Modern Advancements in Post-Detonation Nuclear Forensic Analysis

Deterring nuclear terrorism is a critical national asset to support the preclusion of non-state actors from initiating a nuclear attack on the United States. Successful attribution of a detonated nuclear weapon, which includes locating the source of the radiological materials used in the weapon, allows for timely responsive measures that prove essential in the period following a nuclear event. In conjunction with intelligence and law enforcement evidence, the technical nuclear forensics (TNF) post-detonation community supports this mission through the development and advancement of expertise to characterize weapon debris through a rapid, accurate, and detailed approach. Though the TNF field is young, numerous strides have been made in recent years toward a more robust characterization capability. This work presents modern advancements in post-detonation expertise over the last ten years and demonstrates the need for continued extensive research in this field.


I. Introduction
In recent years, the United States has called upon the scientific community to address gaps in technology to improve the performance of forensics as a deterrent to nuclear terrorism [1]. The Nuclear Forensics and Attribution Act (NFAA) [2], enacted in 2010, is the legislative embodiment of this directive that stresses the technological readiness such a scenario necessitates and has been approached through an interagency and academic collaboration [3,4]. Technical Nuclear Forensics (TNF) has been established as the specialized field of science to enhance this technology and analyze nuclear residues of interdicted (predetonation) and exploded (post-detonation) nuclear materials. Attribution of these materials employs TNF findings in concert with intelligence and law enforcement evidence to locate the source of these materials. As the following pages highlight, the post-detonation arm of TNF has made recent technological strides in identifying weapon characteristics from nuclear debris to supply timely, high-quality data in support of the attribution process.

A. Legal Benchmarks of TNF Data
As in any field of forensics, data supporting the nuclear forensic analysis process may eventually reach judicial review. Though nuclear forensic evidence may not necessarily encounter the judicial process before the President and his/her national security council make an attribution decision in the event of a nuclear attack, any country wishing to attribute a nuclear incident to another sovereign nation or subnational entity will face intense scrutiny, and as such, must have a high standard of legally defensible forensic methodology. The NFAA does not contain language specifically referring to a defined standard; however, it recommends international cooperation and designates investigative agencies that are bound by legal standards.
The standard most relevant to nuclear forensic methods is the Daubert standard, as it applies to the Federal Rules of Evidence, Article 7, Rule 702 [5][6][7]. Based on the Daubert standard, judges are given means by which they can assess an expert's scientific testimony on the grounds of reasoning or methodology. Under this standard, the five factors used to assess the validity of a method are (1) whether the theory or technique in question can be and has been tested, (2) whether it has been subjected to peer review and publication, (3) its known or potential error rate, (4) the existence and maintenance of standards controlling its operation, and (5) whether it has attracted widespread acceptance within a relevant scientific community [6].
For the United States, any research effort seeking broad acceptance and government support must meet this standard.
Application of this forensics standard has rightly received rigorous attention in the scientific community [5,[8][9][10][11]. In addition, the National Institute of Standards and Technology (NIST) and other researchers are establishing certified reference materials (CRMs) and recognized databases of nuclear information that may act as a known standard for other nuclear materials [12]. Both of these standards generally agree with the requirements for competence outlined in International Organization for Standardization (ISO) code 17025.

B. Essential Steps: Nuclear Forensic Analysis
Post-detonation nuclear forensic analysis begins with the collection of materials produced in the extreme temperature and pressure where the weapon detonates. In the aftermath of a detonation, a specialized type of debris is formed that effectively encapsulates weapon components and fission products in a solidified, glassy matrix [13]. This debris, or nuclear melt glass, is essential for nuclear forensic scientists to conclude weapon characteristics during post-detonation nuclear forensic analysis [14]. Analyzing the debris begins with non-destructive physical and radiological characterization and progresses toward dissolution and destructive analysis.  (Table 2), are performed in a similar manner, with radiochemical separations and radiological characterization having the largest contribution to subsequent attribution.  [15] C.

Collection of Species
Collecting ground samples of nuclear fallout debris is the essential first step toward forensic attribution. Samples must be taken from a site sufficiently close to the detonation source or fallout plume to ensure the samples were created in the fireball and encapsulate the necessary fission products, activation products, and anthropogenic materials needed during forensic analysis. Debris collection falls outside the scope of this work and will not be discussed here.

II. Synthetic Nuclear Debris
Rapid sample analysis is essential for forensic attribution in a post-detonation scenario. A recent multiagency effort between the Federal Emergency Management Agency (FEMA), the Department of Homeland Security (DHS), and the Defense Threat Reduction Agency (DTRA) addressed the repercussions of an urban nuclear event and the uncertainty associated with samples of urban nuclear debris for forensic analysis [4]. While samples of nuclear melt glass (both surface and aerodynamic debris) are available to the academic community from the Trinity test, many fission products have decayed and the Trinitite samples are only quasi-representative of the signatures that would be obtained from a newly acquired sample. Therefore, much work is being dedicated to creating realistic synthetic samples of nuclear melt glass for the experimental development of post-detonation analytical techniques. These surrogates began as simple highly enriched uranium (HEU)-doped sol-gel glass, as reported by Carney et al. in 2013 [16]. The glass was impregnated with 93% HEU and neutron irradiated for 15 minutes in order to simulate, on a first-tier basis, the fission and activation products that would be found in nuclear debris.
Many papers followed that advanced the elemental accuracy of synthetic nuclear debris. The need for synthetic nuclear melt glass representative of an urban environment was the next step toward developing analytical techniques for attribution purposes. Giminaro et al. recently addressed this need in a study detailing city-specific formulation techniques to identify the elemental composition of any given city using land use data [14]. Two representative samples (Houson, TX and New York, NY) were modeled and synthesized in order to demonstrate the procedure. The need for synthetic nuclear debris, which can be directly compared to actual debris and those that represent a hypothetical urban event, was addressed for the first time in recent years; efforts to improve the realism of the samples are ongoing. These samples provide a more credible baseline for developing forensic techniques for real postdetonation debris [19].

III. Analysis
Analyzing nuclear debris to characterize its physical, chemical, and radiological signatures is a vital component of the technical nuclear forensics process [20]. The procedure aims to reverse-engineer the design of a detonated weapon using the debris it generates. The community of analytical nuclear forensics has achieved significant strides in recent years toward improving the timeliness and accuracy of these techniques.
Upon the detonation of a nuclear weapon, the resulting debris consists mainly of oxidized materials that contain a variety of radiological and elemental forensics signatures [21,22]. [22,23] [22,23]Elemental and radiological signatures can be detected using a variety of methods. Elemental signatures consist of the elements captured by the heat and pressure of detonation and are incorporated into the final composition of the melt glass. Radioactive signatures consist of unstable elements that have a tendency to decay. It should be noted that many of the techniques used in traditional forensics can be used in nuclear forensics [23]. Table 2 contains a variety of techniques that can be used for postdetonation nuclear forensics [23]. It is important to note that Table 2 does not have the associated time component that Table 1 contains because attribution should proceed as quickly as possible in a postdetonation scenario.

A. Elemental Analysis
The elements found in nuclear melt glass are largely found in their oxide (and occasionally chloride) form due to the excessive oxygen and extreme temperatures found in the toroidal region of the blast [24]. When performing elemental analysis on a debris sample, it is important that the analytical techniques are performed so that the spatial integrity of the sample is preserved prior to interrogation. Physical characterization-requiring largely non-destructive techniques-includes morphology of the sample, texture, stratification, and other statically observable characteristics of the debris. The presence of elements in the sample matrix may be indicative of several weapon characteristics and is an important aspect of physical characterization. Some of the more important constituents are plutonium and uranium; these elements are important because the debris contains trace levels of fissile material resulting from an incomplete detonation-no reaction is one hundred percent efficient-and provide useful indications of the initial state of the fuel. Previous reported literature took advantage of alpha spectroscopy on thin vertical slices of trinitite to identify deposits of U and Pu; however, a recent study by Donohue et al. integrated several additional techniques, including laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS), electron microprobe analysis (EMP), energy dispersive X-ray fluorescence system (XRF), scanning electron microscopy (SEM), and back scattered electron analysis (BSE), in addition to alpha spectroscopy, to obtain a clearer picture of the distribution of elements of interest. Pu deposits were found up to 10 mm deeper in the sample than previously reported [36]. It is clear that more work is still necessary to validate and advance the physical characterization of post-detonation nuclear debris.

B. Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry
When analyzing nuclear melt glass, it is important to gain insight into the amount of homogeneity or heterogeneity of the elements of interest prior to performing techniques requiring destructive analysis. One rapid technique for determining the spatial resolution of matrix elements is scanning electron microscopy/energy dispersive x-ray spectrometry SEM/EDS; however, the limiting factor for SEM/EDS is debated in literature [37,38] and it certainly provides poorer sensitivity than other TNF methods. It is generally agreed that SEM/EDS has the advantage of providing the "whole picture" of elemental dispersion, but has only provided elements with an atomic number greater than 5, targeting boron as a problematic constituent due to its low photon energy, and thus, low x-ray yield [39].

C. Laser Ablation Inductively-coupled Plasma Mass Spectrometry
Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) is an appealing postdetonation analysis technique because of its ability to introduce samples into the instrument without prior sample dissolution. This is a powerful technique that is particularly beneficial for samples that may fully dissolve during a dissolution process. Essentially, the laser evaporates or sublimates a portion of the material for detection, precluding any need for dissolution. LA-ICP-MS is used to determine elements at the ultra-trace level (at concentrations of less than 0.0005 wt. . This is useful for detecting both impurities in a sample and ultra-trace elements of interest to the forensic scientist. Additionally, due to the potent capabilities of ICP-MS, it is possible to distinguish relevant isotopes as discussed in the following section.

D. Isotopic Analysis
It is important to note that isotopic signatures of stable isotopes are a useful source of information when performing the analysis of post-detonation debris. A key interest is determining stable oxygen (  Pb, and 204 Pb with LA-MC-ICP-MS to determine the geographic source of the Pb in the weapon from the Trinity test (the debris is the only openly available weapon debris in existence). The distinct overlapping isotopic ratio in both the trinitite sample and the Buchans mine demonstrated that the lead originated in the Buchans mine in Newfoundland, Canada [27]. These heavy metal isotopes shed light on both the ores used to create components of the device or infrastructure surrounding the device and the heavier elements (uranium, thorium) from which it decayed. 204 Pb is alone in its natural primordial origins; the other three isotopes stem directly from the long-lived decay products of 232 Th, 235 U, and 238 U [27]. Such interrogations are extremely useful during the sourcing process following technical analysis, though an urban detonation would certainly produce a more complex elemental matrix than the relatively simple elemental matrix of a sandy desert. Isotopes can be detected using a variety of instrumentation including ICP-MS, where a sample is broken into its elemental constituents and a mass-to-charge ratio is measured.

E. Oxidation States
Research by Nelson et al. [41] has noted the relevance of x-ray absorption near edge structure (XANES ) when determining the oxidation state ratios of 5+ U/ 6+ U and Fe 2+ /Fe 3+ . Some variation in the oxidation of the iron was found, with the Fe ratios varying from 33-55 percent Fe 2+ , implying reducing conditions and concluding that the dominant species of plutonium was Pu 4+ .

IV. Radiological Signatures
The presence of radiation is a unique characteristic of nuclear forensic samples when compared to forensic samples in other scientific disciplines. Radiation signatures from post-detonation debris can greatly advance the forensic interrogation of the sample.

A. Gamma-ray Spectroscopy
The most common (and typically, first to be employed) form of radioactivity analysis is gamma-ray spectroscopy Am, and 60 Co have been reported, whereas synthetic versions of these materials also showed 24 Na, 140 La, 42 K, 59 Fe, 47 Ca, 132 I, 46 Sc, 95 Zr, 130 I, 133 I, 103 Ru, 131 I, and 132 Te. There have been four studies on the radioactive nuclides in trinitite as measured by gamma-ray spectroscopy-the resulting isotopes and their specific activities (Bq/g) are summarized in Table 3.

B. Alpha Spectroscopy
Direct alpha spectroscopy and preparation of intact debris samples is a useful technique in nuclear forensics; however, its use in studying nuclear melt glass is limited to one study performed by Eaton et al. [43]. Sample preparations consisted of a very thin slice of nuclear melt glass placed in an alpha spectrometer, where a distinct peak at 5.157 MeV was observed, correlating to both 239 Pu and 240 Pu.

Table 3. Reported Radioactive Nuclides in Trinitite Activities in (Bq/g) for Specific Isotopes of Interest
Additional peaks exhibited energies of 5.486 and 5.499 MeV (corresponding to 241 Am and 238 Pu, respectively). The activity ratios were subsequently calculated.

V. Separation Techniques
To perform mass spectrometry analysis of nuclear samples, it is important to reduce isobaric or radiation interferences using analytical separations. A particular concern before performing separations is the dissolution of unique matrices that can encompass a large array of elemental constituents. Post-detonation materials are likely to include a suite of elements that are not found in traditional nuclear melt glasses such as silicon-rich trinitite. Advanced dissolution techniques are imperative for proper dissolution of complex matrices, and several recent publications have introduced innovative approaches to this challenge. Subsequent separations can be performed on these dissolved matrices following proper dissolution of the initial sample. Recent dissolution and separation techniques addressing post-detonation debris are highlighted in the following sections.

A. Dissolution and Laser Ablation
Destructive analysis of nuclear materials provides a useful platform for a variety of analytical methods. These methods provide data on both major and minor elemental parameters, producing information that can help identify the materials' intended uses (radiological dispersion, nuclear weapon, etc.), component/material age and source (do the isotopics point toward a specific mining operation?), reactor information (are there fingerprints indicative of specific reactors?) [44], and production processing [45,46]. From 2005 to 2015, a variety of methodologies for the preparation of destructive samples were investigated; however, the focus has largely highlighted capabilities of laser ablation (LA) and methods of liquid dissolution.
Laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS) has seen a recent increase in application since the introduction of laser-induced breakdown spectroscopy in the 1980s [46-54]. Continuous-wave CO2 lasers remain the preferred method due to their power, wide availability, and wide application. Regarding nuclear melt glass, LA proves useful for specific actinides (e.g. plutonium isotopes), but struggles with many fission fragment nuclides [46,47]. In particular, Ga-Rb and Mo-Cs suffer from volatilization loss with LA [46]. Even with these shortcomings, the appeal of the introduction of a direct sample into a mass spectrometer will continue to drive research; however, the complexity of laser methods appears to keep effective solutions out of reach. . He reports a NaOH fusion process wherein 1g samples are placed in a graphite crucible with 15g of solid NaOH, heated for 15min, and allowed to cool for 10min. The samples are then transferred to a hot plate and H2O was added as needed. After partial dissolution, samples are cooled to ~0 C and the suspended solid residues are separated from the liquid by centrifugation at 3,600rpm for 6min. The solid residue is then dissolved with 60-80mL of 1.5M HCl and diluted to 170mL with 0.01M HCl. After dilution, the samples are treated with 25mL of 28M HF, mixed, and allowed to stand for 10min prior to centrifugation. The supernatant liquid is kept, and the remaining solids are dissolved in 5mL of 3M HNO3 -0.25M H3BO3, 6mL of 7M HNO3, and 7mL of 2M Al(NO3)3. This method shows success with select actinide separation and recommends methods for fission product separations. The separations for this dissolution method are discussed in the subsequent section. To date, the dissolution of trinitite has seen success; however as new surrogates are developed to replicate trinitite and other weapon scenarios, challenges will likely arise and persist with forensically relevant dissolution methods. Unfortunately, present nuclear threats are not likely to produce debris resembling the Alamogordo desert, which is the site of the Trinity test [57].  [62,63]. In this work, the organic compounds 1,1,1,5,5,5-hexafluoroacetylacetone, 2,2,6,6-tetramethyl-3,5-heptanedione, and 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione combined to produce 7 and 9 coordinate compounds that volatilize at temperatures ranging from 140 to 220 C. These complexes show promise in a rapid separation technique using a coupled gas chromatography -ICP-TOF-MS for elemental and isotopic identification and quantification, as well as a separation step for other measurement methods.

VI. Conclusion
Modern research in nuclear forensic technology continues to address the timeliness and accuracy of postdetonation forensic analysis techniques toward an effective in extremis national security capability. As a vital component of the attribution process, it is critical for the TNF community to remain abreast of revolutionary technology to provide increasingly accurate and timely data into the attribution cycle. Evolving analytical approaches toward the rapid analysis of post-detonation materials is essential, and several works presented here have generated innovative solutions to the challenges posed by nuclear forensic science. It is imperative for progress to continue down its current path in support of a robust and rapid analytical TNF capability.

VII. Works Cited
His current research focuses on rapid radiochemical separations of fission and activation products from nuclear detonation debris. Gas-phase chemistry is exploited to develop and improve separations, with a particular emphasis on faster and higher specificity separations. A key research objective is to better constrain the thermodynamic data regarding the interaction of the gas-phase species with the separation column in order to model and optimize a large-scale matrix separation. John D. Auxier II is currently a research assistant professor at the University of Tennessee (UTK) in the nuclear engineering department and the Institute for Nuclear Security. He also holds joint positions with Y-12 National Security Complex, Oak Ridge National Laboratory, and Los Alamos National Laboratory (LANL). Prior to this effort, he worked as a Post-Doc under Dr. Howard Hall in standing up the National Nuclear Security Administration (NNSA) Radiochemistry Center of Excellence (RCoE) at UTK, which focuses on the development of novel nuclear forensic methods for post-detonation scenarios. Dr. Auxier received his Ph.D. from UTK with research efforts that focused on the development of materials for thermal neutron detection, and research efforts at LANL that focused on development of alpha spectroscopy development software. His current interests involve research in nuclear forensics, radiochemical separations, and radiation detection applications. As part of this research he has published numerous scientific articles on nuclear forensics and development of radiation detection material, along with a patent on rapid separations. He has been awarded a number of grants from sponsors including DHS, DTRA, and DOE NNSA in areas related to nuclear forensics and analysis.