Isotope ratio method: state-of-the-art of forensic applications to CBRNE materials

Abstract

The threat of Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) events is a serious challenge worldwide. This threat is aggravated by the prevalence of potential CBRNE materials normally used for industrial and scientific purposes. One potential deterrent to the use of CBRNE materials for nefarious purposes is the ability by law enforcement to attribute interdicted threat materials in terms of their provenance and linking them to people, places, and events. Isotope Ratio Method (IRM) is a technique that utilizes ratios of different isotopes of particular elements present in an investigated material to determine an isotopic signature of that material. A survey of the literature has been conducted in order to consolidate the state of current knowledge on the forensic application of IRM specifically to CBRNE materials. This review is intended for both researchers and policy makers to help identify gaps in knowledge and to determine the strategic direction of research and development to advance the application of IRM in the general arena of public safety and security.

RÉSUMÉ

La menace d’événements chimiques, biologiques, radiologiques, nucléaires et explosifs (CBRNE) constitue un grave problème à l’échelle mondiale. Cette menace est aggravée par la prévalence de matériaux CBRNE potentiels normalement utilisés à des fins industrielles et scientifiques. L’un des moyens de dissuader l’utilisation de matériaux CBRNE à des fins malveillantes est la capacité des autorités policières d’attribuer les matériaux menaçants interceptés en termes de provenance et de les relier à des personnes, des lieux et des événements. La méthode du rapport isotopique (MRI) est une technique qui utilise les rapports de différents isotopes d’éléments particuliers présents dans un matériau étudié pour déterminer la signature isotopique de ce matériau. Une étude de la littérature a été réalisée afin de consolider l’état des connaissances actuelles sur l’application médico-légale de la MRI spécifiquement aux matériaux CBRNE. Cette étude est destinée aux chercheurs et aux décideurs politiques afin de les aider à identifier les lacunes dans les connaissances et à déterminer l’orientation stratégique de la recherche et du développement pour faire progresser l’application de la MRI dans le domaine général de la sûreté et de la sécurité publiques.

Keywords:

• Chemical
• Biological
• explosives
• isotope ratio method
• nuclear
• radiological

MOTS CLÉS::

• Chimique
• biologique
• radiologique
• Nucléaire
• Explosifs
• Méthode du rapport isotopique

Introduction

The threat of Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) events is a serious challenge worldwide. The intent of terrorist organizations and radicalized individuals to launch terrorist attacks continues, and unfortunately, Canada is not excluded as a target. This threat is aggravated by the prevalence of potential CBRNE materials normally used for industrial and scientific purposes. In order to enhance and sustain Canada’s resilience to CBRNE events, all levels of government have collaborated to develop the CBRNE Resilience Strategy for Canada [1]. Its purpose is to provide the policy framework to guide the creation of sustainable capabilities and common standards in CBRNE policies, programs, equipment, and training.

One potential deterrent to the use of CBRNE materials for nefarious purposes is the ability by law enforcement to attribute interdicted threat materials in terms of their provenance and linking them to people, places, and events. Forensic science provides the means to enable this attribution, and advancements in forensic analytical techniques and methodologies in recent years have helped enhance this capability.

The Isotope Ratio Method (IRM) is a technique that utilizes ratios of different isotopes of particular elements present in an investigated material to determine an isotopic signature (isotopic fingerprint) of that material. IRM technology has been in use for many years for a wide variety of applications, including age dating, environmental studies, nutrition studies, archaeology, geochemistry, and paleoclimatology. A recent review of IRM techniques applied to these forensic science applications was completed by Matos and Jackson [2], including applications to explosives and poisons. Application of IRM to classical forensics has also been reported, including the use of carbon and oxygen isotopic data derived from packaging tapes, isotope ratio analysis of heroin and other illicit drugs, use of carbon isotopic analysis to identify petroleum-based accelerants, and others [3–6]. Additional resources for IRM techniques applied to forensic science in the applications listed can be found using the Forensic Isotope Ratio Mass Spectrometry (FIRMS) network [7].

A survey of the literature has been conducted in order to consolidate the state of current knowledge on the forensic application of IRM specifically to CBRNE materials. The literature survey was completed using popular internet search engines and using keywords of “isotope ratio method,” “chemical,” “biological,” “radiological,” “nuclear,” and “explosive,” and their derivatives. Where possible, open-access publications were used to maximize availability of the references to the readers of this review. In some cases, a subscription was required to access references cited in this review. This review was not intended to be entirely comprehensive, but to accurately reflect the state-of-the-art of IRM.

This review is intended for both researchers and policy makers to help identify gaps in knowledge and to determine the strategic direction of research and development to advance the application of IRM in the general arena of public safety and security. To our knowledge, this is the first review combining IRM in all aspects of CBRNE in a single source. This paper documents the results of this literature survey.

Isotope ratio method

An isotope ratio is a ratio of abundances of two isotopes from either the same element or two different elements. The measurement of isotope ratios is described in “Isotope ratio measurements” section and a framework for the application of IRM to forensics is given in “Framework for forensic application of IRM” section.

The standard method for expressing deviation of isotopic fractions from a naturally occurring standard is the delta (δ) [8] (1) $$\mathit{Delta}\mathrm{ }\left(\delta \right)=1000\left(\frac{\mathit{ratio}\mathrm{ }of\mathrm{ }\mathit{sample}}{\mathit{ratio}\mathrm{ }of\mathrm{ }\mathit{standard}}-1\right)$$(1) where “ratio of sample” is the abundance ratio of two isotopes measured in a sample and “ratio of standard” is the abundance ratio of the equivalent isotopes measured in a standard. International reference materials, or laboratory reference materials that are calibrated against international standards, are normally used for the measurement of ratio of standard. The isotope ratio method utilizes both the ratio of sample as well as the delta, depending on the application.

Throughout this paper, specific ratios are listed where only the ratio in the material is used (i.e., does not use δ). Also, δ^xA refers to the δ of the ratio of isotope ^xA (where x is the atomic mass and A is the element identity) to the most abundant isotope of element A (e.g., δ²H refers to δ²H/¹H).

Isotope ratio measurements

An isotope ratio can be measured by a variety of methods and often depends on the specific nuclides of interest and their relative ratio. Although measurements and methodology are not the focus of this review, a brief description of measurements is provided in this section.

For stable isotopes, the isotopic abundancies of many elements were fixed when the earth was form and have largely not changed since [9]. However, stable isotopes are fractionated in the environment by physical, chemical, and biological processes such as [8]

• Mass dependent processes (e.g., diffusion, ultrafiltration)

• Isotopic exchange reactions involving redistribution

• Unidirectional mass dependent reactions (e.g., freezing)

• Biological processes (e.g., sulphate reduction by bacteria, degradation kinetics of fossil fuels)

Mass spectrometry (MS) is the most prominent measurement technique for IRM of stable isotopes. In MS, samples are converted to gaseous form, ionized with an electron beam, and then mass-to-charge ratios are measured. If a bulk analysis is desired (i.e., independent of pure element, single chemical or complex mixtures), dual-inlet, elemental analyzer, and flow injection analysis MS are common methods. If the isotope ratio in some or only individual compounds in a sample are required, an additional separation step is required. Gas Chromatography (GC) and liquid chromatography MS are common methods of compound-specific isotope analysis [9]. In the case of non-stable isotopes, MS may be used, but gamma and alpha spectrometry are also common [10].

In any method, certified reference materials (CRMs) are needed for the calibration of instruments and traceability. They are also needed to express measurements in terms of the delta (Eq. (1)(1) $$\mathit{Delta}\mathrm{ }\left(\delta \right)=1000\left(\frac{\mathit{ratio}\mathrm{ }of\mathrm{ }\mathit{sample}}{\mathit{ratio}\mathrm{ }of\mathrm{ }\mathit{standard}}-1\right)$$(1) ). Often the CRM needs to be the same or similar to the sample being analyzed, both in terms of chemical form and the magnitude of the ratio being measured. Therefore a large and wide ranging inventory of CRMs permits a greater range of samples to be analyzed.

Framework for forensic application of IRM

The goal of forensic investigation is to identify and differentiate a given sample of interest and draw inference on its identifying characteristics such as its origin, processing information (e.g., treatments, production process), and/or similarity to related materials. IRM complements other forensic technologies as it may differentiate samples with otherwise identical chemical composition.

A framework for forensic application of IRM of stable isotopes has been developed by Ehleringer et al. [11] and is shown in Figure 1. Using isotope ratios of physical evidence, a comparative approach may be used to determine if a given sample is similar to other samples from the same forensic incident. If a database for the material is available, it can be determined if a given sample is similar to previous known samples within a database. The ability of this approach in determining the origin of a given sample is related to the size and detail in the database on sample origins.

Figure 1. Framework for the application of isotopic signatures in forensic investigations, adapted from Ehleringer et al. [11].

A predictive model approach builds upon the comparative approach. Additional information in the environment where the sample was collected is needed as well as a first-principle or process-based models of stable isotopes fractionation. The ability of this approach in determining the origin or processing information of a given sample is related to the size and detail in the database on sample origins and the fractionation effects in the material and the accuracy of the model.

Geographic Information System (GIS) models go one step further to help predict the geographic area from which the sample may have originated, based on climate drivers. GIS models require database layers for the climate drivers and can produce spatial maps that predict originating regions. Such geographic projections of isotopic compositions could provide a sophisticated tool for forensic analysis. The models need to be validated against measured data and need to be developed for each material type desired.

To apply IRM to forensics, precise measurements with low uncertainty are needed. Overlapping uncertainties inhibits the ability to differentiate samples. Additionally, precise models with low, quantifiable uncertainties are also needed in order to use predictive model or GIS model approaches. Ultimately, the usefulness of IRM to forensics is dependent on the precision of the analytical measurement, the heterogeneity within a sample, heterogeneity within the greater population, and the size and quality of databases used [12].

It is important to note that, to our knowledge, IRM has not yet been used in a court of law.

Forensic applications of IRM to CBRNE threats

Chemical

Chemical weapons are toxic chemicals and their precursors (except for non-prohibitive purposes), munitions and devices specifically designed to cause death or harm through their toxic properties, and any equipment specifically designed for use in connection with said munitions and devices [13]. The main types of chemical weapons are blister agents (e.g., mustard gas and lewisite), nerve agents (e.g., sarin, soman, and VX), choking agents (e.g., chlorine, phosgene), and blood agents (e.g., hydrogen cyanide).

Forensic application of IRM to chemical threats is traditionally focused on analysis of isotopes of light elements such as hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and sulphur (S) in a sample [14]. Table 1 lists a summary of forensic applications of IRM to various chemical threats compiled from literature.

Table 1. Summary of forensic applications of IRM to various chemical threats.

Application	Ratios	References
KCN and NaCN (cyanide salts) geographic origin	δ^13C δ^15N	[14, 15]
Methylphosphonic dichloride (sarin precursor) synthetic environment	δ^13C	[16]

While IRM is a promising forensic tool, the forensic application of IRM directly to chemical threat agents has been demonstrated only by a limited number of studies. Specific examples described in this section suggest differentiation by origin country and synthetic environment (including possible time window of synthesis) is possible. While work directly on chemical threat agents appears limited, knowledge of isotopic compositions of reagents and the fractionation effects of synthetic processes may lead to predictions of final isotopic compositions.

Mirjankar et al. [14] analyzed 27 distinct potassium cyanide (KCN) and sodium cyanide (NaCN) samples from five different countries. They could differentiate the country of origin into one of two origin country clusters using δ¹³C combined with a suite of other chemical identifiers. Tea et al. [15] had similar results for KCN and NaCN with a smaller sample size analyzed for δ¹³C and δ¹⁵N. Moran et al. [16] analyzed δ¹³C of 13 distinct methylphosphonic dichloride samples (a sarin precursor) and could differentiate samples into one of three synthetic environment clusters. If samples can be grouped into synthetic environment clusters, it could indicate which samples were made together. This could indicate they are made at the same time, in the same batch, with the same feedstock, or at the same facility. Further information beyond association with each other requires a database of other samples, a process-based model, or a GIS model.

Biological

Biological weapons are living organisms or materials derived from living organisms to cause disease or harm in humans, animals, or plants. They include bacteria, viruses, parasites, fungi, prions, and toxins [13]. Examples of biological agents include Ricin, botulism, Ebola, smallpox, and anthrax. To some extent, chemical and biological weapons can overlap, such as the use of toxins produced by living organisms.

Similar to chemical threats, forensic application of IRM to biological threats is also traditionally focused on analysis of isotopes of light elements [14]. Table 2 lists a summary of forensic applications of IRM to various biological threats compiled from literature.

Table 2. Summary of forensic applications of IRM to various biological threats.

Application	Ratios	References
Water source geographic origin, growth medium source, and identify co-cultured samples of spores	δ^2H δ^18O δ^13C δ^15N	[17–22]
Source seeds of Ricin	δ^15N	[23]

Research to date on application of IRM directly to biological threat agents has focused on the effects of growth medium, source water, and preparation techniques on the isotopic composition of a biological agent or biological species similar to threat agents. The examples described in this section suggest all of these factors may be influential, and differentiation of biological threat agents by IRM may be possible.

Kreuzer-Martin et al. [17] compared the stable isotopes of H, C, and N in water and growth media to the stable isotope composition of the resulting bacterial culture, and concluded that the growth environment directly influences the isotopic signature of the spore. The overall contribution of δ²H in the spores resulting from growth media versus the water source was modeled [18] alongside the C signature, demonstrating that the δ¹³C can be used to constrain the possible range of δ²H from the original source water. This range can then be used to identify possible geolocations for the water source. δ¹⁸O was found to vary with the type of growth environment (solid growth media versus liquid) due to its sensitivity to evaporation, and therefore has limited usefulness without knowledge of the type of growth medium [19]. Overall, these relationships between source water, growth media and the resulting spores can be used as a tool in forensic investigations to exclude incompatible growth environments, narrow the possible geolocations of the source materials or alternatively to confirm whether seized material could have been produced in a suspected location or using suspected water sources.

Kreuzer-Martin and Jarman [20] measured δ¹³C, δ¹⁵N, δ¹⁸O and δ²H of 247 separate cultures of genetically identical Bacillus subtilis spores in 32 culture mediums and with water of varying isotopic composition. Results showed that isotope ratios have the potential to associate co-cultured spores (i.e., same culture, medium, and water). Again, δ¹³C and δ¹⁵N were unable to identify the specific culture medium, but could reveal general sources of the C and N (e.g., animal vs. yeast-based). Other research by Kreuzer-Martin et al. [22] successfully identified the geographic origin of water sources from five different regions in the USA used to culture identical cultures of Bacillus subtilis via δ¹⁸O and δ²H. Horita and Vass [21] analyzed Bacillus globigii and Erwinia agglomerans for δ²H, δ¹³C, and δ¹⁵N and found samples could be differentiated by water and culture medium. The cultures used in these studies are benign surrogates, but the results may be applicable to more dangerous spores such as anthrax (see “Anthrax” section for a case study on anthrax).

Kreuzer et al. [23] analyzed δ¹³C, δ¹⁵N, δ¹⁸O, and δ²H of Ricinius communis and of Ricin prepared from the seeds. They found δ¹⁵N measurements were virtually identical between Ricin and the source seeds independent of its preparation technique, providing a source for correlation.

Radiological and nuclear

Nuclear weapons use a nuclear fission or fusion reaction to create their explosive power [13]. They generally involve the use of enriched uranium or plutonium. Radiological weapons use conventional explosives or other means to spread radioactive material to cause short- or long-term health issues [13]. As the materials used to produce both of these threats often overlap, they are combined in this section.

Radiological and nuclear threat materials differ from other CBRNE materials as their primary hazards generally relate to their non-stable isotopes. Accordingly, the processes used to generate or isolate the non-stable isotopes in these types of threat materials typically have a large influence on their isotopic composition. Relative to chemical and biological threat materials, the application of IRM to radiological and nuclear threat materials is more extensive and is often applied directly to the threat materials themselves, rather than a surrogate. Applications to nuclear fuel are typically focused on analysis of fissile and fertile isotopes, while applications to other materials span a wide range of analysed elements. Table 3 is a summary of forensic applications of IRM to various radiological and nuclear materials. Demonstrated applications are described in greater detail in this section and have been grouped into unirradiated fuel materials, irradiated materials, and radioactive sources and reactor components.

Table 3. Summary of forensic applications of IRM to various radiological and nuclear materials.

Application	Ratios	Reference
Unirradiated fuel materials
Uranium production date	^234U/^230Th ^235U/^231Pa ^232Th/^228Th ^226Ra/^234U ^227Ac/^235U	[24–29] [30–34]
Uranium geographic origin	^207Pb/^206Pb ^208Pb/^206Pb ^87Sr/^86Sr δ^98Mo ^143Nd/^144Nd ^34S/^32S δ^18O ^234U/^238U	[35–48]
UOC-UFx conversion detection	^238U^19F/^238U ^19F/^238U	[49, 50]
Uranium enrichment (^235U content)	^235U/U	[27, 51–56]
Uranium enrichment process	^234U/^238U ^235U/^234U	[54, 57, 58]
Irradiated fuel materials
Uranium source (mined or reprocessed)	^236U/^238U ^233U/^238U ^232U/^238U	[54, 57, 59]
Fuel burnup	various U various Pu ^100Ru/^104Ru	[60–63]
Plutonium production date	^238Pu/^234U ^239Pu/^235U ^240Pu/^236U ^242Pu/^238U ^241Pu/^241Am ^238Pu/^239Pu ^241Pu/^239Pu ^242Pu/^239Pu	[64–69]
Fuel age	^244Cm/^245Cm ^244Cm/^246Cm	[70]
Plutonium geographic origin	^18O/^16O	[66]
Reactor type	various Pu ^136Ba/^138Ba, ^137Cs/^133Cs, ^134Cs/^137Cs, ^135Cs/^137Cs, ^154Eu/^153Eu, ^150Sm/^149Sm, ^152Sm/^149Sm ^100Ru/^104Ru, ^105Pd/^110Pd	[63, 65, 71–77]
Nuclear weapon detonation detection	various Xe various I	[78, 79]
Radioactive sources and reactor components
Radioactive source age	^60Co/^60Ni ^241Am/^233Pa ^137Cs/^137Ba ^90Sr/^90Y/^90Zr	[80–84]
Graphite moderator fluence	^10B/^11B δ^7Li δ^13C ^48Ti/^49Ti ^235U/^238U ^236U/^238U ^240Pu/^239Pu ^241Pu/^239Pu ^242Pu/^239Pu δ^41Ca δ^59Ni	[59, 85–88]
Fluence of other reactor components	^10B/^11B δ^7Li δ^59Ni δ^63Ni ^49Ti/^48Ti ^114Cd/^113Cd ^158Gd/^157Gd ^94Zr/^96Zr	[59, 89–92]

Unirradiated fuel materials

IRM has been applied to many nuclear and radiological materials that have not been irradiated. Research typically focuses on various materials of the nuclear fuel cycle. In brief, the fuel cycle consists of mining the uranium from the ground (uraninite), milling to form uranium ore concentrate (UOC), conversion to UF₆, enrichment via gaseous centrifuge or diffusion, and conversion to UO₂ powder prior to being formed into pellets. Aspects of the fuel cycle can be determined via IRM, such as production date, geographic origin, methods of chemical conversion, enrichment (²³⁵U content), and enrichment process.

The age (production date) of uranium compounds such as UOC, UO₂ powders, and highly enriched uranium has been determined, and/or techniques improved, by several studies [24–34]. The concentration of daughter nuclides builds up over time by natural decay, acting as a natural chronometer. If a material is assumed to contain zero daughter nuclides on the date of its last purification, its age can be determined by the ratio of parent to daughter nuclides. Chronometer ratio pairs include ²³⁴U/²³⁰Th, ²³⁵U/²³¹Pa, ²³²Th/²²⁸Th, ²²⁶Ra/²³⁴U and ²²⁷Ac/²³⁵U.

Uranium-containing samples can possess different characteristics depending on their geographic origin. One such characteristic is the U isotopic composition. Richter et al. [35] analysed the uranium isotopic composition of six samples collected worldwide from various uranium ore mining facilities. For all ratios (i.e., ²³⁴U/²³⁸U, ²³⁵U/²³⁸U, and ²³⁶U/²³⁸U) they found significant differences for samples of different origin and concluded that, with some constraints, this set of data can be considered as isotopic “fingerprints” for their geographic origin. However, they added that using only one isotope ratio makes it impossible to distinguish between most of the samples. Svedkauskaite-LeGore et al. [36] utilized ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios to determine the geographic origin of UOC samples from 35 mines (14 countries) and 10 yellow-cakes. Lead ratios are good candidates for IRM as the decay chain of ²³⁵U terminates at ²⁰⁷Pb, ²³⁸U terminates at ²⁰⁶Pb, and ²³²Th terminates at ²⁰⁸Pb [36]. ²⁰⁴Pb is not radiogenic and can be used to determine the amount of natural lead co-present in a sample. Varga et al. [37] found the ⁸⁷Sr/⁸⁶Sr ratio less prone to variation than ²⁰⁷Pb/²⁰⁶Pb. A related study by Varga et al. [38] showed that different isotope ratios (e.g., various ratios of Pb, Sr, Nd, O, and other trace and rare earth element concentrations) are useful to verify the declared origin of UOCs. Finally, δ⁹⁸Mo has also been identified by Migeon et al. [39] as having potential to assist in tracing UOC material to the original uranium ore if used in conjunction with other indicators.

Moreover, these data have potential application to help identify the origin of unknown UOCs intercepted from illicit trafficking. Other ratios applied to UOC geographic origin include δ¹⁸O in combination with rare earth element ratios (e.g., La/Yb [40], ¹⁴³Nd/¹⁴⁴Nd [41], and ³⁴S/³²S [42]). δ¹⁸O has been used to determine the geographic origin of uranium oxides [43, 44]. Recently, links have been made between δ¹⁸O values of secondary hydration minerals formed on the surface of U₃O₈ with its surrounding environment during mineralization [45], thereby providing possible geolocation information from the material history. In another study, Balboni et al. [46] report that for 11 uranium ore (uraninite) samples and one UOC from various US deposits, trace element data and Sr isotopic ratios indicate distinct signatures that are highly dependent on the mode of mineralization and host rock geology. Comparison of rare earth element patterns between uraninite and corresponding UOC from the same area indicates identical patterns at different absolute concentrations. Similarly, Spano et al. [47] reported that U isotopic ratios for uraninite and UOC overlap and confirm a lack of significant isotopic fractionation during the fabrication process. These results confirm the importance of establishing geochemical signatures of uranium ore materials for attribution purposes in the forensic analysis of intercepted nuclear materials. Fujikawa et al. [48] utilized ²³⁴U/²³⁸U to determine the geographic origin of uranium in water samples. Varga et al. [93] followed the production from a UOC to a U₃O₈ product, measuring isotopic ratios. They found ²⁰⁷Pb/²⁰⁸Pb, ²⁰⁶Pb/²⁰⁸Pb, and ⁸⁷Sr/⁸⁶Sr change over the production process and cannot be used to trace a product back to its UOC.

The compound UF₆ is commonly associated with uranium conversion and enrichment [49]. It reacts vigorously with water to form hydrogen fluoride and uranium oxyfluoride particles (UO₂F₂). Any leak from sealed equipment will result in deposit of particulate UF₄ or UO₂F₂ on various surfaces within and outside the conversion or enrichment facility, which can be used to detect nuclear activities and verify them with the facility’s declaration (e.g., applied as part of safeguards inspections). Faure et al. [49] used the ratio of ²³⁸U¹⁹F to ²³⁸U to determine if uranium conversion occurred prior to sampling by comparison to CRMs of UF₄ particles and UOC particles. Further, they showed storage in a plastic bag at ambient temperatures in the dark sufficiently preserved the samples for up to four years. Kips et al. [50] showed no significant changes of the isotope ratio of ¹⁹F/²³⁸U in UO₂F₂ laboratory-prepared particles exposed to various ultraviolet (UV)-light, humidity, and temperatures simulating natural environmental conditions, over a period of seven weeks.

Uranium enrichment is a differentiating characteristic of nuclear fuels and an important component of materials accounting and nuclear safeguards. ²³⁵U/U is a direct measure of enrichment of ²³⁵U in uranium oxide and is the prime measurement employed by the International Atomic Energy Agency (IAEA) world-wide network of analytical laboratories [94]. Enrichment has been measured in hot particles from Chernobyl soils [52], highly enriched uranium up to 80% [52], known samples in a Nuclear Forensics International Technical Working Group (ITWG) forensic exercise [53–55], and actual forensic investigations [27] (described in greater detail in “Nuclear materials” section). Buchmann et al. [56] has even applied this technique to uranium extracted from pine needles and could differentiate those in the vicinity of an enrichment facility and those more than 20 km away.

²³⁴U/²³⁸U and ²³⁵U/²³⁴U can be used to identify the enrichment technology employed and, in the case of tailings, the degree of enrichment may be implied (low vs. highly enriched) [57, 58]. Processes have been developed to measure ²³⁴U/²³⁸U [95–98]. In the ITWG forensic exercise, ²³⁴U/²³⁸U versus ²³⁵U/²³⁸U measurements concluded that three unique samples were enriched by the same process [54].

Efforts to refine methods for uranium isotopic analysis, including reducing the time required for analysis while maintaining analytical precision, are continually being examined. This was demonstrated by Krachler et al. [96] with their paper on isotopic analysis of uranium using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), offering a faster, relatively non-destructive technique to determine the isotopic signature of a uranium sample. This improved turn-around is beneficial for the small reporting windows associated with eventual forensic investigations.

Irradiated fuel materials

When a material is irradiated, fission and transmutation can occur. The resulting isotopic distribution contains many characteristics that can be used to uniquely identify aspects of the irradiation process [99]. IRM has been shown to be capable of determining uranium source (mined or reprocessed), fuel burnup, production date of materials, type of reactor where the material was irradiated, and detect the detonation of a nuclear weapon, with references given in Table 3.

The ratios ²³⁶U/²³⁸U and ²³³U/²³⁸U can be used to determine if the material was recovered from irradiated material as ²³⁶U and ²³³U are normally only produced in significant quantities from irradiation [57–59]. Processes have been developed to measure ²³⁶U/²³⁸U [95–98, 100]. In the ITWG forensic exercise, ²³⁶U/²³⁸U measurements concluded that two of three samples contained recovered uranium from irradiated fuel [54]. Tumey et al. [101] developed a method for highly sensitive ²³³U/²³⁸U measurements for potential use in determining if uranium was recovered from irradiated material.

Fuel burnup is a measure of the amount of energy produced during irradiation from a given fuel per fuel mass unit [99]. Various U and Pu ratios can be measured during post-irradiation examination of fuel to benchmark burnup values predicted by reactor physics models for fuel depletion [60, 61]. Konegger-Kappel and Prohaska [62] determined isotope ratios ²⁴²Pu/²³⁹Pu and ²⁴⁰Pu/²³⁹Pu of Pu-containing environmental particles collected from the Chernobyl area. The measured Pu isotopic ratios were found in good agreement with the expected ratios typical for the fuel burnup in a Russian RBMK reactor at the time of the accident.

Similar to uranium, the age (production date) of refined plutonium-containing compounds can be determined by the concentration of daughter nuclides built up over time by natural decay, acting as a natural chronometer [64]. Due to the long half-lives of most Pu isotopes, highly accurate techniques have been developed in order to measure the low concentrations of daughter nuclides [102–106]. Common ratios used for Pu age dating include ²³⁸Pu/²³⁴U, ²³⁹Pu/²³⁵U, ²⁴⁰Pu/²³⁶U, ²⁴¹Pu/²⁴¹Am, ²³⁸Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu, and ²⁴²Pu/²³⁹Pu [64–68]. While ²⁴²Pu/²³⁸U can be used theoretically, the long half-life of ²⁴²Pu and its low abundance makes this ratio impractical [65]. ²⁴¹Pu/²⁴¹Am can be measured by gamma spectrometry and therefore does not require separation necessary for MS measurements [64]. The need for Pu CRMs, specifically with a certified date of production for use in radiochronometry analyses was identified [69]. Some studies have focused on producing consensus dates for already existing CRMs [69].

Christl et al. [70] investigated the use of ²⁴⁴Cm/²⁴⁵Cm and ²⁴⁴Cm/²⁴⁶Cm ratios as a method to age date irradiated fuel, owing to the short half-life of ²⁴⁴Cm relative to ²⁴⁵Cm and ²⁴⁶Cm. A precision of five years was reported with potential for improvement.

Similar to uranium oxides, Tamborini et al. [66] used δ¹⁸O to determine the geographic origin of plutonium oxides.

The isotopic distribution of Pu in irradiated nuclear fuel is dependent on fuel enrichment, neutron flux energy spectrum, and fuel burnup, all of which are determined by the reactor type. For example, ²³⁸Pu production increases with ²³⁵U enrichment, a faster neutron flux results in relatively more light mass isotopes, ²³⁸Pu and ²⁴²Pu increase with burnup, and high ²³⁹Pu is indicative of low burnup fuel [65]. After correcting for age since discharge from reactor via the ²³⁹Pu/²³⁵U ratio, Wallenius et al. [65, 71] analysed the isotopic composition of 5 Pu samples and differentiated the type of reactor in which they were generated (see Figure 2). In a similar study, Dimayuga et al. [72] reported potential isotopic signatures of fuels irradiated in heavy water reactors. Reactor physics calculations were used to determine the theoretical isotopic ratios, which were then compared to measured values. Results indicated that correlations of specific isotopic ratios can be employed to determine unique trends that can differentiate these fuels from others irradiated in other reactor types. In another study, insoluble stable isotopes produced in the noble metal phase were used to correlate irradiated fuel and reactor type [63]. Palmer et al. [63] found that ¹⁰⁰Ru/¹⁰⁴Ru ratio had a linear relationship with burnup across all reactor types, and therefore could be used as an indicator of burnup of an unknown irradiated fuel. Further, when plotted against the ¹⁰⁰Ru/¹⁰⁴Ru ratio, ¹⁰⁵Pd/¹¹⁰Pd demonstrated potential to discriminate reactor type of low burnup, weapons grade irradiated fuel.

Figure 2. Reactor type identification by Pu isotopic correlation, adapted from Wallenius et al. [65].

A nuclear forensics methodology was developed in order to determine the reactor type used to produce weapons grade Pu, as well as the fuel burnup and time since discharge. It is intended to be used on interdicted Pu that has been chemically separated from the irradiated fuel. A series of studies document the development, experimental validation and sensitivity testing of this methodology [73–77]. It uses Pu ratios (²⁴¹Pu/²³⁹Pu, ²⁴⁰Pu/²³⁹Pu, ²⁴²Pu/²³⁹Pu) as well as intra-element fission product ratios (¹³⁶Ba/¹³⁸Ba, ¹³⁷Cs/¹³³Cs, ¹³⁴Cs/¹³⁷Cs, ¹³⁵Cs/¹³⁷Cs, ¹⁵⁴Eu/¹⁵³Eu, ¹⁵⁰Sm/¹⁴⁹Sm, ¹⁵²Sm/¹⁴⁹Sm) to identify the most likely reactor to produce the sample from within a library. Future work identified in this series includes expanding the reactors in the library and expanding parameters of interest with additional target isotope ratios.

The source of radioactive species in the atmosphere may be determined by IRM and has been developed for enforcement of the Comprehensive Nuclear Test Ban Treaty. Zhang et al. [78] measured activity concentration of ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe at the Bruce B nuclear power station in Canada. They also obtained similar data from a monitoring station near the medical isotope production facility in Chalk River, Canada. They were able to differentiate emissions from medical isotope production and nuclear power reactors, and also expect to be able to differentiate from the ratios expected from a nuclear weapon. Kalinowski et al. [79] differentiated emission from nuclear power stations and nuclear weapons by modelling the activity concentrations of ¹³⁵I, ¹³³I, and ¹³¹I and validating the data against measured data from a Nevada Test Site explosion.

Radioactive sources and reactor components

The isotopic composition of radioactive sources can be used to determine their production age, and is often a key component of source attribution. One study demonstrated radiochronometry of ⁶⁰Co sources, developing a reference material and standard operating procedure, and validation through a series of inter-comparison exercises [80]. In another study, Vesterlund et al. [81] employed gamma spectrometry to identify isotopic signatures in ²⁴¹Am sources. The authors noted that in addition to age, the presence of impurities, such as ²³⁹Np indicating the presence of ²⁴³Am or ²³Na from the fused glass matrix of the source, can help differentiate the investigated sources. Improvements in ¹³⁷Cs/¹³⁷Ba radiochronometry are presented by Steeb et al. [82]. Finally, a method to determine the production date of ⁹⁰Sr sources was developed [82] and further refined [84] using commercially available materials and equipment, allowing this method to be more widely implemented in laboratories for forensic applications.

IRM has potential as a forensic safeguards verification tool via analysis of isotopic ratios in reactor components. Isotopes present in reactor structural materials, such as pressure tubes and vessels, flow channels, control and instrument equipment, moderators, etc., are transmuted during irradiation. The resulting isotope ratios can be measured to deduce the material’s lifetime neutron fluence. Since neutron fluence is directly related to the amounts of fissile materials produced (e.g., plutonium), IRM has potential to confirm reactor performance against the facility’s declaration [86].

Analysis of trace elements in graphite moderators to determine fluence has been studied by Pacific Northwest National Laboratory [85, 86]. The fluence range has an impact on the precision of some isotope ratios as shown in Table 4. Gerlach et al. [87] also used ¹⁰B/¹¹B to estimate fluence in the graphite moderator in the British Experimental Pile Zero (BEPO) reactor at Harwell, UK. Fetter [59] proposed estimating neutron fluence via δ⁴¹Ca and δ⁵⁹Ni in pure graphite moderators. In addition, Remeikis et al. [88] reported good agreement between simulated and measured δ¹³C in the irradiated graphite of the central part of the RBMK-1500 reactor core, indicating that the neutron flux was modelled accurately.

Table 4. Isotopic ratio measured in graphite moderator for fluence predictions [86].

Element	Measured ratios	Fluence range
Boron	^10B/^11B	Low
Lithium	^6Li/^7Li	Low-Intermediate
Titanium	^48Ti/^49Ti	Intermediate-High
Uranium	^235U/^238U, ^236U/^238U	Low-High
Plutonium	^240Pu/^239Pu, ^241Pu/^239Pu, ^242Pu/^239Pu	Low-High

A structural material commonly used in nuclear reactors is zirconium alloys, typically as structural elements of fuel assemblies, fuel element cladding, and fuel channels [89]. In one study, Gerlach et al. [89] reported that based on their preliminary results, Ti isotope ratios, particularly ⁴⁹Ti/⁴⁸Ti ratios, measured in irradiated Zircaloy samples (taken from the channel that surrounds Boiling Water Reactor (BWR) fuel assemblies) vary in a self-consistent manner with reported fluence values.

The fluence of other reactor components has also been determined via IRM. Cristie et al. [90] used the isotope ratio of ¹⁰B/¹¹B and δ⁷Li to estimate fluence in Al₂O₃/B₄C composites, radioactive liquid samples, and borosilicate glasses. Fetter [59] proposed estimating neutron fluences via δ⁵⁹Ni and δ⁶³Ni in steel components. Tomiyushi et al. [91] used ¹¹⁴Cd/¹¹³Cd and ¹⁵⁸Gd/¹⁵⁷Gd to measure neutron fluences; these elements are commonly used in control rods and moderator poisons, respectively. Simonits et al. [92] used ⁹⁴Zr/⁹⁶Zr to evaluate thermal-to-epithermal neutron flux ratios.

Explosives

An explosive is anything that is made, manufactured, or used to produce an explosion, detonation, or pyrotechnic effect [107]. Their destructive force comes from the rapid expansion of gases they create. Examples of explosives are given in Tables 5 and 6.

Table 5. Description of the fractions of an explosive [108].

Explosive fraction	Purpose	Common examples
Explosive	Rapidly expand gases to destroy surrounding objects	RDX
		HMX
		PETN
		TNT
		AN
Binder	Decrease sensitivity of the explosive	SBR (styrenebutadiene)
		PIB (polyisobutylene)
Plasticizers and oils	Easier to mold and safer to handle	BEHA (bis(2-ethylhexyl) adipate)
		BEHS (bis(2-ethylhexl) sebacate)
		DIBP (diisobutyl phthalate)
		Tributyl citrate

May also contain small amounts of antioxidants, dyes, and/or taggants.

There are many chemical properties of explosives that can be used for forensic applications, including moisture content, acidity, explosive content, and non-explosive component identity and quantity. IRM provides a way to further discriminate samples of identical chemical composition. Typically, δ²H, δ¹³C, δ¹⁵N, and/or δ¹⁸O, sometimes in combination with the quantification of components, provide a good basis for discrimination [108].

A framework for an explosives IRM investigation was developed by Chesson et al. [108]. This stepwise process starts by analysing the “bulk” explosive, namely all the components in a single measurement. Next, any explosive recovered as a pure material is analyzed. Then, the explosive is separated into its different fractions for analysis, namely, the explosive, binder, plasticizer/dye/oil, and other insoluble fractions. Finally, fractions are separated by individual component for analysis, if required (e.g., multiple binder compounds in the binder fraction). Table 5 is a general description of the fractions of an explosive. Table 6 is a list of the common names of explosives used in this paper and their respective chemical names.

Table 6. List of explosives and their chemical names.

Common name/Abbreviation	Chemical name/Description
AN	Ammonium nitrate
BP	Black powder (mixture of potassium nitrate, sulfur, and charcoal)
ETN	Erythritol tetranitrate
HMTD	Hexamethylene triperoxide diamine
HMX	Cyclotetramethylene-tetranitramine
LX-10	HMX with 5% fluorocarbon binder “Viton”
LX-14	HMX with 5% polyurethane binder “Estane”
LX-17	TATB with 7.5% chlorofluorocarbon binder “KeL-F 800”
OCTOL	75% HMX with 25% TNT
PETN	Pentaerythritol tetranitrate
RDX	Cyclotrimethylenetrinitramine
TATB	Triamino trinitrobenzene
TATP	Triacetone triperoxide
Tetryl	2,4,6-trinitrophenyl methyl nitramine
TNT	2,4,6-Trinitrotoluene

The Chesson et al. framework [108] is shown in greater detail in Figure 3. The framework shows separation pathways, separation methods, the characteristics determined, and the chemical analyses completed alongside various types of isotope ratio mass spectrometry (IRMS) analyses. These analyses include GC, quadrupole mass spectrometry (Q-MS), high temperature conversion elemental analyzer (TCEA)-IRMS, gas chromatography thermal conversion (GCTC)-IRMS, elemental analysis (EA)-IRMS, and high performance liquid chromatography (HPLC). This framework was validated by Howa et al. [109], who evaluated the isotopic signatures of raw material (before separation) and the separated components. While the δ¹³C and δ¹⁵N in the raw material and RDX were not able to discriminate between samples, the isotopic signatures of the separated non-explosive material were able to do so, thereby providing additional forensic information through the separation process.

Figure 3. Detailed framework for forensic analysis of explosives, adapted from Chesson et al. [108].

The forensic application of IRM to explosives materials has been demonstrated by many studies. Table 7 lists a summary compiled from literature. Benson et al. [110, 111] differentiated the geographic origin of various samples of undetonated AN, TATP, and PETN using δ¹⁸O, δ²H, δ¹³C, and δ¹⁵N. Howa et al. [113] conducted a survey of acetone (TATP precursor) from 12 countries and found that the country of origin was the dominant source of variation in δ¹³C and δ²H in the acetone samples. Moreover, Howa et al. [113] synthesized and measured TATP and determined that the isotopic ratios of TATP were similar to that of its acetone precursor. Accordingly, the variation in δ¹³C and δ²H in acetone could possibly be used as a basis for geographic correlation of TATP samples. Howa et al. [114] determined the manufacturing process of RDX and HMX samples from δ¹³C and δ¹⁵N. Gentile [112] differentiated the manufacturer, type, and/or batch of undetonated AN and BP via δ¹³C, δ¹⁵N, and δ¹⁸O. For HMX, LX-10, LX-14, TNT, OCTOL, TATB, and LX-17, McGuire et al. [8] found large differences in δ¹³C between aromatic (e.g., TNT, TATB) and nonaromatic (e.g., HMX) explosives that were consistent post-detonation. Lock et al. [115] synthesized HMTD from precursor hexamethylenetetramine in laboratory and field conditions and found δ¹³C to be similar to precursor fractions. They also found δ¹⁵N to be similar to precursor fractions only when synthesis conditions were controlled. In a recent study, Bezemer et al. [116] studied δ¹³C, δ¹⁵N, δ²H and δ¹⁸O in ETN and its precursors in order to discriminate between samples and established linkages with its precursors.

Table 7. Summary of forensic applications of IRM to various explosives threats.

Application	Ratios	References
AN geographic origin	δ^18O, δ^2H, δ^15N	[110, 111]
AN manufacturer and type	δ^13C, δ^15N, δ^18O	[112]
TATP geographic origin	δ^13C, δ^2H, δ^18O	[110, 111, 113]
PETN geographic origin	δ^13C, δ^15N	[110, 111]
RDX and HDX manufacturing process	δ^13C, δ^15N	[114]
Aromatic or nonaromatic determination	δ^13C	[8]
BP manufacturer, type, and batch	δ^13C, δ^15N, δ^18O	[112]
HMTD precursor	δ^13C, δ^15N	[115]

In addition to the studies described in this section, valuable information has been obtained for implementing IRM as a forensic technique for explosives on a wide scale. Howa et al. [117] demonstrated that for AN, the separation of ammonium and nitrate permits two independent sources of N isotopic fractions for discrimination. Sisco et al. [118] investigated the isotopic stability of ETN, PETN, RDX, HMX, TNT, and tetryl under various environmental conditions. They found a temperature of −4 °C had minimal effect on all samples and may be the ideal storage conditions for preservation of evidence thought to contain explosive material. The isotopic stability of some explosive compounds was found to be sensitive to temperature, humidity, UV-light, and ozone (O₃) [112, 118].

The largest challenges to the forensic application of IRM to explosives are related to post-detonation analysis. Benson et al. [110, 111] found δ¹⁵N of AN samples were not maintained post-blast, limiting the IRM application to undetonated explosive material only. McGuire et al. [8] found a trend of δ¹⁵N increasing upon detonation of HMX, LX-10, LX-14, TNT, OCTOL, TATB, and LX-17, and theorised some reaction with atmospheric N. McGuire et al. [8] found contamination of organic matter (including detonators and mounting material) in post-detonation residues influenced the isotopic fraction of HMX, LX-10, LX-14, TNT, OCTOL, TATB, and LX-17, even in tests designed to minimize the potential for contamination.

Case studies

Nuclear materials

Wallenius et al. [27] present results of IRM used on nuclear material as part of two actual forensic investigations. A brief summary is provided here as both investigations provide excellent examples of isotopic ratios being used to attribute material.

The first investigation involved four fuel pellets received at the Institute for Transuranium Elements (ITU) in Germany from Lithuania. Gamma spectrometry was used to measure the ²³⁵U enrichment. As all four pellets had similar enrichment and dimensions, only one was dissolved for measurement of select isotopic ratios by both thermal ionisation mass spectrometry (TIMS) and MC-ICP-MS. The ratio of ²³⁴U/²³⁰Th was used to determine the age of the sample using the half-life of ²³⁴U. The dimensional information and measured enrichment were cross-referenced against a database of nuclear fuel fabrication information and the results were consistent with an RBMK-1500 reactor, of which the only operational units in the world were at the Ignalina station in Lithuania. Further, there was only one fuel manufacturer for the Ignalina station and the measured age of the fuel was consistent with the operational period of the fuel manufacturer. Cross-referencing the results with an IAEA database of illicit nuclear material trafficking indicated this fuel may have originated from a reported case of stolen fresh fuel from the Ignalina station in 1992.

The second investigation involved four samples of uranium powder received by ITU from the Czech Republic. Gamma spectrometry was again used to measure the ²³⁵U enrichment. This time the enrichment was greater than 88% ²³⁵U. Part of each sample was dissolved for analysis by TIMS and MC-ICP-MS and its age determined using the same method as the fuel pellets. As the enrichment of the powder was higher than that used in power reactors, the fuel fabrication database was not useable for these samples, but did indicate the powders were weapons-useable. The age and impurities measured were consistent with another seizure of highly-enriched powder in Germany in 1994, supporting the conclusion that both seizures may have originated from the same batch.

Both case studies demonstrate the methodology given in Sec. 2. The measured isotope ratios were first used to compare samples to determine if they were consistent with one another. The use of a database could then be used to determine if the samples were consistent with other previously measured samples. By understanding the process of uranium purification, including the removal of ²³⁰Th, the age of the material could also be determined. Finally, a database of fuel manufacturers permitted the attribution of the fuel pellets to a particular location. It should be noted that IRM was not used in isolation, but in conjunction with other properties of the samples in it attribution, such as the physical dimensions and impurities.

Anthrax

In 2001, letters containing anthrax were mailed to several news offices and two senators in the United States. As part of the investigation, stable isotopes were measured in one anthrax sample, a variety of spore culture grown in the area, and tap water sample from 18 cities [119]. Envelopes used to mail the anthrax were also tested, but are not CBRNE material and do not impact the isotopes ratios in CBRNE material and therefore are not discussed further.

The sample of anthrax and spore cultures were tested for δ²H, δ¹³C, δ¹⁵N, and δ¹⁸O. The tap water samples were tested for δ²H and δ¹⁸O. The δ¹⁸O values indicated the anthrax sample was inconsistent with the other tested spores. To understand the results, a model of δ²H was developed based on the medium and water used. Using water from Dugway as an input to the model, it was determined that is was unlikely that Dugway water was used to prepare the anthrax. Further, the relationship between δ²H and δ¹⁸O suggested the sample was not grown in a liquid medium with tap water, but rather an agar medium [119].

Although the evidence was not used in court, Ehleringer and Matheson [12] detail how such IRM evidence could be applied in court.

Summary and recommendation for future research

In recent years, research using IRM has been applied to CBRNE materials. The state-of-the-art of IRM’s forensic application shows great potential to be useful to law enforcement, but is currently rarely used in actual forensic investigations. To our knowledge, IRM has not yet been used in a court of law. The literature review documented in this paper highlights the need of further development before IRM can become widely used.

There are several areas of needed research and development that are common to all of the CBRNE materials. First is the need for precise measurement techniques with low uncertainties. The specific measurement equipment, technique, and required uncertainty will largely be driven by the analyzed sample material and variability in the isotopic ratios of interest. Second is the need for large and extensive databases. Often, a database is required to correlate an IRM result with identifying characteristics of the material (e.g., geolocation). Detailed databases of many data points spanning many materials and fractionation processes will assist in establishing the greatest degree of identifying characteristics of material from IRM results. Advanced data analytics (e.g., chemometrics) work best with larger datasets for classification/characterisation purposes. Third is the need to quantify uncertainties and naturally occurring variability. In order for IRM to be used in prosecution, the degree of confidence in both measurements and correlation will need to be established. Fourth is the need for CRMs. Standards are needed for most analyses and common standards will permit comparing results from various laboratories. These four needs for IRM research and development span the application to all CBRNE materials, but there are also more specific future research recommendations for each CBRNE material sub-group.

The application of IRM to chemical and biological threat materials is the least mature of all the CBRNE threat materials. The nature of these materials makes direct research on them difficult and limited. However, the research so far using chemical and biological materials shows the potential of IRM to differentiate between different sources of the same material.

Only a limited number of studies have been done exploiting IRM to identify the attribution signatures of various chemical threat agents. Stable light element (such as H, N, O, C, and S) isotope ratios are particularly useful for fingerprinting chemical threat materials. Focusing the IRM of the less hazardous precursor compounds and understanding the fractionation effects from synthesis and other treatments will lead to the quickest gains in this area. Isotope fractionation refers to physical, chemical, or biological processes that change the relative abundances of stable isotopes of an element. Hence, the understanding of mechanisms (such as differences in reaction rates) that causes isotopic fractionation will be useful for the analysis and interpretation of isotope ratio data.

The isotope ratio method has promise as a useful tool in microbial forensics to establish the origin and transmission route of a particular microbial strain. For biological threats, focus on the fractionation effects of growth media, water source, preparation techniques, and biological fractionation processes will lead to the quickest gains in this area. It is only after these fractionation effects are understood that IRM can be used for attribution. The hydrogen and oxygen isotope ratios of growing cells and spores are related to media water, hence these values can be useful to trace the geographic origin of microbial culture. The carbon and nitrogen isotope ratio values are related to the growth medium. The isotope ratio analysis will have to be performed with original material, since sample preparation in another environment will cause the organisms to take the signature of that environment. A standardized sample handling procedure needs to be developed for IRM techniques for microbial forensics.

The current state-of-the-art of IRM applied to chemical and biological hazards may be useful for linking samples from related events to the same production of the chemical or biological threat materials as well as for comparative approaches. Before more widespread field application however, the application of IRM to the actual chemical and biological hazardous materials will need to be completed and the fractionation effects verified.

The state of IRM application to radiological and nuclear materials is the most developed relative to the other CBRNE threat materials. The applications are the most diverse, most capable of uniquely identifying characteristics of RN materials, and most likely to be used first in prosecution. It should be noted, however, that the majority of the previous work has focused on nuclear materials, such as fuel supply products and reactor materials, with a limited amount of work on radioactive sources. Although research on isotopic signatures of nuclear materials is still warranted given the need to develop multiple signatures of diverse material types to enable effective attribution, a greater emphasis is required on exploring isotopic signatures of radioactive sources. This is supported by the observation that the majority of thefts and losses reported to the IAEA’s Incident and Trafficking Database involve radioactive sources used in industrial and medical applications [120]. This research can be in the form of demonstrating useful radiochronometers for various sources, as well as novel signatures, such as impurities from feedstock materials. Also, as research discovers additional isotopic signatures, development of more complete databases or libraries containing signatures of the materials will be beneficial. Uncertainties in correlations (e.g., correlations between precursor and threat materials, correlations between threat materials and geolocations, etc.), including model-derived correlations, will need to be better defined before the IRM technique can be deployed in prosecution. Models to correlate IRM results with flux profiles and fluence levels are also needed for both fuel and reactor structural materials.

The application of IRM to explosives is developing. Gentile [112] theorized that (1) a large international database will be essential for success of forensic applications of IRM to explosives, (2) intra- and inter-sample variability are important factors to discrimination, and (3) the quantification of uncertainties make prosecution difficult (e.g., court conviction). In its current stage of development, IRM can be useful during investigation (e.g., identify patterns, trends, series, suspects, crime execution, sequence of events, etc.). Therefore, more research into identification of isotopic signatures of the major components of explosives, including investigation of methods to minimize uncertainties in analytical results, is recommended. Also, understanding of the effects of detonation on fractionation will allow pre-detonation research to apply more broadly to post-detonation cases.

In conclusion, IRM of CBRNE materials is a promising technique for forensic investigations and worth of continued research and funding. Further guided and strategic research and development could lead to the widespread use of IRM in law enforcement and eventually in court.

Acknowledgements

This study was funded by Atomic Energy of Canada Limited, under the auspices of the Federal Nuclear Science and Technology Program. The research was conducted at the Canadian Nuclear Laboratories.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This study was funded by Atomic Energy of Canada Limited, under the auspices of the Federal Nuclear Science and Technology Program. The research was conducted at the Canadian Nuclear Laboratories.