Technical Capabilities

The Nuclear Forensics Toolbox

Nuclear forensics is a scientific discipline that makes use of a wide variety of instruments to measure specific material properties (e.g. physical and/or chemical characteristics) that form the basis of discriminating nuclear forensics signatures in interdicted radioactive and nuclear (R/N) materials. Many of these instruments are also used in other scientific disciplines, most notably geochemistry. These instruments make up the components of a “toolbox” available to the nuclear forensic analyst.

This page describes of some of the techniques that may be used in nuclear forensic material analysis. Please note that other instruments or measurement techniques may be available, and the choice of techniques should be selected by what is available, as well as what types of data are required to reach forensics conclusions about an interdicted R/N material.

What instrument(s) should be used?

The choice of instrumentation used in the analysis of the interdicted R/N material depends primarily on the questions that are being asked as part of the nuclear forensics investigation, for example: “What is the material?,” “What could it be used for?,” “How was it made?,” and “Has a legal statute been violated?” The choice of instrument is typically driven by a number of considerations, including the nature of the material, the required analytical precision, the cost, and the timeframe within which data must be obtained. Fortunately, for most R/N material analysis signatures, more than one technique can be used, and many instruments are relatively inexpensive to procure, maintain, and use in an R/N materials examination. However, all instruments require skilled, experienced, and attentive analysts to operate them.

Destructive vs. Non-Destructive

One key consideration in choice of analytical technique is whether the R/N sample needs to remain intact or whether small fractions of the material can be consumed during the course of the measurements. Non-destructive techniques, such as gamma spectrometry, is a passive technique used to measure the gamma radiation released by an R/N material, and therefore the material analyzed remains intact. Destructive techniques, such as mass spectrometry, generally require small amounts of the sample to be dissolved prior to analysis. Nuclear forensic analysis is an iterative process - the results of initial, low-precision, non-destructive analyses may reveal key characteristics that can be further examined using higher-precision, destructive measurement techniques. Typically, non-destructive analyses are performed first, and the data and results from these analyses can then be used to inform if destructive analyses are necessary. An additional consideration is sample heterogeneity, particularly for destructive analyses, where usually only a small portion of an R/N material is analyzed In the case of inhomogeneous samples, the choice of which portion to analyze becomes critical. A nuclear forensics analytical plan should be sufficiently flexible to allow for modifications, based on initial analytical findings.

In the field of nuclear forensics the term ‘signatures’ describes material characteristics such as isotopic abundances, elemental profile, physical and chemical form, physical dimensions etc. that may be used to link a material to its origin or process history.

Optical Microscopy

Scanning Electron Microscopy

Gamma Spectrometry

X-ray Diffraction Spectrometry

X-ray Fluorescence Spectrometry

Optical Microscopy

When a sample is received by the laboratory for nuclear forensic investigation, a thorough and detailed examination of its physical state is the first step. In many cases, highly relevant information about the nature of the sample can be obtained from physical examination aided by optical microscopy. Fundamental (and rather simple) measurements, such as determination physical dimensions of sample, can be taken with high precision. Some samples, particularly fuel cycle materials such as fuel pellets, may present diagnostic markings, such as serial numbers or other identifying information, and often are designed with dimensional parameters specified for a specific type of nuclear reactor. Sealed radioactive sources may contain diagnostic information on the casing. Samples that have undergone transit outside of regulatory control may contain “wear and tear” markings, such as scratches or other imperfections, which may provide information regarding their history.

Physical examination does not require sophisticated microscopy equipment. Most optical microscopes can be equipped with a high-resolution digital camera, which can be used to record physical features for interpretation in the context of a nuclear forensic investigation. Perhaps most importantly, physical examination using microscopy can be used to guide sub-sampling for additional analytical techniques. Many high-precision analytical techniques, such as mass spectrometry, require destruction of a small portion of the sample prior to analysis. Optical microscopic examination can aid selection of one or more areas of the sample to be selected for removal and dissolution for destructive analysis, either to ensure that a representative fraction of the sample is used, or to assess sample heterogeneity through the use of several sub-samples.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a technique used for examining the polished or unpolished surface of a nuclear forensics sample for its micro-scale physical topography and chemical composition. SEM instruments offer a range of magnifications greatly exceeding those that can be achieved by optical (light) microscopy, allowing for a more detailed and high-resolution qualitative and quantitative analysis of physical properties such as sample texture, porosity, grain size, and other micro-scale features. SEM images can be interpreted qualitatively, resulting in physical descriptions of features such as grain morphology (e.g. “platy”, “acicular”, etc.). In some cases, images can be processed in a manner that allows for quantitative measurements of features such as grain dimension and shape, as well as dimensions of specific physical features, such as the length of a marking on the sample.

Most SEM instruments are equipped with several types of detectors, including: (i) back-scattered electron detectors, allowing for the spatially-resolved determination of the chemical composition of the sample, based on the average atomic number (Z) of each region of the sample, on the micro-scale, and (ii) secondary electron detectors, allowing for high-resolution mapping of physical features. Ideally, the sample is polished prior to backscattered electron analysis, allowing for quantitative measurements to be taken at a high spatial resolution, relative to standard reference materials. Backscattered electron images can provide great insight into the degree of chemical heterogeneity present in the sample, which can guide future sub-sampling campaigns for destructive analysis. Some SEM instruments are equipped with energy dispersive spectroscopy (EDS) detectors, allowing for semi-quantitative to quantitative spatially-resolved measurements of chemical composition, down to a detection limit of 10s to 100s of parts per million.

Gamma Spectrometry

Gamma spectrometry measurements takes advantage of the gamma radiation produced by a nuclear forensics samples to determine the activity of the gamma-emitting radionuclides present in the sample. This passive analytical technique does not require alteration of the sample, making it a key non-destructive analysis that is a core measurement in any nuclear forensic investigation. The sample is simply placed in the detector for a length of time sufficient to allow for sufficient counting statistics to be achieved, or for qualitative determination of the inventory of radionuclides present. Several types of gamma detectors are available, differing in terms of energy resolution and efficiency. In nuclear forensics, the most commonly-used gamma spectrometer is the high-purity germanium detector, which allows for maximum energy resolution for radionuclide determination. Gamma spectrometry analysis also has the advantage of a short analytical time, on order of minutes to hours. Longer analytical times are possible, and can provide more information regarding the radionuclide inventory of the sample..

Samples can also be analyzed for gamma spectrometry following alteration of the sample, to idealize the geometry for quantitative analysis. In some cases, fractions of the dissolved sample can be analyzed in solution form, allowing for a standard container size to be used. Following this approach, semi-quantitative to quantitative determination of radionuclide concentration and/ or isotope ratio can be performed after calibration using radioactive standard reference materials.

X-ray Diffraction Spectrometry

In X-ray diffraction (XRD) spectrometry, a nuclear forensics sample can be analyzed for presence and abundance of specific crystalline phases. Samples are loaded into the spectrometer typically in powder form with a randomized grain arrangement. The sample is rotated while exposed to a primary X-ray beam of known wavelength, and the angle of the beam is slowly adjusted through a pre-determined range of angles. On the other end of the sample is an X-ray detector; when Bragg’s equation is satisfied for the wavelength of the primary X-ray beam and the crystal lattice spacing of one or more crystalline phases present in the sample, the X-ray beam is diffracted off the sample, resulting in detection. The resultant spectrograph consists of a series of peaks, characteristic of one or more crystalline phases present in the sample. Typically, these characteristic peaks are matched to a database of known phases, and a determination is made. Some XRD instruments can be calibrated for the determination of the relative abundance of each crystalline phase present in the sample. XRD is particularly suited to crystalline nuclear forensics samples such as uranium ores, uranium ore concentrates, and uranium and plutonium oxides, as well as metals and other industrial materials. XRD spectrometry can also be used to demonstrate that a nuclear forensics sample is amorphous (glassy).

X-ray Fluorescence Spectrometry

X-ray fluorescence (XRF) spectrometry is an analytical technique that can be used to determine the chemical composition of a nuclear forensics sample, destructively or non-destructively. In XRF spectrometry, a nuclear forensics sample is exposed to a primary X-ray beam, generated by an X-ray tube with a cathode consisting of a heavy metal such as rhodium or molybdenum, capable of generating a monochromatic X-ray beam that is used to excite the electrons of the constituent elements of the sample. The wavelengths of the secondary X-rays generated by the sample are characteristic of the chemical composition of the sample, and are detected by one or more detectors, depending on the type of XRF instrument. In energy-dispersive XRF spectroscopy, the detector itself is used to resolve the differences in energy of the incident secondary X-rays. In wavelength-dispersive XRF spectroscopy, a series of crystals with characteristic crystal lattice spacing are used to diffract only those characteristic X-rays that solve for Bragg’s equation, allowing for isolation of specific X-ray energies and a more precise and accurate analysis of the X-ray signal. In either case, the XRF instrument can be used to determine the concentration of elements present in the nuclear forensics sample ranging from the very lightest elements to the actinides. Detection limits range from weight percent to 10s of parts per million, depending on the instrument used and the element of interest; lighter elements typically have higher detection limits than heavier elements.

Since nuclear forensics samples can be analyzed non-destructively via XRF spectrometry (provided the sample fits in the spectrometer), these analyses are often among the first performed during a nuclear forensics investigation, revealing the bulk chemical composition of the sample. For more precise, quantitative analyses, the sample can be analyzed destructively by XRF. Typical XRF preparation methods include fluxing (melting and quenching with additives) to make glass beads and glass discs; binding with a binding agent; and/or pressing into a pellet. Most XRF instruments can operate “standardless”, obviating the need for matrix-matched standard reference materials. But for more accurate analyses, standard reference materials can be used to prepare calibration curves for specific elements of interest.

Quadrupole – Inductively Coupled Plasma – Mass Spectrometry (q-ICP-MS)

The quadrupole inductively coupled plasma mass spectrometer (q-ICP-MS) is a class of mass spectrometers that makes use of a quadrupole lens to separate ions on the basis of their mass-to-charge ratios. Ions are produced by injection of a nuclear forensics sample (typically as a dissolved liquid, though other introduction methods are possible) into a plasma, which atomizes and ionizes the chemical compounds that constitute the sample. The positively-charged ions are then accelerated by an electromagnetic field through a quadrupole lens, which performs the mass-to-charge ratio analysis. This method of mass spectrometry has several advantages, including: (i) the technology is inexpensive relative to other types of mass spectrometry, and more robust; (ii) the quadrupole lens can be used to scan the entire periodic table in timescales on the order of milliseconds, and (iii) analyses do not typically require chemical separation. However, q-ICP-MS is less precise than other mass spectrometry methods that are tailored for the measurement of specific elemental and isotopic systems. q-ICP-MS can also be used to determine isotope ratios for some isotopic systems, but with less precision than isotope ratio mass spectrometers designed specifically for this purpose.

In nuclear forensics, q-ICP-MS is typically used to determine the major, minor, and trace element concentration of the sample. In order to calculate quantitative concentrations from measured intensities, a calibration curve is required for each element of interest, usually developed on the day of analysis, prior to the measurement of the sample through the analysis of solution standards. Alternatively, the “standard addition” method can be used, which can increase accuracy and precision, but at the expense of additional effort and cost. Non-metals are difficult to analyze via q-ICP-MS, and some semi-metals (such as silicon) can be a challenge. Furthermore, some isotopes of elements of interest are interfered with by other isotopes and/or polyatomic compounds (e.g. 40Ar16O on 56Fe), which require interference correction (or selection of a different isotope) to avoid erroneous results.

Alpha Spectrometry

Alpha spectrometry is a robust, reliable, and time-tested method for determining the isotopic composition of one or more elements present in a nuclear forensic sample. For accurate alpha spectrometry analysis, chemical purification of the element of interest is usually required. Following purification, the sample can either be electroplated onto a planchette, or precipitated onto a filter paper, for analysis using an alpha spectrometry system. Alpha spectrometry systems consist of a series of alpha detectors in sealed, evacuated chambers, to allow for the measurement of one or more samples simultaneously, in an environment allowing for unimpeded transit of the alpha particles to the detector. Count times for alpha spectrometry are typically on the order of hours to days, depending on the specific activity of the sample. Incident alpha particles are detected and binned by energy, resulting in a series of peaks, the area of which is a function of radionuclide activity. If the sample has been spiked with a known quantity of an isotopic tracer not present in the sample prior to purification chemistry, it can be possible to back-calculate the concentration of each isotope present, with high precision. Alpha spectrometry is a cost-effective method for performing isotopic composition measurements in cases where mass spectrometry measurements are impractical, or for high-activity isotopes such as 238Pu that are difficult to measure by mass spectrometry.

Multi-collector – Inductively Coupled Plasma – Mass Spectrometry (MC-ICP-MS)

MC-ICP-MS systems take advantage of the use of multiple detectors to simultaneously detect ion beams for high precision isotope ratio measurements. These instruments are typically designed specifically for isotope ratio measurements, and lack the versatility of single-collector systems such as q-ICP-MS. However, what they lack in versatility they make up for in terms of accuracy and precision for isotope ratio measurements, as well as concentration measurements using the isotope dilution technique. Positively-charged ions are produced using an inductively coupled plasma, and accelerated down a flight tube using an electromagnetic field. MC-ICP-MS instruments make use of a magnet situated alongside a curved tube to perform the separation of ion beams on the basis of mass-to-charge ratio, with heavier isotopes less deflected than light isotopes around the curved path. Nuclear forensics samples typically require chemical purification prior to analysis, and each element of interest must be purified and measured separately. Furthermore, isotope ratio standards must be analyzed alongside samples in order to ensure accurate measurements are performed.

MC-ICP-MS is the gold standard, alongside thermal ionization mass spectrometry (TIMS), for the measurement of isotope ratios in nuclear forensic samples. MC-ICP-MS is particularly suitable for the analysis of U and Pu isotope ratios. Care must be taken to ensure that sufficient wash-out time is allowed between analyses, as insufficient cleanliness can generate biased isotope ratios.

Thermal Ionization Mass Spectrometry (TIMS)

Like MC-ICP-MS, modern TIMS instruments typically utilize multiple detectors to simultaneously detect ion beams for isotope ratio measurements. However, in the TIMS technique, ions are produced via thermalization after loading onto a filament (typically rhenium) in a high-vacuum chamber, then accelerated by an electromagnetic field and separated on the basis of mass-to-charge ratio using a magnet, similarly to MC-ICP-MS. TIMS measurements can be even more precise than equivalent measurements performed by MC-ICP-MS. However, TIMS measurements typically take longer, with individual measurements occurring over a period of several hours, to achieve the highest possible precision. TIMS measurements require correction for biases in isotope ratios that result from preferential thermalization of the light isotope. Isotopic systems such as Pb, Sr, and Nd have been measured in nuclear forensics samples via TIMS.

Secondary Ion Mass Spectrometry (SIMS)

SIMS is used for elemental and isotopic analysis of solid samples. SIMS is a destructive technique, although the amount of sample consumed during analysis is usually quite small. The greatest strength of SIMS is the ability to analyze very small areas (as small as 50 nm using the NanoSIMS, for example) and to generate high-spatial resolution maps of the distribution of elements and isotopes within the sample. SIMS may also be applied for the analysis of single micrometer sized particles. SIMS is a surface sensitive technique, since the secondary ions at any point in the analysis derive from the top 3-5 monolayers of material. However, repetitive analyses can be performed over longer periods as the primary ion beam sputters away the sample (to a depth of several microns in some instances) in a technique known as “depth profiling”. The measurement of the isotopic composition of sample is usually straightforward, only requiring the analysis of the sample and that of an isotopic reference material for determination of the mass bias of the instrument. Quantification of elements, however, involves the analysis of matrix matched standards for the determination of the relative sensitivity factor (a function of both the element to be analyzed and the matrix). SIMS is can be used in nuclear forensics for exploring the heterogeneity of the material on fine spatial scale.

Isotope ratio mass spectrometry (IRMS)

Isotope ratio mass spectrometry (IRMS) is used to analyze variations in the natural abundance of isotopes from the same element. This approach is usually applied to stable isotopes of light elements such as hydrogen, oxygen, nitrogen, carbon and sulfur. IRMS instruments are designed analyze simple gases, so for sample types such as liquids or solids, the sample must first be converted to a gas using online peripheral instruments or offline vacuum manifolds. Furthermore, the gases generated from samples must be purified or separated by chromatographic or cryogenic methods so that only one simple gas such as N2 or CO2 enters the IRMS at a time. IRMS systems use a magnetic sector mass analyzer to separate different isotopic compositions of the same simple gas and then analyze the abundance ratios of these isotopes on two or more separate collectors. IRMS methods rely on relative comparisons of isotope ratios between unknown samples and a known standard reference material.

IRMS analysis of stable isotope ratios is a relatively new area of research in nuclear forensics. Light stable elements are present in most nuclear materials, and in many cases the isotope ratios of these elements are related to the chemical processes used to produce the material. In addition, nuclear materials may inherit the isotope compositions of elements such as carbon or sulfur from precursors in the production process. For materials that are produced using local water or are exposed to local water vapor, the oxygen and hydrogen in the nuclear material may reflect the global variation in the isotope compositions from the water cycle.

Technical Capabilities

The Nuclear Forensics Toolbox

What instrument(s) should be used?

Destructive vs. Non-Destructive

Instruments & Tools