Nuclear forensics is the investigation of
nuclear materials
Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", con ...
to find evidence for the source, the trafficking, and the enrichment of the material. The material can be recovered from various sources including dust from the vicinity of a nuclear facility, or from the radioactive debris following a
nuclear explosion
A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, t ...
.
Results of nuclear forensic testing are used by different organisations to make decisions. The information is typically combined with other sources of information such as law enforcement and
intelligence information.
History
The first seizures of nuclear or otherwise radioactive material were reported in Switzerland and Italy in 1991. Later, reports of incidents of nuclear material occurred in Germany, the Czech Republic, Hungary and other central European countries. Nuclear Forensics became a new branch of scientific research with the intent of not only determining the nature of the material, but also the intended use of the seized material as well as its origin and about the potential trafficking routes. Nuclear forensics relies on making these determinations through measurable parameters including, but not limited to chemical impurities, isotopic composition, microscopic appearance, and microstructure. By measuring these parameters, conclusions can be drawn as to the origin of the material. Identification of these parameters is an ongoing area of research, however, data interpretation also relies on the availability of reference information and on knowledge of the fuel cell operations.
The first investigative radiochemical measurements began in the early days of nuclear fission. In 1944, the US Air Force made the first attempts to detect
fissiogenic 133Xe in the atmosphere in order to indicate the production of plutonium through the irradiation of uranium and chemical reprocessing in an effort to gather intelligence on the status of the
German nuclear program
The Uranverein ( en, "Uranium Club") or Uranprojekt ( en, "Uranium Project") was the name given to the project in Germany to research nuclear technology, including nuclear weapons and nuclear reactors, during World War II. It went through seve ...
. However, no
133Xe was detected. In the subsequent years it became increasingly valuable to gather information on the
Soviet nuclear weapons program, which resulted in the development of technologies that could gather
airborne particles
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
in a
WB-29 weather reconnaissance plane. On September 3, 1949, these particles were used to determine that the detonation time of the first Soviet atomic test, "
Joe 1
The RDS-1 (russian: РДС-1), also known as Izdeliye 501 (device 501) and First Lightning (), was the nuclear bomb used in the Soviet Union's first nuclear weapon test. The United States assigned it the code-name Joe-1, in reference to Joseph S ...
".
Further analysis revealed that this bomb was a replicate of the "
Fat Man
"Fat Man" (also known as Mark III) is the codename for the type of nuclear bomb the United States detonated over the Japanese city of Nagasaki on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the fir ...
", which was the bomb dropped on
Nagasaki
is the capital and the largest city of Nagasaki Prefecture on the island of Kyushu in Japan.
It became the sole port used for trade with the Portuguese and Dutch during the 16th through 19th centuries. The Hidden Christian Sites in the ...
in 1945. This investigative methodology combined radiochemistry and other techniques to gather intelligence on nuclear activities.
The first seizures of nuclear materials from trafficking in the early 1990s allowed the nuclear forensic methodology to be adopted by a wider scientific community. When scientific laboratories outside the weapons and intelligence community took an interest in this methodology was when the term "Nuclear Forensics" was coined. Unlike standard forensics, nuclear forensics focuses mainly on the nuclear or radioactive material and aims to provide knowledge of the intended use of the materials.
In 1994 560 grams of plutonium and uranium oxide were intercepted at
Munich airport
Munich International Airport- Franz Josef Strauß (german: link=no, Flughafen München) is an international airport serving Munich and Upper Bavaria. It is the second-busiest airport in Germany in terms of passenger traffic after Frankfurt A ...
in an airplane coming from Moscow.
The precise composition was 363 grams plutonium (87% of which was
Plutonium-239
Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main ...
) and 122 grams of uranium.
It later emerged through a German parliamentary enquiry that the purchase had been arranged and financed by the German
Federal Intelligence Service
The Federal Intelligence Service (German: ; , BND) is the foreign intelligence agency of Germany, directly subordinate to the Chancellor's Office. The BND headquarters is located in central Berlin and is the world's largest intelligence head ...
.
Chronometry
Determining a nuclear material's age is critical to nuclear forensic investigations. Dating techniques can be utilized to identify a material's source as well as procedures performed on the material. This can aid in determining the information about the potential participant in the "age" of the material of interest.
Nuclide
A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state.
The word ''nuclide'' was coined by Truman ...
s, related through radioactive decay processes will have relative sample concentrations that can be predicted using parent-daughter in-growth equations and relevant half-lives. Because radioactive isotopes decay at a rate determined by the amount of the isotope in a sample and the half-life of the parent isotope, the relative amount of the decay products compared to the parent isotopes can be used to determine "age". Heavy element nuclides have a 4n+2 relationship, where the mass number divided by 4 leaves a remainder of two. The decay network begins with
238Pu and proceeds through the in-growth of long-lived
234U,
230Th, and
226Ra. If any member of the 4n+2 decay chain is purified it will immediately begin to produce descendant species. The time since a sample was last purified can be calculated from the ratio of any two concentrations among the decaying nuclides.
Essentially, if a nuclear material has been put through a refinement process to remove the daughter species, the time elapsed since purification can be "back-calculated" using radiochemical separation techniques in conjunction with analytical measurement of the existing parent-daughter ratios. For example, the α decay of
239Pu to
235U can be used as an example of this procedure. with the assumption of a perfect purification time ''T''
0 then there will be a linear relationship between the in-growth of
235U and time elapsed since purification. There are, however, various instances where the correlation is not as clear. This strategy may not apply when the parent-daughter pair achieve secular equilibrium very rapidly or when the half-life of the daughter nuclide is significantly shorter than the time that has elapsed since purification of the nuclear material, e.g.
237Np/
233Pa. Another possible complication is if in environmental samples, non-equivalent metal/ion transport for parents and daughter species may complicate or invalidate the use of chronometric measurements. Special age-dating relationships exist, including the commonly employed
234U/
230Th and
241Pu/
241Am chronometers. In special circumstances, parent-granddaughter relationships can be used to elucidate the age of nuclear materials when the material is intentionally made to look older through the addition of daughter nuclides.
Chronometry is based on the concept that the composition of the nuclear material changes as samples are prepared and analyzed. This barrier can be substantial for species that decay quickly or whose daughter products put forth spectral interferences. The decay of
233U, for example, has a t
1/2~1.6x10
5years which is rapid in comparison to many species and yield
229Th, which emits an α particle that is isoenergetic, having the same energy, as the parent. To avoid this, freshly prepared samples as well as complementary analysis methods are used for confident nuclear materials characterization. The decay of nuclear samples makes rapid analysis methods highly desirable.
Separations
Chemical separation techniques are frequently utilized in nuclear forensics as a method of reducing the interferences and to facilitate the measurement of low level radionuclides. Purification that occurs rapidly as progeny in-growth begins immediately following purification is ideal.
Anion Exchange
Anion exchange separation methods are widely used in the purification of
actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
s and actinide bearing materials through the use of resin columns. The anionic actinide complexes are retained by anion exchange sites that are on the resin and neutral species pass through the column unretained. Then the retained species can be eluted from the column by conversion to a neutral complex, typically by changing the mobile phase passed through the resin bed.
Anion exchange-based separations of actinides, while valued for there simplicity and widely used, tend to be time-consuming and are infrequently automated. Most are still dependent on gravity. Speeding up the flow of the mobile phase tends to introduce problems such as impurities and jeopardize future investigations. Hence, there is still a need for development of this technique to satisfy the nuclear forensic research priorities.
Co-Precipitation
Actinide isolation by co-precipitation is frequently used for samples of relatively large volumes to concentrate analytes and remove interferences. Actinide carriers include
iron hydroxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which ...
s,
lanthanide fluoride
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...
s/
hydroxides
Hydroxide is a polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization ...
,
manganese dioxide
Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cell ...
, and a few other species.
Analysis
A wide range of instrumental techniques are employed in nuclear forensics. Radiometric counting techniques are useful when determining decay products of species with short half-lives. However, for longer half-lives, inorganic mass spec is a powerful means of carrying out elemental analysis and determining isotopic relationships. Microscopy approaches can also be useful in characterization of a nuclear material.
Counting Techniques
Counting techniques of α,β,γ or neutron can be used as approaches for the analysis of nuclear forensic materials that emit decay species. The most common of these are alpha and gamma spectroscopy. β counting is used infrequently because most short lived β-emitters also give off characteristic γ-rays and produce very broad counting peaks. Neutron counting are found more rarely in analytical labs due in part to shielding concerns should such neutron emitters be introduced into a counting facility.
Alpha-particle spectroscopy
Alpha-particle spectroscopy is a method of measuring the radionuclides based on emission of
α particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 atomic nucleus, nucleus. They are generally produced in the process of alpha decay, but may ...
s. They can be measured by a variety of detectors, including liquid scintillation counters, gas ionization detectors, and ion-implanted silicon semiconductor detectors. Typical alpha-particle spectrometers have low backgrounds and measure particles ranging from 3 to 10 MeV.
Radionuclide
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...
s that decay through
α emission tend to eject α particles with discrete, characteristic energies between 4 and 6 MeV. These energies become attenuated as they pass through the layers of sample. Increasing the distance between the source and the detector can lead to improved resolution, but decreased particle detection.
The advantages of alpha-particle spectroscopy include relatively inexpensive equipment costs, low backgrounds, high selectivity, and good throughput capabilities with the use of multi-chamber systems. There are also disadvantages of alpha-particle spectroscopy. One disadvantage is that there must be significant sample preparation to obtain useful spectroscopy sources. Also, spectral interferences or artifacts from extensive preparation prior to counting, to minimize this high purity acids are needed. Another disadvantage is that measurements require a large quantity of material which can also lead to poor resolution. Also, undesired spectral overlap and long analysis times are disadvantages.
Gamma Spectroscopy
Gamma spectroscopy
Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics.
Most radioactive sources produce gamma rays, which are of various energie ...
yields results that are conceptually equivalent to
alpha-particle spectroscopy
Alpha spectrometry (also known as alpha(-particle) spectroscopy) is the quantitative study of the energy of alpha particles emitted by a radioactive nuclide that is an alpha emitter.
As emitted alpha particles are mono-energetic (i.e. not emitt ...
, however, can result in sharper peaks due to reduced attenuation of energy. Some radionuclides produce discrete
γ-rays
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nucleus, atomic nuclei. It consists of the shortest wavelength electromagnetic wav ...
that produce energy between a few KeV to 10 MeV which can be measured with a
gamma-ray spectrometer
A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon.
The study and analysis of gamma-ray spectra for scientific and technic ...
. This can be accomplished without destroying the sample. The most common gamma-ray detector is a semiconductor germanium detector which allow for a greater energy resolution than
alpha-particle spectroscopy
Alpha spectrometry (also known as alpha(-particle) spectroscopy) is the quantitative study of the energy of alpha particles emitted by a radioactive nuclide that is an alpha emitter.
As emitted alpha particles are mono-energetic (i.e. not emitt ...
, however gamma spectroscopy only has an efficiency of a few percent. Gamma spectroscopy is a less sensitive method due to low detector efficiency and high background. However, gamma spectroscopy has the advantage of having less time-consuming sample procedures and portable detectors for field use.
Mass Spectrometry
Mass spec techniques are essential in nuclear forensics analysis. Mass spec can provide elemental and isotopic information. Mass spec also requires less sample mass relative to counting techniques. For nuclear forensic purposes it is essential that the mass spectrometry offers excellent resolution in order to distinguish between similar analytes, e.g.
235U and
236U. Ideally, mass spec should offer excellent resolution/mass abundance, low backgrounds, and proper instrumental function.
Thermal Ionization MS
In
thermal ionization mass spectrometry
Thermal ionization mass spectrometry (TIMS) is also known as surface ionization and is a highly sensitive isotope mass spectrometry characterization technique. The isotopic ratios of radionuclides are used to get an accurate measurement for the ele ...
, small quantities of highly purified analyte are deposited onto a clean metal filament. Rhenium or tungsten are typically used. The sample is heated in a vacuum of the ion source by applying a current to the filaments. A portion of the analyte will be ionized by the filament and then are directed down the flight tube and separated based on mass to charge ratios. Major disadvantages include time-consuming sample preparation and inefficient analyte ionization.
Multi-Collector Inductively Coupled Plasma-Mass Spectrometry
This is a frequently used technique in nuclear forensics. In this technique a purified sample is nebulized in a spray chamber and then aspirated into a plasma. The high temperature of the plasma leads to sample dissociation and high efficiency of ionization of the analyte. The ions then enter the mass spectrometer where they are discriminated based on mass based on a double focusing system. Ions of various masses are detected simultaneously by a bank of detectors similar to those used in the thermal ionization mass spec. MC-ICP-MS has a more rapid analysis because it does not require lengthy filament preparation. For high quality, however, there is a requirement for extensive sample cleanup. Argon plasma is also less stable and requires relatively expensive equipment as well as skilled operators.
Secondary-Ion MS
SIMS
Sims, sims or SIMS may refer to:
Games
* ''The Sims'', a life simulation video game series
** ''The Sims'' (video game), the first installment, released in 2000
** ''The Sims 2'', the second installment, released in 2004
** '' The Sims 3'', th ...
is a micro-analytical technique valuable for three-dimensional analysis of a materials elemental composition and isotopic ratios. This method can be utilized in characterization of bulk materials with a
detection limit
The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the ...
in the low
parts per billion
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...
(10
−9 or ng/g) range. Particles as small as a few hundreds of nanometers can be detected.
Ion production in this technique is dependent on the bombardment of solid samples with a focused beam of primary ions. The sputtered, secondary ions are directed onto the
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
system to be measured. The secondary ions are a result of kinetic energy transfer from the primary ions. These primary ions penetrate into the solid sample to some depth. This method can be used to detect any element, however the sputtering process is highly matrix dependent and ion yields vary.
This method is especially useful, because it can be fully automated to find uranium particles in a sample of many million particles in a matter of hours. Particles of interest can then be imaged and further analyzed with very high
isotopic precision.
Additional Nuclear Forensic Methods
Numerous additional approaches may be employed in the interrogation of seized nuclear material. In contrast to previously mentioned analysis techniques, these approaches have received relatively low attention in recent years in terms of novel advancement, and, typically, require greater quantities of sample.
Scanning electron microscope
The
scanning electron microscope can provide images of an object's surface at high magnification with a resolution on the order of nanometers. A focused beam of energetic electrons is scanned over the sample and electrons that a backscattered or emitted from the sample surface are detected. Images are constructed via measuring the fluctuations of electrons from the sample beam scanning position. This data is useful in determining what process may have been employed in the materials production and to distinguish between materials of differing origins. Measurement of backscattered electrons elucidate the average atomic number of the area being scanned. The emitted, or secondary electrons provide topological information. This is a relatively straight forward technique, however samples must be amenable to being under a vacuum and may require pre-treatment.
X-Ray Fluorescence
X-ray fluorescence
X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...
offers rapid and non-destructive determination of the elemental composition of a nuclear material based on the detection of characteristic
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s. Direct sample irradiation allows for minimal sample preparation and portable instrumentation for field deployment. The
detection limit
The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the ...
is 10 ppm. This is well above mass spectrometry. This technique tends to be hindered by matrix affects, which must be corrected for.
Neutron Activation Analysis
Neutron activation analysis
Neutron activation analysis (NAA) is the nuclear process used for determining the concentrations of elements in many materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on atomic ...
is a powerful non-destructive method of analyzing elements of mid to high atomic number. This method combines excitation by nuclear reaction and the radiation counting techniques to detect various materials. The measurement of characteristic radiation, following the bombardment completion, is indicative of the elements of interest. The equation for the production product is given by:
where
is the starting analyte,
is the incoming neutron,
is the excited product and
is the detected radiation that results from the de-excitation of the product species.
The advantages of this technique include multi-element analysis, excellent sensitivity, and high selectivity, and no time-consuming separation procedures. One disadvantage is the requirement of a nuclear reactor for sample preparation.
X-Ray Absorption Spectroscopy
X-Ray absorption spectroscopy (XAS) has been demonstrated as a technique for nuclear forensic investigations involving uranium speciation. Both the lower energy
near-edge (XANES) and higher energy
fine structure
In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to electron spin and relativistic corrections to the non-relativistic Schrödinger equation. It was first measured precisely for the hydrogen atom ...
(EXAFS) analytical methods may be useful for this type of characterisation. Typically, XANES is employed to determine the oxidation state of the absorbing uranium atom, while EXAFS can be used to determine its local atomic environment. This spectroscopic method, when coupled with
X-Ray diffraction
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
(XRD), would be of most benefit to complex nuclear forensic investigations involving species of different oxidation states.
Objective Colour Analysis
Objective colour analysis can be performed using digital images taken with a digital camera, either in the field or in a laboratory. This method was developed to replace subjective colour reporting, such as by-eye observations, with quantitative RGB and HSV values. The method has previously been demonstrated on the thermal treatment of uranyl peroxide powders, which yield distinctive yellow to brown hues.
[{{Cite journal, last1=Thompson, first1=Nathan B. A., last2=O’Sullivan, first2=Sarah E., last3=Howell, first3=Robert J., last4=Bailey, first4=Daniel J., last5=Gilbert, first5=Matthew R., last6=Hyatt, first6=Neil C., date=2020-12-25, title=Objective Colour Analysis from Digital Images as a Nuclear Forensic Tool, url=http://www.sciencedirect.com/science/article/pii/S0379073820305405, journal=Forensic Science International, volume=319, language=en, pages=110678, doi=10.1016/j.forsciint.2020.110678, pmid=33444895, s2cid=231612412 , issn=0379-0738] Hence, this method is noted as particularly useful in determining thermal processing history, especially where colour changes occur in uranium compounds of various oxidation states.
References
Forensic techniques
Nuclear interdisciplinary topics