding the particulates. It is expected that the optical emission from burning particulates will be related to their burn intensity and rate. For explosives, the characteristic emission should be a high-intensity light burst in a short time period. For a nonexplosive, the characteristic emission, if any, should be a low-intensity light emission over a longer time period. This is exactly what is observed in the experiments described in U.S. Pat. No. 5,638,166 for "Apparatus and Method for Rapid Detection of Explosives Residue from the Deflagration Signature thereof," which issued to Herbert O. Funsten and David J. McComas on Jun. 10, 1997. Optical emission can be measured using, for example, a photomultiplier tube, charge coupled device (CCD), or photodiode. In comparing signals from the optical emission of burned explosive and nonexplosive particulate, for the same magnitude pulse height, the pulse from an explosive is expected to have one or a combination of the following: a narrower width, different line emission, and/or extension of the continuum optical radiation spectrum to shorter wavelengths relative to nonexplosives. These signatures permit the measurement of explosive residues using pulse shape analysis of the optical emission of the burning residue. The present invention makes use of the strong ultraviolet emission from energetic materials to distinguish these materials from nonexplosive materials undergoing deflagration.

Reference will now be made in detail to the present preferred embodiment of the invention, examples of which are illustrated in the accompanying drawings. Similar or identical structure is identified using identical callouts. Turning now to FIG. 1, where the apparatus of the present invention is schematically illustrated, explosive particulates (and other particulates as well) can be collected by vacuuming or swiping, as examples. Shown are particles, 10, being collected by means of vacuum hose, 12. The particles are caused to flow past hot surface, 14, in enclosure, 16, by vacuum pump or impeller, 18, which exhausts into the atmosphere, 20. Some of the particles impinge on hot surface, 14. Controller, 22, regulates the temperature of surface, 14, to where particulate deflagration can occur. Ultraviolet photodetector, 24, controlled by detector electronics, 26, receives the optical output of the deflagrating particles, which is subsequently analyzed by optical pulse analyzer 28. In manual operation, a sample swipe of residue would be transferred onto hot surface, 14, within enclosure, 16, where deflagration is initiated in the particulates and the resulting ultraviolet output is detected and analyzed. It is conceivable that some explosive particles may be attached to nonexplosive dust particulates.

Various other methods for bringing the particulate to their deflagration temperature are also contemplated. Deflagration of the explosive particulate in the sample can also be initiated by using laser or other radiation, or by means of frictional agitation. The temperature at which deflagration occurs can be reduced by increasing the ambient pressure. Suitable photodetectors include, for example, ultraviolet photomultiplier tubes, photodiodes, and image intensifier tubes, or CCD detectors if infrared and visible light filters are employed. The ultraviolet pulse corresponding to the particulate deflagration may be analyzed using the ultraviolet signal pulse width, pulse magnitude, continuum spectrum (in part or in whole), or characteristic line emission(s).

Having generally described the invention, the following specific examples are given as further illustration thereof.

FIG. 2 hereof, illustrates the apparatus used in initial experiments for characterizing optical signatures from explosive particulates. Ultraviolet photomultiplier tube (PMT), 24, was mounted on one end of cylindrical enclosure, 16. The PMT was energized using high voltage power supply 26, and Sample particulates, 10, were placed in contact with hot plate, 30, such that the sample particulates were heated to a temperature of greater than 500.degree. C. (measured using a thermocouple probe, not shown). No attempt was made to exclude ambient light, 15, from entering the enclosure. Although the room lights were extinguished, sufficient light entered the room through closed window blinds to enable the experiments to be conducted. Within several seconds, explosive particulates were observed to burn, and a photograph was taken of the ultraviolet emission pulse as detected by PMT, 24. Preamplifier, 32, was found to be unnecessary since the ultraviolet photodetector signals could be viewed directly using oscilloscope, 34.

Individual pellets of nitrocellulose gunpowder were cleaved into particulates having an approximate size 4.times.10.sup.-6 cm.sup.3 (0.2.times.0.2.times.0.1 mm), which corresponds to about 7 .mu.g (assuming a density of 1.7 g/cm.sup.3). An ultraviolet signature was observed within two seconds of contacting the particulates with the hot plate. FIGS. 3a and 3b represent typical ultraviolet emission signatures nitrocellulose particles (approximately 7 .mu.g) that were ignited using a hot plate temperature of approximately 505.degree. C. The ultraviolet photomultiplier employed had a response between 100 and 350 nm using a CsTe photocathode, and a MgF.sub.2 window. FIG. 3a shows the unamplified ultraviolet signal from a single deflagration as a function of time displayed on an oscilloscope. The vertical scale is 100 mV per box, while the horizontal scale is 2 ms per box. To determine the effects of ambient light incident on the photomultiplier, a filter which blocks visible and infrared radiation was employed. This filter has a 20% transmission between 170 and 270 nm. FIG. 3b shows a similar deflagration using the ultraviolet filter placed in front of the photomultiplier tube. The vertical scale is now 20 mV per box and reflects the sharp decrease in signal, while the horizontal scale remains the same at 2 ms per box. By contrast, as is shown in Funsten et al., supra, nonexplosive materials have deflagration times of seconds. For nonexplosive samples, the oscilloscope trigger level using the unamplified photomultiplier tube was set at 15.6 mV. At this level, no triggers (oscilloscope traces) were observed, even when a flame from burning paper was produced. For comparison, the signal level of the explosive particulate under identical conditions was approximately 1.4 V. At similar trigger levels using the light-tight configuration of Funsten et al., with a photomultiplier tube sensitive to visible light, comparable signals were observed for the gunpowder, but nonexplosive particulate signatures were observed on the order of several tens of mV. In the experiments of Funsten et al., however, the quantity of nonexplosive employed was insufficient to generate a flame.

These results show that explosive particulates can be uniquely detected and differentiated from nonexplosive material due to their ultraviolet signatures. It is expected that different explosive materials can be identified by pulse shape analysis of the observed ultraviolet pulse produced by the emitted light upon deflagration. Signatures from explosive materials will be observed as spikes without background, there being no observed ultraviolet signal for nonexplosive particulates.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.