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The United States has seen an increased mandate for forensic tools to combat terrorist activities, in part due to an increased use of improvised explosive devices (IEDs), both home and abroad. In Afghanistan in September 2009, there were 806 IED incidents, of which 106 were classified as 'effective attacks' (Schogol, 2009). Within the borders of the United States, from 2004-2006, there were 10,957 incidents with 546 injuries and 68 deaths but only 2,174 investigations were referred for prosecution (ATF Fact Sheet, 2008). Yet the number of tools utilized for analyzing shrapnel remnants from IEDs, including pipe bombs, is minimal. Current forensic methods require a large percentage of shrapnel to be recovered and reconstruction of the bomb (or IED), either physically or on paper, is a primary method by which bomb construction is characterized (Oxley et al., 2001). In many instances, such as after an IED-triggered ambush, collection of the preponderance of fragments is not feasible while soldiers are under fire. If additional methods were available to characterize a bomb from only a few fragments, more bombs could be tracked to their source, thus increasing the number of cases brought before the courts, and helping prevent further casualties.
This proposal outlines a method by which Auger electron spectroscopy (AES), scanning X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and other nanoscale methods will be used to analyze small fragments of model exploded bombs in an effort to pinpoint microscopic indicators to classify various components of bombs, including construction and energetic filler, even when only a small amount of material is retrieved. Analysis of multiple examples will enable determination of a 'fingerprint' that pinpoints a particular manufacturing 'signature'. NIST is already at the forefront of explosives residue detection in cooperation with the Department of Homeland Security (DHS), the Transportation Security Administration (TSA), and various other law enforcement agencies. NIST scientists have used secondary ion mass spectrometry (SIMS) to identify and count explosive particles with application to trace explosive collectors (Gillen et al., 2006). Verkouteren (2007) utilized various techniques including scanning electron microscopy (SEM) to characterize high explosives such as RDX and PETN. Using multiple measurement sciences, this study will complement NIST's formidable forensic trace explosives research effort and address a critical national need by contributing to the creation of a synthesis of services to forensic and law enforcement communities ' standards of analysis from pre-explosion detection through to shrapnel and residual aftermath.
Oxley et al. (2001) undertook an intensive investigation of pipe bomb fragmentation, in which 56 different pipe bombs were exploded using eight different energetic fillers, three pipe types, two different ignition systems, and varying degrees of fill. It was demonstrated that fragmentation size distribution was reproducible among identically prepared bombs; based on fragmentation alone, a bomb could be classified as low-energy or high-energy. While this is a significant and comprehensive report on fragmentation, the method relies heavily on recovery of a large percentage of shrapnel; average recovery for this study was 87% ' not always feasible in the field. Furthermore, this study neglected to analyze two potentially key pieces of evidence: chemical residue left by exploded filler material, and microscopic analysis of the fragments themselves.
Improvised explosive devices contain a wide range of energetic filler, many of which are a home-made mix; forensic examination of these explosive residues is well-documented (Beveridge, 1992; Dahl, 1987; Garner et al., 1986). The residues left behind by energetic filler material may contain a mixture of combustion products, unexploded material, and traces of the containers they occupied throughout the process. Analysis of this chemical residue can provide a determination of the original filler material and traces of particulates unique to a given bomb-maker. Any containers used in processing may transfer traces of themselves to the mix that can be detected in the residue; these may be indicated by metallic crystallites impregnated near the surface or impurities present in the residue. The contamination that becomes included within the bomb as a result of the particular bomb-maker's processing makes possible a better forensic 'fingerprint' ' identification and quantitation of these nanoscale traces is thus vitally important. A study on pyrotechnic residues using SEM with an energy dispersive spectrometer (EDS) showed that it was indeed possible to link the residues directly back to starting composition (Phillips, 2001). However, this study failed to specifically analyze the metal of the pipe post-explosion, though consistent damage to the pipes was noted. Changes in near-surface structure or particulates embedded into this metal may have provided more complete information. In addition, SEM/EDS analysis is only able to provide an elemental analysis at a significant depth (?m) and cannot distinguish between bulk bomb casing material and contaminated surface coatings. Additional methodology, such as AES, XPS, and TEM, is necessary to provide a comprehensive characterization at higher depth resolution of surface residues and nanoscale changes in near-surface structure.
During a pipe-bomb explosion, the container is subjected to a high-degree of shock, which can result in significant structural alteration of the metal surface, including recrystallization (Feng et al., 1996), deformation twinning (Pappu and Murr, 2000), shock hardening (Moin and Murr, 1979), and phase transformation (Barbieri and Montanari, 1991). Two different studies used metallographic techniques to classify post-explosive microstructure changes in hardness, compositional/phase changes (austenite to martensite), and grain size and texture (Daifalla and Bahgat, 2007; Walsh et al., 2003). Both studies showed an increase in hardness and an increase in martenization post-explosion. Walsh et al. (2003), using micrographs at magnifications of 50X-400X, could detect no consistent change in grain size but did detect correlations between amount of deformation and detonation velocity and pressure. Daifalla and Bahgat (2007), using a digital imaging system, noted a reduction in grain count for detonations with high explosives. Neither study provided definitive compositional or nanostructural analysis of the surface.
Similar to shock waves generated by explosive forces, railroad cars exert tremendous forces upon rails, creating changes in the surface microstructure. Studies by Wild et al. (2003) and Lojkowski et al. (2001) investigated in detail many types of surface and sub-surface modifications in both structure and composition utilizing TEM, differential scanning calorimetry, SEM, X-ray diffraction (XRD), synchrotron X-ray diffraction, and small angle synchrotron X-ray scattering. In both studies, they were able to document many microstructural changes such as plastic deformation, dislocations, grain-size refinement, expansion of the lattice, and topography. These studies demonstrate that microscale changes precipitated by extreme force can be characterized by using micro- and nanoscale analytical techniques.
There is no one research study that has encompassed a full range of analytical techniques to a suite of bomb shrapnel or similar objects. Many techniques have been utilized, including SEM/EDS, TEM, and XRD, but there remains a need to provide an amalgamation of methodology whereby multiple established analyses are coupled with results acquired via complementary analysis methods such as AES, FIB-TEM and XPS. This proposal seeks to provide that synthesis.
The first step to begin the task of comprehensive analysis and classification of shrapnel is to create or gather test specimens (i.e. to explode model 'bombs' and collect their fragments) so that the surface nanostructure modification hypothesis can be tested. There are various locations throughout the United States, such as the National Law Enforcement and Corrections Technology Center's Explosive Detection and Neutralization Program in New Mexico, where various law enforcement personnel are trained in detection and disposal of explosives. At these locations, small controlled explosions are detonated on metal plates; it is expected that damage to the plates will vary with the type of explosive material used and also with the composition of the plate itself. Collection of several plates for each type of explosive material will create a reference 'library' of model 'bomb' fragments upon which to begin testing.
To begin, analyses will be applied to multiple plates composed of the same material, with the same type and amount of explosive used in detonation. Electron beam microscopic methods, including SEM/EDS, scanning AES, XPS, and TEM combined with focused ion beam (FIB) milling and cross-sectioning, all of which are readily accessible at NIST, will be used to identify nanoscale morphological and compositional features that are consistent and reproducible among identical explosions. Results will be compared to analysis of the surface of identical metal plates which were not subjected to an explosion. Changes in that structure and composition will then be catalogued to create a 'fingerprint' for that particular explosive material's interaction with a given type of metal plate.
Auger electron spectroscopy is ideal for compositional surface analysis such as this. AES is a surface science technique with an information depth of a few nanometers. Furthermore, its lateral resolution is limited by the probe spot diameter for modern instruments, on the order of ~10 nanometers. This combination of fine spatial and depth resolving capabilities allows for highly detailed elemental analysis mapping of the surface. AES is sensitive to most elements; carbon is expected to be a major contributor to any phase changes detected and AES is ideal to identify this. Also, the spectral features of many elements are readily distinguishable from carbon features, so identification of trace contaminates or near-surface phase segregation should not be hindered by interference from the carbon signal. Because AES is sensitive to just the first few nanometers of the surface, any compositional changes detected will represent recent structural alteration and not bulk material. Since analyses focus on metallic shrapnel fragments, electron beam-induced specimen charging is not expected to complicate the analysis.
X-ray photoelectron spectroscopy has a larger spot size than AES, ~10 ?m, but still remains sensitive to only the first few nanometers of the surface. Compared to AES, it is much more sensitive to chemical environment, such as oxidation state, and quantification of elements is more straightforward. Using XPS, the Auger parameter can be determined; this eliminates the charge bias that is created between spectrometer and sample. The line shapes of AES spectra can be re-evaluated in light of the XPS chemical shifts. Combined with AES, spatially resolved chemical state analysis is thus fully realized on the nanoscale.
To determine nanoscale structure accurately, another method is necessary. A focused ion beam (FIB) can be utilized to thinly slice a cross-section of the shrapnel specimen which can then be analyzed with TEM. Using transverses of the sample, transmission electron spectroscopy can identify crystal structure on the atomic scale as a function of depth, providing information on the force transferred to containment material by the explosion. TEM can also identify, with depth, any particulates that have been embedded in the sample. Interesting structural features can be compositionally evaluated using electron energy loss spectroscopy (EELS) and transferred back to the scanning Auger microscope for chemical analysis.
In contrast to Walsh et al. (2003) and Daifalla and Bahgat (2007), AES combined with XPS and TEM can resolve submicrometer structures, grain boundary effects, and micro-depth profiling. These previous studies were only able to look at micrographs or digital images to gain a coarse understanding of large-scale grain effects such as deformation or changes in grain size. This project will look at individual grains to determine structural changes on a much finer scale. These fine-scale observations will then be compared to both bulk structure and pre-explosion structure to pinpoint changes brought about by the explosion itself. It is expected that examination of surfaces both in contact with and not in contact with the explosive will reveal chemical and structural markers that can be used to clearly distinguish the 'inside' from the 'outside' of the bomb when few fragments are recovered in the field. In addition to looking at the metal structure of exploded fragments, examination of the thin layer of explosive residue, both unexploded particles and combustion products will be undertaken, similar to research performed by Phillips (2001). SEM/EDS techniques will provide a starting point, but AES and XPS can also provide data on residues left from the explosion, from a thickness of as little as a few nanometers. Utilizing a complete suite of electron beam and x-ray techniques, a total 'fingerprint' of composition, structure, morphology, nature, identity and depth of forcibly embedded species, and explosively modified containment material will be fully realized. In this manner, this study hopes to provide information on bomb filler material, energetics, and structural details so that a common origin can be determined from only a handful of fragments.
This study will concentrate on the first set of samples, or the control group of model 'bombs', using a suite of identical plates with identical explosive material and amount and determining whether nanoscale changes can be consistently tied to a common origin, thereby proving the principle that a few pieces of shrapnel contain sufficient information to characterize their origin. Completing and cataloguing an entire reference library will exceed the two year time frame for this study; indeed, such a library would be expanded continuously as the data from previously unidentified materials and construction features are added over time. There is every expectation that this can be a far-reaching study; making generic modifications to the model bomb, such as by altering plate material, modifying the explosive composition, adding contaminants, increasing the amount of explosive material, and varying the type of containment (e.g. capped vs. uncapped) could keep an investigator busy for an entire career. The goal of this study is to build the methodologies by which such libraries will be generated.
This project will comprise a synthesis of techniques to create a 'complete picture' of shrapnel fragments from explosive residue to changes in the microstructure. Compiling a catalog of shrapnel classified by explosive filler, metal containment, and other variables, whereby common features can be clearly identified as to manufacturing signature, will facilitate more effective tracking of perpetrators by law enforcement and military intelligence, improve the conviction rate for offenders, enable the criminal justice system to bring more investigations to prosecution, and reduce future attacks. Confirmation that a complex set of chemical, compositional, and microstructural information can be obtained from just a few shrapnel fragments will aid those situations in which collection of a large percentage of fragments is prohibitive, such as soldiers under fire.
This study is broadly applicable to other areas of research in addition to generating new forensics methodology. High-performance engine designers, whose products include a contained space where repeated energetic explosions occur, may benefit from the information gleaned from these studies. Spacecraft designers, concerned about high-velocity impacts from micrometeorites, may be interested in the parallels of how various metals react to explosions and how they expect spacecraft to endure in space. High energy materials research, wherein materials are subjected to extremes including temperature, pressure, stress, and strain, may benefit from information on nanoscale changes detected in this study. Aircraft materials engineers may find the methods developed here beneficial when considering designs that would remediate the effects of volcanic ash or other particulates on their machines in flight. In short, this study which addresses a crucial forensic need utilizing multiple measurement science techniques may have far-reaching implications.