IUPAC Periodic Table of the Elements and Isotopes (IPTEI) for the Education Community (IUPAC Technical Report)

4.1.1 Hydrogen isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of hydrogen possess slightly different physical and chemical properties and they are commonly fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights (Fig. 4.1.1). Hydrogen has the largest relative mass difference among its isotopes and consequently exhibits the largest variation in isotopic composition of any element that does not have radioactive or radiogenic isotopes. Ranges in the stable isotopic composition of naturally occurring hydrogen-bearing materials are shown in Fig. 4.1.1. These variations enable hydrogen isotopes to be used as tracers in environmental studies [13].

Fig. 4.1.1:

Variation in atomic weight with isotopic composition of selected hydrogen-bearing materials (modified from [13], [17]).

A primary use of stable hydrogen isotopes is in isotope hydrology. Although the evolution of the stable hydrogen and oxygen isotopic composition of precipitation begins with the evaporation of water from the oceans, their local and global relationship arises primarily from equilibrium isotopic fractionation of heavier (²H and ¹⁸O) and lighter (¹H and ¹⁶O) isotopes of hydrogen and oxygen during condensation as a tropospheric vapor mass follows a trajectory to higher latitudes and over continents [14], [15]. As a consequence, the hydrogen isotopic composition of precipitation, rivers, and tap waters varies with elevation, season, and distance from the ocean-continent boundary. Figure 4.1.2 shows the variation in the atomic weight of hydrogen in water from rivers across the United States. These variations in the hydrogen isotopic composition of environmental water are often combined with stable oxygen isotopic compositions and have been used to identify the origin of water samples and to investigate the interaction between groundwater and surface water (e.g. lakes, streams, and rivers) [16].

Fig. 4.1.2:

Variation in atomic weight of hydrogen in river waters across the continental United States (modified from [16]). Blue color indicates waters most depleted in ²H (resulting in lower atomic weight of hydrogen) and brown color indicates those most enriched in ²H (resulting in higher atomic weight of hydrogen).

4.1.2 Hydrogen isotopes in forensic science and anthropology

Measurements of relative ²H abundances are used to determine the breeding grounds of many species of migrant songbirds. These species of songbirds only grow their feathers before migration, and they grow them on or close to their breeding grounds. Therefore, the isotopic composition of a bird’s feathers correlates to the isotopic composition of the growing season’s precipitation [18], [19], [20].

Measurements of relative ²H abundances of human hair samples collected at archeological sites are used to determine the geographic region in which a subject lived based on the hydrogen isotopic composition of the water they drank. This is possible because hair stores a daily record of the hydrogen isotopic composition of intake water, which correlates to local meteoric water [18], [21].

4.1.3 Hydrogen isotopes in geochronology

³H (tritium), with a half-life of 12.31 years, decays to ³He. The relative variations in n(³He)/n(³H) ratios can be interpreted in terms of elapsed time for dating purposes. The dates of groundwater recharge (water moving downward from the surface), where large amounts of ³H were received from precipitation following thermonuclear bomb test periods, come from the elapsed time since a water mass became isolated from the atmosphere in the time range from the mid-1950s to the present [15].

4.1.4 Hydrogen isotopes in industry

³H is used for self-luminous exit signs in aircraft and commercial buildings. It is found in luminous dials, gauges, wristwatches, and luminous paints [22]. ²H, in the form of heavy water, is used in CANDU (CANada Deuterium Uranium) nuclear reactors as a moderator and coolant [23].

4.1.5 Hydrogen isotopes in medicine

²H is used for the isotopic labeling of drugs and nutrients to trace their uptake and metabolism in the human body [24], [25]. ²H, in the form of heavy water, is used to study human metabolism. For example, ²H is used in combination with ¹⁸O (double labeled water) to measure energy expenditure [26].

4.2.1 Helium isotopes in geochronology

³He is a product of the radioactive decay of ³H (half-life of 12.31 years). The relative variations in the amount ration(³He)/n(³H) can be interpreted in terms of elapsed time. This has been especially useful in aquatic systems, including oceans, lakes, and aquifers, that received large inputs of ³H from precipitation following thermonuclear bomb test periods. ³H-³He dating provides the elapsed time since a water mass became isolated from the atmosphere in the time range from the mid-1950s to the present. Such studies are important for establishing the sustainability of groundwater resources in shallow aquifers [27], [28].

⁴He is a product of radioactive decay in the uranium and thorium decay series. As a result, ⁴He concentration is used to estimate the relative ages of minerals and groundwater. In closed systems (systems that do not exchange matter with their surroundings), relative variations in the amount ratio n(⁴He)/n(U) can be interpreted in terms of elapsed time, although other processes can alter the distribution of helium, which is highly mobile in terrestrial environments [29], [30].

⁴He concentrations commonly increase along groundwater flow paths through a cumulative release from aquifer materials. This rate of accumulation is used to estimate the time since the groundwater was recharged at the surface. The ⁴He accumulation method of groundwater dating is typically used in deeper aquifers, where groundwater is relatively old and the ³H-³He method cannot be used because of the relatively short half-life of 12.31 years for ³H [30].

4.2.2 Helium isotopes in industry

³He has a large absorption cross section for neutrons, which makes it especially useful for radioactivity detection [31], [32]. In this application, neutrons produced by the radioactive decay of elements, such as uranium and plutonium, enter the detector, where the reaction ³He (n, p) ³H produces ¹H and ³H atoms. This induces further collisions and the release of electrons, which interact with charged surfaces to generate an electric current. Large amounts of ³He are used to produce neutron detectors in portal monitors for detecting illicit radioactive materials at ports, border crossings, and airports (Fig. 4.2.1). Unfortunately, the isotope³He is rare and there is a need to incorporate alternative gases for use in neutron detectors. ³He neutron detectors are also used in devices that determine the proportions of water, oil, and gas in wells drilled for energy production. Other important uses of ³He include lasers, gyroscopes used for missile stability and guidance, and cryogenic research (ultra-low temperature, less than 1 K).

Fig. 4.2.1:

Radiation detectors are installed in many areas to screen people, vehicles, and cargo for radioactive materials. ³He detectors are sensitive to thermal neutrons and are used to detect isotopes of uranium and plutonium that might be used in nuclear weapons, along with other sources that produce neutrons by radioactive decay. (Image Source: U.S. Government Accountability Office) [33].

The global supply of ³He available for research and practical applications has become severely limited in recent years, such that prices have increased substantially and some uses have been curtailed [31], [32]. A major source of ³He is from nuclear weapons containing ³H, recovered when the warheads are reconditioned or dismantled. ³He accumulates in such devices as a radiogenic product of ³H decay. The annual supply of new ³He has decreased with reductions in nuclear arsenals.

4.2.3 Helium isotopes in medicine

³He is used as an inhalant to improve magnetic resonance imaging (MRI) of the lungs [34].

4.3.1 Lithium isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of lithium possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. Natural terrestrial materials show a substantial variation in lithium isotopic abundance (Fig. 4.3.1), and these natural isotopic abundances have been used to determine sources of dissolved lithium and to investigate environmental processes [13], [35].

Fig. 4.3.1:

Variation in atomic weight with isotopic composition of selected lithium-bearing materials (modified from [13], [17]).

Variations in isotope-amount ratiosn(⁷Li)/n(⁶Li) can help determine the source of some water. Because the relative abundances of lithium isotopes can change during hydrothermal processes, isotopic analysis of lithium in water can help distinguish water derived from marine sedimentary rocks from water derived from hydrothermally altered igneous rocks (Fig. 4.3.2) [36], [37].

Fig. 4.3.2:

Diagram of submarine hydrothermal vent processes. (Image Source: Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration) [38].

4.3.2 Lithium isotopes in industry

⁷Li, as hydroxide monohydrate (⁷LiOH•H₂O), is used to maintain the pH level of the coolant used in pressurized water reactors in the nuclear power industry [39], [40]. Lithium plays a role in the construction of a thermonuclear bomb, which differs from a fission weapon in that it uses the energy released when two light atomic nuclei (i.e. deuterium (²H) and tritium (³H)) fuse to form helium and a high energy neutronvia this DT reaction. ⁶Li is used, in the form of ⁶Li deuteride (⁶Li²H), as fusion fuel capable of producing tritium when bombarded with neutrons within the weapon via the reaction ⁶Li (n, ³H) ⁴He [41].

Li-based laboratory reagents have found their way into surface water and can be easily identified. Although a military secret in the 1950s, it is now known that substantial amounts of ⁶Li (normally having an isotopic abundance of 0.076) were removed from chemical reagents to be used in nuclear weapon development. Reagents containing the remaining lithium depleted in ⁶Li (having an isotopic abundance as low as 0.025) were sold to both chemical manufacturers and to laboratory chemists for their use [42]. The distinctive isotopic signature of depleted ⁶Li, having a n(⁷Li)/n(⁶Li) ratio of 39, compared to a ratio of 12 in naturally occurring terrestrial materials, enables easier detection of this lithium source in polluted waterways and the environment [35], [37].

4.3.3 Lithium isotopes in medicine

⁷Li is a decay product of the ¹⁰B (neutron, alpha) ⁷Li reaction, which has a peak value for room temperature neutrons. Brain tumor cells are typically found some 5 to 7 cm below the surface of the skull. After ¹⁰B has been introduced to or entered the tumor cells, a beam of neutrons of energy slightly above room temperature is introduced to the affected areas. The energy of these neutrons is reduced to room temperature by the time they react with the ¹⁰B, which then disintegrates into high energy charged particles (⁷Li and ⁴He), which deposit their kinetic energy in nearby (predominately cancerous) cells and destroys them. Any adjacent normal cells are unaffected [43].

4.4.1 Beryllium isotopes in geochronology

Cosmogenic¹⁰Be and ⁷Be isotopes are produced in the atmosphere, largely by cosmic-ray spallation of nitrogen and oxygen. Because of its relatively short half-life (⁷Be, half-life t_1/2=53 d, compared to that of ¹⁰Be, half-life t_1/2=1.39×10⁶ a, where the unit symbol “d” stands for day and “a” stands for year), measurements of cosmogenic ⁷Be, and especially the isotope-amount ration(⁷Be)/n(¹⁰Be), have been used to study rates of atmospheric circulation, mixing, formation of aerosols (fine solids or liquids suspended in a gas; e.g. smoke and mist are aerosols), and particle deposition [44]. Cosmogenic atmospheric beryllium isotopes (⁷Be and ¹⁰Be) are deposited on the Earth’s surface, where they accumulate in soils, sediments, and snow while decaying away. Measurements of cosmogenic beryllium isotopes in such deposits are used to explore rates of soil formation, erosion, sedimentation, and snow accumulation on time scales ranging from months (⁷Be) to millions of years (¹⁰Be) [45], [46]. The minerals in rocks at the Earth’s surface interact with cosmic rays and form substantial quantities of ¹⁰Be and ⁷Be, thus providing a tool to determine the ages of geologic processes. In some situations, it is possible to estimate “exposure ages” for rocks in eroding terrains [47], [48], [49]. By comparing measured ¹⁰Be concentrations with estimated rates of in situ cosmogenic ¹⁰Be production, the rate of rock erosion and formation of canyons and other geologic features can be determined (Fig. 4.4.1).

Fig. 4.4.1:

Variability in ¹⁰Be production as a result of the interaction of cosmic rays with exposed rocks at three sites on the Level 2 terrace in upper Holtwood Gorge, Pennsylvania, approximately 50 km upstream of Chesapeake Bay [49].

Anthropogenic¹⁰Be was produced by nuclear bomb explosions largely through the reaction of fast neutrons (neutrons produced by nuclear fission having high kinetic energy) with ¹³C via the ¹³C (n, alpha) ¹⁰Be reaction in atmospheric CO₂. Although the quantity of ¹⁰Be produced in this way is small, its presence above natural background concentrations in some environmental samples can potentially provide information about bomb-related processes and contamination [50].

4.5.1 Boron isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of boron possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. Natural terrestrial materials show a substantial variation in boron isotopic abundance (Fig. 4.5.1). The relative abundances of ¹⁰B and ¹¹B have been used in a variety of environmental tracer applications [51], [52]. The isotope-amount ration(¹¹B)/n(¹⁰B) of boron in a water sample depends on the source of the water and region through which the water flows, and it may also be affected by some types of contamination, such as dissolved borate in domestic wastewater. Different water sources may have their own distinct boron isotopic composition, e.g. seawater versus water from continental sources (Fig. 4.5.1).

Fig. 4.5.1:

Variations in atomic weight with isotopic composition of selected boron-bearing materials (modified from [13]).

4.5.2 Boron isotopes in industry

The large value of the absorption cross section of ¹⁰B for thermal neutrons makes this isotope useful for counting neutrons. ¹⁰B is being studied as a potential replacement for ³He in radiation detectors [32], [53], [54]. The large thermal absorption cross section of ¹⁰B makes the isotope useful in control rods (Fig. 4.5.2) [55].

Fig. 4.5.2:

Diagram of a typical pressurized water reactor, which shows where the boron control rods can be inserted or withdrawn from the core (1). (Diagram Source: U.S. Nuclear Regulatory Commission) [55].

4.5.3 Boron isotopes in medicine

¹⁰B has a high thermal neutron absorption cross section and can readily absorb neutrons via the reaction ¹⁰B+n→⁷Li+α. The alpha particles resulting from this reaction carry away a relatively large kinetic energy and are useful for the treatment of malignant tumors in cancer patients [56], [57], [58].

4.6.1 Carbon isotopes in biology

Because of above-ground nuclear bomb testing, the neutrons released reacted with CO₂ to increase atmospheric ¹⁴C via the ¹⁴N (n, p) ¹⁴C reaction, and ¹⁴C started rising in about 1955 (Fig. 4.6.1) and reached a peak in the mid-1960s [59]. With the curtailment of above-ground nuclear testing in the 1960s, the atmospheric ¹⁴C concentration has since been decreasing exponentially (Fig. 4.6.1). This variation in ¹⁴C concentration is used to establish when cells in biology were born and how quickly they are renewed [60]. This technique is commonly called carbon-14 bomb pulse biology and it has provided information on the age of cells and their regeneration. Figure 4.6.2 shows the average age of selected cells in a 30-year-old human.

Fig. 4.6.1:

Global relative average atmospheric ¹⁴C concentration (isotope-amount ration(¹⁴C)/n(¹²C)) between 1950 and 2010 (modified from [59]).

Fig. 4.6.2:

Average age of selected cells in a 30-year-old human, determined with ¹⁴C produced primarily in the 1960s as a result of above-ground, nuclear-weapons testing (figure compiled from published data [60], [61], [62], [63], [64], [65]). This technique commonly is called carbon-14 bomb pulse biology. Occipital neurons in the cortex of the adult human brain are as old as the individual [60]. Lens crystallines are special proteins in the eye lens and are formed almost exclusively at birth with a very small, and decreasing, continuous formation throughout life [65]. Achilles tendon cells do not regenerate after the first 17 years of life [61]. Half of human fat cells are replaced about every 8 years [63].

4.6.2 Carbon isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of carbon possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. Carbon in natural terrestrial materials shows a substantial variation in isotopic abundance (Fig. 4.6.3), providing many different ways of distinguishing sources of materials and processes affecting them [13]. Variations in the isotope-amount ration(¹³C)/n(¹²C) in tree rings and in CO₂ trapped in ice cores have been used to study causes of variations in atmospheric CO₂ levels [66]. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) and in the ¹⁴C concentration of surface ocean waters have been used to trace the incorporation and movement of atmospheric CO₂ in the ocean [66].

Fig. 4.6.3:

Variation in atomic weight with isotopic composition of selected carbon-bearing materials (modified from [13], [17]).

4.6.3 Carbon isotopes in forensic science and anthropology

Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of biological products can be observed using isotope-ratio mass spectrometry (IRMS) to detect adulteration (the addition of inferior ingredients) in honey and other food products.

The isotope-amount ratio n(¹³C)/n(¹²C) can fluctuate between carbon sources, for example C₃ plants (found in temperate climates and which use atmospheric carbon dioxide to make a 3-carbon molecule during photosynthesis — examples include rice, potatoes, tomatoes, and sugar beets), C₄ plants (found in hot climates and which use atmospheric carbon dioxide to make a 4-carbon molecule during photosynthesis — examples include corn and sugar cane), animal carbon, atmospheric CO₂, etc. This commonly makes it possible to detect whether these different carbon sources have been mixed by using isotope or mass balance to distinguish, for example, between beet sugar and cane sugar. Complications in source identification can arise with plants that open stomata at night to collect carbon dioxide to use a third mechanism to fix atmospheric carbon dioxide (CAM or crassulacean acid metabolism). The isotope-amount ratio n(¹³C)/n(¹²C) of CAM plants overlaps that of C₃ or C₄ plants — examples include pineapples and jade plants. The following adulterations are commonly detected using stable carbon isotope IRMS:

1. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of honey are used to detect the addition (and potential adulteration) of high fructose corn syrup, corn, or sugar cane [67].

2. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of fruit juice have been used to detect the addition of a sugar [67].

3. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of natural vanilla extract have been used to detect the addition of artificial vanillin or p-hydroxybenzaldehyde [67].

4. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of beer are used to detect C₄ carbon, which would indicate that a beer company may have added ingredients that are not traditionally used in brewing beer. Therefore, this ratio is used to detect the misrepresentation of a product as being pure [67], [68].

Stable carbon IRMS has been used to determine if the botanical origin of an alcoholic spirit has been mislabeled and if chaptalization (the process of adding sugar to increase the alcoholic content) of wine has occurred [67], [68]. ¹⁴C scintillation counting has been used to determine the age of wine and alcoholic spirits [67], [68]. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of urine has been used to determine if steroids in urine are natural or of synthetic origin. These measurements enable anti-doping laboratories to perfect their methods for detecting steroid doping in athletes [69], [70], [71]. Variations in the isotope-amount ratio n(¹³C)/n(¹²C) of marijuana can provide information to determine if the plants were grown “inside” a building or greenhouse or were “open grown” (Fig. 4.6.4). Plant carbon isotopic compositions are controlled by atmospheric CO₂ and the supply and demand of CO₂ in photosynthesis (the process used by plants to convert light energy from the sun into chemical energy). “Open grown” plants are grown in an area that is well ventilated and receives natural CO₂. In contrast, plants grown “inside” receive supplemented CO₂ and the photosynthesis process is more confined. Additionally, CO₂ from a tank of compressed gas used to augment atmospheric CO₂ to increase the growth of marijuana plants is commonly highly depleted in ¹³C as a refinery by-product. These differences change the carbon isotope ratios of the plants and the ratios vary enough to enable the determination of the growing and cultivation process of marijuana [72], [73].

Fig. 4.6.4:

Variations in the isotope-amount ration(¹³C)/n(¹²C) of marijuana have been used to determine if the plants were grown inside a building or greenhouse or were “open grown.” (Image Source: U.S. Drug Enforcement Administration and U.S. Department of Justice) [74].

4.6.4 Carbon isotopes in geochronology

Radioactive ¹⁴C is the basis for the radiocarbon dating method to determine the ages of carbon-bearing materials. ¹⁴C is formed naturally in the atmosphere by cosmic-ray interactions and was also released by above-ground, nuclear weapons testing (Fig. 4.6.1). Atmospheric ¹⁴C is incorporated into plants, animals, soils, groundwater, and ocean water, and it decays with a half-life of ~5700 years. This makes it useful for dating objects, such as archaeological remains and water masses in oceans and aquifers, on time scales ranging from hundreds of years to tens of thousands of years [15]. Plants and animals living since the 1950s can be identified by bomb-peak ¹⁴C in their cells (see Section 4.6.1).

4.6.5 Carbon isotopes in medicine

¹⁴C is used to create isotopically labeled drugs to study their uptake and metabolism in humans [75], [76], [77]. ¹³C is used in breath tests to detect Helicobacter pylori bacteria (bacteria in the stomach linked to ulcers), which can cause cancers [78].

4.7.1 Nitrogen isotopes in biology

Isotopic fractionation can cause the isotope-amount ration(¹⁵N)/n(¹⁴N) to increase systematically through food chains through assimilation of nitrogen compounds in biomolecules such as proteins. When lower-order organisms are ingested by higher-order organisms, ¹⁵N may be selectively retained and ¹⁴N may be selectively excreted such that higher-order organisms tend to have higher n(¹⁵N)/n(¹⁴N) ratios than their food sources. Isotopic fractionation occurs as a result of assimilation, storage, and excretion of proteins and other nitrogen compounds. Biologists can use isotope-amount ratio n(¹⁵N)/n(¹⁴N) measurements to test hypotheses about predator-prey relations and detect disruptions to trophic structure of ecosystems that might be caused by toxic contaminants, invasive species, or harvesting of organisms. Similar principles are used to detect differences in diets among animals, including humans, both today and in the distant past [79], [80], [81].

Artificially enriched ¹⁵N tracers are used to study movement and transformation of nitrogen in biological and environmental systems, such as the uptake and loss of nitrogen fertilizers by crops (Fig. 4.7.1). A common experiment involves introducing an isotopically labeled compound into the environment and then analyzing various samples taken from the environment for the presence of the enriched isotope to determine where the labeled compound moved and whether it transformed into other compounds (Fig. 4.7.2). Artificially enriched ¹⁵N is used to study uptake and dispersal of nitrogen in feed supplies used in food production industries such as aquaculture [82].

Fig. 4.7.1:

Variation in nitrogen stable isotopes has been used to track fertilizer nitrogen into plants, soils, and infiltrating groundwater in experiments to improve agricultural efficiency and reduce impacts on the environment. This aerial photograph shows experimental agricultural fields where different amounts of excess nitrogen from fertilizer and plant residues can be found in groundwater. (Photo Source: Böhlke, J.K., U.S. Geological Survey).

Fig. 4.7.2:

Tracer experiments with the stable isotope¹⁵N have been used to track excess dissolved nitrate in groundwater and streams and to determine to what extent the dissolved nitrate is removed by natural processes, such as conversion to harmless N₂ gas before entering nitrogen-sensitive ecosystems [83]. (Photo Source: Böhlke, J.K., U.S. Geological Survey).

4.7.2 Nitrogen isotopes in Earth/planetary science

The stable isotopes of nitrogen are subject to isotopic fractionation by physical, chemical, and biological processes. Variations in the isotope-amount ratio n(¹⁵N)/n(¹⁴N) are substantial (Fig. 4.7.3) and commonly are used to study Earth-system processes, especially those related to biology because nitrogen is a major nutrient for growth [84]. For example, isotope fractionation occurs when dissolved solutes, such as nitrate (NO₃^-), are transformed to more reduced compounds (i.e. nitrogen gas) because nitrate with higher ¹⁴N abundances tends to be more readily broken down. This leaves the residual unreacted nitrate with a higher n(¹⁵N)/n(¹⁴N) ratio than the initial ratio prior to reaction. Changes in the isotopic composition of biologically reactive compounds can be used to detect such reactions in aquatic environments, which are important mechanisms for removing reactive contaminants like nitrate [85], [86].

Fig. 4.7.3:

Variation in atomic weight with isotopic composition of selected nitrogen-bearing materials (modified from [13], [17]).

Variations in the isotope-amount ratio n(¹⁵N)/n(¹⁴N) are used to determine sources of nitrogen contamination in the atmosphere, oceans, groundwater, and rivers, where the isotopic composition of a contaminant molecule preserves evidence of the nitrogen sources and processes involved in its creation. An example is nitrate derived from artificial fertilizer, manure, power-plant emissions, or natural sources [87], [88], [89].

Artificially enriched ¹⁵N tracers have been used to determine rates of movement and natural remediation of nitrogen-bearing contaminants in aquifers and rivers [83], [90].

4.7.3 Nitrogen isotopes in forensic science and anthropology

Stable hydrogen, carbon, and nitrogen isotopic compositions are used to determine the origin of pseudoephedrine from seized methyl-amphetamine made from the pseudoephedrine (drug used as a nasal decongestant or as a stimulant) [91].

4.8.1 Oxygen isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of oxygen possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of oxygen in natural terrestrial materials (Fig. 4.8.1). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [13].

Fig. 4.8.1:

Variation in atomic weight with isotopic composition of selected oxygen-bearing materials (modified from [13], [17]).

A primary use of stable oxygen isotopes is in isotope hydrology. Although the evolution of the stable hydrogen and oxygen isotopic composition of precipitation begins with the evaporation of water from the oceans, their local and global relationship arises primarily from equilibrium isotopic fractionation of heavier (²H and ¹⁸O) and lighter isotopes (¹H and ¹⁶O) of hydrogen and oxygen during condensation as a tropospheric vapor mass follows a trajectory to higher latitudes and over continents [14], [15]. As a consequence, the isotopic composition and atomic weight of oxygen in precipitation, rivers, and tap waters varies with elevation, season, and distance from the ocean-continent boundary. Figure 4.8.2 shows the variation in stable oxygen isotopic composition of water from rivers across the United States. These variations in oxygen isotopic composition of environmental water are often combined with hydrogen isotopic compositions and have been used to identify the origin of water and to investigate the interaction between groundwater and surface water (e.g. lakes, streams, and rivers) [16].

Fig. 4.8.2:

Variation in atomic weight of oxygen in river waters across the continental United States (modified from [16]). Blue color indicates waters most depleted in ¹⁸O (resulting in lower atomic weight of oxygen) and brown color indicates those most enriched in ¹⁸O (resulting in higher atomic weight of oxygen).

4.8.2 Oxygen isotopes in forensic science and anthropology

Measurements of relative ¹⁸O abundances have been used to determine the breeding grounds of many species of migrant songbirds. These species of songbirds only grow their feathers before migration, and they grow them on or close to their breeding grounds. Therefore, the isotopic composition of a bird’s feathers correlates to the isotopic signature of the growing season’s precipitation [19], [20].

Measurements of relative ¹⁸O abundances of human hair or nail samples collected at archeological sites have been used to determine the geographic region in which a subject lived based on the oxygen isotopic composition of the water they drank (Fig. 4.8.3). This is possible because hair stores a daily record of oxygen isotopic composition of intake water, which correlates to local meteoric water [92].

Fig. 4.8.3:

Cross plot of mole fractions of ²H and ¹⁸O of human nail samples from a variety of global sites (modified from [93]). The hydrogen and oxygen isotopic compositions reflect the oxygen and hydrogen isotopic compositions of water consumed, and generally they decrease with increasing latitude, increasing elevation, and distance inland from the ocean-continent boundary [14], [15].

4.8.3 Oxygen isotopes in medicine

¹⁶O is used to produce radioactive ¹³N via the ¹⁶O (p, ⁴He) ¹³N reaction for imaging in positron emission tomography (PET) and to study blood flow through the heart (myocardial perfusion) [94], [95].

¹⁷O has been used as a tracer to study cerebral oxygen utilization [96]. Variations in stable oxygen and hydrogen isotopes are used in energy expenditure studies in animals and humans. The subject is administered a dose of doubly labeled water (water enriched in both ²H and ¹⁸O). Measurements of the elimination rates of ²H and ¹⁸O in the subject over time through regular sampling of body water (by sampling saliva, urine, or blood) provide information on energy expenditure because the hydrogen isotopic composition of body water is affected primarily by water loss (mainly urination), but the oxygen isotopic composition is affected by both respiration and water loss [97].

4.9.1 Fluorine isotopes in medicine

¹⁸F is a radioactive fluorine isotope that is used in an ¹⁸F-FDG compound (¹⁸F-labeled, fluoro-deoxy glucose) for imaging the organs, bones, tissues, and brain of the body with a technique called positron emission topography (PET). The ¹⁸F-FDG compound is injected and the isotopically labeled glucose is consumed by any cell requiring glucose as a source of energy [98], [99].

1. ¹⁸F emits positrons that collect in tissue and interact with regular negative electrons when injected into the body. The positrons and electrons annihilate each other, producing two gamma rays that are emitted in opposite directions. The radiation is detected on a PET camera, which generates a picture of the body part being examined (Fig. 4.9.1).

2. Because ¹⁸F has a short half-life of about 110 min, there is little chance of radiation damage to the patient.

Fig. 4.9.1:

An ¹⁸F-FDG PET scan is used to observe the differences in brain activity between a sober and an intoxicated brain. (Image Source: National Institute on Alcohol Abuse and Alcoholism (NIAAA)) [100].

4.10.1 Neon isotopes in Earth/planetary science

Neon is subject to stable isotopic fractionation by physical processes, such as exchange between gas, liquid, and solid phases. Small variations in the isotope-amount ration(²²Ne)/n(²⁰Ne) have been used to examine gas-liquid exchange processes during groundwater recharge (water moving downward from the surface) and discharge [29], [101], [102].

4.10.2 Neon isotopes in geochronology

Some ²¹Ne and ²²Ne form naturally in the Earth’s crust largely by reactions of ¹⁸O and ¹⁹F in minerals with neutrons and alpha particles emitted from uranium and thorium decay, called nucleogenic neon isotopes [29], [101]. In addition, neon isotopes can form at the surface of the Earth and in extraterrestrial bodies by cosmic-ray-induced spallation reactions on magnesium, silicon, aluminum, and sodium [103], [104]. Analyses of all three stable neon isotopes may be used to distinguish these sources from primordial neon. The relative amounts of atmospheric neon and crustal nucleogenic neon isotopes in deep groundwaters and natural gases have been used in studies of solid-water-gas interactions and migration (Fig. 4.10.1). The cosmogenic component is mainly detected in ²¹Ne and can be used to determine cosmic-ray exposure ages of rock samples, including meteorites exposed during travel through space and boulders exposed by melting of glacial ice (Fig. 4.10.1).

Fig. 4.10.1:

Neon-isotope ratios of water from fractures in quartzite (open diamonds) and water from fractures in vein quartz (solid circles) from the deep gold mines of the Witwatersrand Basin, South Africa [105]. The isotope-amount ration(²¹Ne)/n(²²Ne) depends upon the amount ratio of oxygen to fluorine in the ~40-μm reaction range of alpha particles from uranium and thorium. Lippmann-Pipke et al. [105] show that the neon end-member represents a fluorine-depleted fluid component that was trapped in fluid inclusions in vein quartz more than 2×10⁹ years ago.

4.10.3 Neon isotopes in industry

Masers (Microwave Amplification by Stimulated Emission of Radiation) containing ²⁰Ne have been used to study quantum physics. ²¹Ne may also play a role in maser studies of quantum physics [106].

4.10.4 Neon isotopes used as a source of radioactive isotope(s)

²²Ne is used to produce the radioisotope²²Na via the reaction ²²Ne (p, n) ²²Na [107]. ²⁰Ne has been used to produce the radioisotope ¹⁸F via the reaction ²⁰Ne (d, ⁴He) ¹⁸F [107].

4.11.1 Sodium isotopes in biology

Both ²²Na and ²⁴Na have been used as radioactive tracers to study electrolytes in the human body [108], [109], [110].

4.11.2 Sodium isotopes in geochronology

²²Na is a cosmogenic isotope with a half-life of 2.6 years that has been used to study the residence time of water in freshwater basins. It has been used for dating of young (up to a few decades old) surface water and groundwater (Fig. 4.11.1) [111].

Fig. 4.11.1:

Variation of ²²Na atmospheric fallout rate at St. Petersburg, Russia (modified from [111]); note that an abbreviation “yr” is used instead of the unit symbol “a” for year. This variation makes ²²Na useful for determining residence time of lakes and other surface water bodies.

4.11.3 Sodium isotopes in medicine

²²Na is used as a source to calibrate positron emission tomography (PET) imaging scanners to check that the instruments are functioning properly [112].

4.12.1 Magnesium isotopes in biology

Natural magnesium enriched in the stable isotopes²⁵Mg and ²⁶Mg has been used as tracers in human studies to assess absorption, excretion, distribution, and utilization of magnesium in basic and applied research [108], [113], [114].

4.12.2 Magnesium isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of magnesium possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of magnesium in natural terrestrial materials (Fig. 4.12.1). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [13], [17], [115].

Fig. 4.12.1:

Variation in atomic weight with isotopic composition of selected magnesium-bearing materials (modified from [13], [17]).

4.12.3 Magnesium isotopes in geochronology

²⁶Mg is a stable isotope and is the radiogenic product of ²⁶Al decay. ²⁶Al is produced by cosmic rays in space and in the atmosphere, and it was present in the primordial solar nebula. The anomalous abundance of ²⁶Mg in meteorite inclusions indicate that this material must have been formed early in the development of the Solar System before all primordial ²⁶Al (with half-life of 7.1×10⁵ years) had decayed [116].

4.13.1 Aluminium isotopes in biology

²⁶Al is a radioactive isotope (half-life of 7.1×10⁵ years) that can be detected at the ultra-trace level (attogram range; 10⁻¹⁸ g levels) using accelerator mass spectrometry. ²⁶Al is used as a tracer to study the uptake, distribution, and retention of aluminium in plants, animals, and humans under different physiological conditions [117], [118].

4.13.2 Aluminium isotopes in geochronology

²⁶Al is produced from spallation reactions of protons, produced by cosmic rays, on argon. ²⁶Al has been used for dating geological samples, such as marine sediments, manganese nodules, rocks, and meteorites [119], [120]. The abundances of ²⁶Al to ¹⁰Be have been used to study erosion and transport of soil and sediments on a thousand- to million-year time scale, because production rates of ²⁶Al to ¹⁰Be are greatest at the surface and decrease exponentially with depth (Fig. 4.13.1) [121], [122].

Fig. 4.13.1:

²⁶Al and ¹⁰Be content in forty sediment samples from eastern Mojave Desert, California (modified from [121]). These results enabled investigators to determine that sediment moves down the piedmont in an active transport layer, which is 20 to 30 cm thick.

Intense cosmic-ray bombardment in space produces ²⁶Al in meteorites and other bodies, such as the Moon. After a meteorite falls to Earth, ²⁶Al production ceases due to atmospheric shielding; the decay of ²⁶Al to ²⁶Mg has been used to determine the terrestrial age of a meteorite (i.e. the time elapsed since the meteorite fell to Earth) [119].

4.14.1 Silicon isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of silicon possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of silicon in natural terrestrial materials (Fig. 4.14.1). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [13], [17]. Diatoms, a major group of algae, need silicon to build up their opaline shells and prefer ²⁸Si while taking up Si(OH)₄, which is the biologically available form of silicon in the marine environment. This progressively enriches surface waters with ²⁹Si and ³⁰Si [123]. ³²Si-labeled silicic acid of high specific radioactivity is used to measure uptake rates of Si and estimate marine sedimentation of biogenic (created by living organisms) silica (by diatoms and sea shells). By performing uptake kinetic experiments, the ³²Si activity can be measured as ³²P using counting of Cherenkov radiation (radiation produced by charged particles passing through a medium at a speed greater than that of light through the same medium — after Soviet physicist Pavel A. Cherenkov) with a liquid scintillation analyzer (measuring ionizing radiation using the interaction of radiation on a material and counting the resulting photon emissions).

Fig. 4.14.1 :

Variation in atomic weight with isotopic composition of selected silicon-bearing materials (modified from [13], [17]).

4.14.2 Silicon isotopes in geochronology

Cosmogenic³²Si has a half-life of about 150 years and is produced by cosmic-ray spallation of argon in the stratosphere and troposphere [124]. ³²Si in dust is precipitated in snow, making it possible to date dust in snow and glacial ice (Fig. 4.14.2). Glaciers are archives for global climate history because they contain a variety of proxies (imprints of past environmental conditions used to interpret paleoclimate) for climate forcing and climate response. Cosmogenic ³²Si that is stored in glaciers and ice-core samples can be analyzed using accelerator mass spectrometry to date when sections of glaciers formed [125], [126].

Fig. 4.14.2:

³²Si activity concentrations in three snow samples from Jungfraujoch, a glacial pass in the Bernese Alps at an elevation of approximately 3.5 km above sea level (modified from [127]). Sample Jungfraujoch 2 contains Saharan dust and has a substantially higher concentration of ³²Si than snow samples not containing Saharan dust.

4.14.3 Silicon isotopes in industry

At Keio University in Japan, the Itoh Research Group has developed a method that utilizes ²⁹Si to store and process information. The Itoh Research Group focused on manipulating the nanostructure of materials at an atomic level, especially with semiconductors such as silicon. Their manipulations and observations demonstrate that differences in the nuclear spin and mass of an isotope affects the ease of further manipulation of the isotope [128], [129].

Silicon crystals enriched to higher than 99.99 percent purity of ²⁸Si are being used in the Avogadro Project. This project is intended to remeasure the Avogadro constant (N_A), which is the proportionality factor between the amount of substance and number of elementary entities [130].

4.15.1 Phosphorus isotopes in biology

³²P (half-life of 14.3 days) is a radioactive isotope of phosphorus that is used to help understand the biological and chemical processes in plants. It is chemically identical to other isotopes of phosphorous and can be substituted in biological and chemical reactions. For example, a phosphate solution containing ³²P (which has the identical behavior of non-radioactive ³¹P) can be inserted into the roots of a plant and its movement can then be tracked throughout the plant with the use of a Geiger counter. This movement detection study helps scientists to better understand how plants use phosphorous to reproduce and grow [131], [132].

At the molecular level, ³²P can substitute for ³¹P in nucleotides of DNA or RNA (ribonucleic acid, a single stranded molecule that regulates genes). Radioactive probes can be created to help identify the presence, absence, and quantity of genes in a system [133], [134].

4.15.2 Phosphorus isotopes in Earth/planetary science

³²P has been used as a tracer to help determine phosphorus nutrient cycling in eutrophied lakes (lakes rich in organic and mineral nutrients commonly leading to the excessive growth of phytoplankton, a self-feeding water organism) (Fig. 4.15.1). In one experiment, phosphoric acid labeled with ³²P was added to a lake that had been experimentally eutrophied. ³²P was measured in microphytoplankton (plankton visible only with a microscope), phytoplankton, and zooplankton (tiny animals that live suspended in fresh or salt water), and the amount of incorporated ³²P was determined [132].

Fig. 4.15.1:

Partitioning of ³²P among water layers, the sediments, and outflow during the 105 days following addition of ³²P to the upper layer of stratified Lake 227 (northwestern Ontario) to trace the lake’s phosphorus cycle during lake stratification and fall overturn (modified from [132]).

³³P has been used to better understand phosphorus dynamics in the environment at the sediment-surface level. Phosphorus is a necessary nutrient for many biota (the plant and animal life of a particular habitat, region, or geological period). Understanding bioavailability and sorption (bonding) of this nutrient to particles in soil is important for understanding ecosystem health. Organic and inorganic phosphorus substrates isotopically labeled with ³³P can be tracked within a sediment system to determine their transport properties and availability to biota [135].

4.15.3 Phosphorus isotopes in industry

³²P was added to tires in the 1950s by Goodrich Laboratories to help determine the location and depth of tire wear in performance tests [136].

4.15.4 Phosphorus isotopes in medicine

Beta emissions from the radioactive isotope ³²P can be used in drug therapy of cancerous bone masses. By injecting a patient with a ³²P pharmaceutical, tumors and other cells can be targeted for cell death, which also helps to alleviate pain [137], [138]. For example, Polycythemia vera is the condition of having excess red blood cells in the bone marrow: ³²P can be used to treat this condition by reducing the number of red blood cells. However, there is no cure for this condition [139]. Using a ³²P labeled bio-silicone product, ³²P has been used as the radioactive target in brachytherapy of solid tumors in the lung [140]. Depending on the type of ³²P-labeled compound (antibody or pharmaceutical drug), when it is ingested or injected into the body, specific body parts (blood, tumors, joints, or bones) can be targeted for visualization and imaged using a gamma camera. This is useful for imaging cancer sites and for treatment monitoring of oncologic patients [133], [134], [138].

4.16.1 Sulfur isotopes in biology

The stable sulfur isotope-amount ration(³⁴S)/n(³²S) has been used to distinguish whether animal tissues grew in freshwater or in marine ecosystems. The isotopes do not fractionate (separate) substantially with trophic influences (the movement of sulfur through and into plant and animal systems), and the isotope-amount ratio n(³⁴S)/n(³²S) is usually substantially different between freshwater and marine environments. As an example, by analyzing sulfur isotope-amount ratios in bird feathers, the environment in which the bird was living when these feathers developed can be determined. This enables one to track bird habitats and migration patterns throughout the year (Fig. 4.16.1) [141].

Fig. 4.16.1:

Variation in atomic weight with isotopic composition of selected sulfur-bearing materials (modified from [13], [17]).

4.16.2 Sulfur isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of sulfur possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of sulfur in natural terrestrial materials (Fig. 4.16.2). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [13], [17]. The isotope-amount ratio n(³⁴S)/n(³²S) can be used to trace natural and anthropogenic sources of sulfur. Examples include studies of acid mine drainage, the cycling of sulfur in agricultural watersheds, groundwater contamination from landfills, and sources of salinity in coastal aquifers [142], [143], [144].

Fig. 4.16.2:

Sulfur isotopic abundances of Tianyuan 1 early modern human found in Eurasia, three terrestrial animals from Tianyuan Cave (Tianyuandong) in the Zhoukoudian region of China, and two fish from Donghulin (modified from [146]). Based on sulfur, carbon, and nitrogen isotopic analyses of bones from the early modern human and the associated animals in Tianyuan Cave and the Donghulin site, Hu et al. [146] conclude that the human most likely obtained a substantial portion of its protein from a freshwater ecosystem, probably from freshwater fish.

4.16.3 Sulfur isotopes in forensic science and anthropology

The isotope-amount ratio n(³⁴S)/n(³²S) can be used to authenticate the dietary source of cattle. First, stable isotopes are measured to infer the dietary source of the cattle. Once the source of the diet is found, the isotopic compositions can be traced in certain muscle groups of the cattle and can be used to determine if the diet of the animal has been changed or if the feed is consistent with what the animal has been claimed to have been fed [145].

4.16.4 Sulfur isotopes in geochronology

³⁵S has a half-life of 87 days, which is an ideal duration for use as a conservative tracer in atmospheric processes. ³⁵SO₂ gas is produced as a natural product of argon exposure to cosmic rays in the atmosphere. Because ³⁵SO₂ gas is present in the atmosphere and then precipitates and falls as moisture in the form of ³⁵SO₄^2-, ³⁵S can act as a tracer to study air mass transport dynamics and atmospheric oxidation capacity [147]. Analyses of ³⁵S in lake water and precipitation can also be used as a tracer to monitor contributions of sulfur that originated in precipitation to surface waters. If a water tests positive for the isotope ³⁵S, it provides evidence that the water had been affected by recent (<~1 year) precipitation [148], [149], [150]. ³⁵S is used in direct labeling of elemental sulfur or sulfate sources to trace the fate of sulfur in fertilizers [142].

4.17.1 Chlorine isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of chlorine possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of chlorine in natural terrestrial materials (Fig. 4.17.1). These variations are useful for investigating the origin of substances and studying environmental, hydrological, and geological processes. Chlorine is subject to isotopic fractionation by physical and chemical processes. Variations in isotopic compositions of stable chlorine isotopes provide evidence for ultrafiltration and crystallization of brines and indicate sources of chlorine-bearing contaminants, such as solvents and rocket fuels, in the environment [151], [152].

Fig. 4.17.1:

Variation in atomic weight with isotopic composition of selected chlorine-bearing materials (modified from [13], [17]).

4.17.2 Chlorine isotopes in forensic science and anthropology

Analyses of chlorine isotopes and other environmental tracers can help to identify whether an environmental contaminant is of anthropogenic origin or naturally occurring. For example, perchlorate (ClO₄^-) can be of anthropogenic origin and is also found naturally. Perchlorate is a widespread groundwater contaminant that can interfere with hormone production in the thyroid gland by displacing iodide. Both the stable chlorine isotope-amount ration(³⁷Cl)/n(³⁵Cl) and the mole fraction of ³⁶Cl, n(³⁶Cl)/n(Cl), can provide useful information about origins of perchlorate in the environment (Fig. 4.17.2). Such information may be important for legal reasons and for remediation of contaminated areas [152], [153].

Fig. 4.17.2:

By analyzing the isotopic composition of chlorine and oxygen in perchlorate in groundwaters of Long Island, NY, sources of perchlorate contamination could be identified [154]. Isotopic compositions indicate that wells in different parts of Long Island were contaminated by different sources. The agriculture source of perchlorate (upper photo) is identified as nitrate fertilizer from Chile, where natural perchlorate-bearing nitrate salt deposits were mined and processed for export. The synthetic source is attributed to contamination from a fireworks disposal area (lower photo). (Image Source: J.K. Böhlke, U.S. Geological Survey).

4.17.3 Chlorine isotopes in geochronology

Radioactive ³⁶Cl provides a useful tool to determine ages in geology and hydrology. Some radioactive ³⁶Cl is cosmogenic and enters the terrestrial environment in precipitation. Because of its long half-life of 3.01×10⁵ years, the level of ³⁶Cl in aquifers can be measured and used to estimate ages (on the order of 10⁵ to 10⁶ years) of old meteoric groundwater (water that was originally precipitation) [155].

Thermonuclear bomb tests in the ocean produced large amounts of ³⁶Cl by neutron reactions with ³⁵Cl in seawater. This was especially prevalent in the late 1950s. Large amounts of this anthropogenic ³⁶Cl were distributed throughout the atmosphere, deposited with precipitation, and incorporated into terrestrial soils and groundwater. This enriched ³⁶Cl has been used as a tracer of meteoric water from that era [156].

4.18.1 Argon isotopes in Earth/planetary science

Argon’s chemically inert properties and three stable isotopes make it an ideal tracer of Earth processes [101], [157], [158], [159], [160], [161], [162], [163], [164], [165], [166], [167]. Measurements and models of the isotope-amount ration(⁴⁰Ar)/n(³⁶Ar) can provide insights about the evolution of the atmosphere and orogenic (mountain-building) history of the Earth. The comparison of results from potassium-argon and n(⁴⁰Ar)/n(³⁹Ar) isotope-amount-ratio dating methods with results from other dating methods has been used to study temperature histories of rocks through differences in apparent ages caused by excess argon or partial argon gas loss. The isotope-amount ratio n(⁴⁰Ar)/n(³⁶Ar) of dissolved argon in groundwater can provide hydrologic information, such as rates of crustal degassing and relative groundwater age. ³⁸Ar produced by cosmic-ray bombardment of rocks and soils at Earth’s surface can provide information about surface exposure history and erosion rate.

4.18.2 Argon isotopes in geochronology

Argon isotopes are used to date rock samples, especially volcanic rocks, using two related techniques (Fig. 4.18.1) [101], [168], [169], [170].

Fig. 4.18.1:

Studying the ratios of argon isotopes can provide insight into the origins and movement of magma and the ages of volcanic rock. ⁴⁰Ar begins increasing in concentration once lava has solidified. (Image Source: U.S. Geological Survey Hawaiian Volcano Observatory, Kilauea) [174].

1. The first technique is potassium-argon dating (K-Ar), which is based on the decay of radioactive ⁴⁰K to stable ⁴⁰Ar. By comparing the concentrations of potassium and ⁴⁰Ar in a sample, it is possible to determine how long the sample has been accumulating radiogenic⁴⁰Ar to determine the “age” of the sample. The half-life of ⁴⁰K is approximately 1.25×10⁹ years, making this a useful tool for dating rocks range in age from about 10⁶ to 10⁹ years.

2. A modification of the potassium-argon dating technique is the n(⁴⁰Ar)/n(³⁹Ar) isotope-amount-ratio technique, in which a sample is irradiated in a nuclear reactor to produce ³⁹Ar from ³⁹K. The isotope-amount ratio n(⁴⁰Ar)/n(³⁹Ar) is then determined, and from this, the approximate age of the rock can be calculated (Fig. 4.18.2).

Fig. 4.18.2:

The U.S. Geological Survey ⁴⁰Ar/³⁹Ar geochronology laboratory in Denver, Colorado uses a custom-built argon extraction line connected to a Mass Analyzer Products (MAP) 215-50 mass spectrometer with a differentially pumped dual laser setup. (U.S. Geological Survey ⁴⁰Ar/³⁹Ar Geochronology Laboratory) [175].

The study of ³⁷Ar (half-life of 35 days), ³⁹Ar (half-life of 268 years), and ⁴⁰Ar concentrations in groundwater can provide information about the production and release of these isotopes from rocks and other sources into groundwater and the relative ages of different groundwaters [159], [164], [165], [171], [172], [173].

4.18.3 Argon isotopes in industry

³⁸K (half-life of 7.6 min), which is produced by the reactions ³⁸Ar (p, n) ³⁸K and ⁴⁰Ar (n, 3n) ³⁸K, is a widely used blood-flow tracer. Because ³⁸Ar is more expensive, ⁴⁰Ar, which also offers many additional advantages as a target, is more commonly used to produce ³⁸K for medical purposes [176], [177]. ⁴¹Ar (half-life of 1.82 h) is used as an industrial gas-flow tracer to help track the movement of gases because its inert properties, half-life, and gamma radiation make it well suited for this purpose [177].

4.19.1 Potassium isotopes in biology

The mole fraction of ⁴⁰K, n(⁴⁰K)/n(K), is used to study the effects of potassium in soil on the growth of plants. Plants need potassium to promote growth and reproduction, and potassium also helps plants resist drought and diseases. The mole fraction of ⁴⁰K is being studied at different depths in several soil types to determine how soil properties affect the fractionation of ⁴⁰K [178].

4.19.2 Potassium isotopes in geochronology

The amount ration(⁴⁰K)/n(⁴⁰Ar) is used in potassium-argon dating by geologists, archaeologists, and paleoanthropologists to determine the age of rocks. This dating method is based on the radioactive decay of ⁴⁰K, having a half-life of 1.248×10⁹ years, to ⁴⁰Ar. When lava crystalizes, ⁴⁰Ar can no longer escape and begins increasing in concentration in a rock (Fig. 4.19.1) [179], [180].

Fig. 4.19.1:

Deeper, older igneous rocks will have a higher ⁴⁰Ar concentration than younger igneous rock, and this technique requires rocks older than 1×10⁵ years in order that sufficient ⁴⁰Ar has accumulated.

4.19.3 Potassium isotopes in medicine

³⁸K, which has a half-life of 7.6 min and is produced by a nuclear reaction involving ³⁸Ar and ⁴⁰Ar as targets, is a widely used blood-flow tracer. Because ³⁸Ar is more expensive, ⁴⁰Ar, which also offers many additional advantages as a target, is more commonly used to produce ³⁸K for medical purposes [75], [176], [181].

4.20.1 Calcium isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of calcium possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights (Fig. 4.20.1). The isotope-amount ration(⁴⁴Ca)/n(⁴⁰Ca) is used to quantify the calcium cycle (sources and sinks of calcium) in the ocean. Calcium isotopes fractionate (separate) in terrestrial and marine environments owing to biological and inorganic processes, which discriminate against heavy calcium isotopes. The calcification process controls the removal of calcium from the ocean, which is mostly balanced by hydrothermal and riverine calcium input. Calcium has a long residence time, symbol τ, in seawater (τ_Ca about 1 to 2 million years) relative to the short mixing time of the global ocean (about 1000 years), which has allowed the calcium isotopic composition of modern seawater to homogenize globally. This was likely the case in the geological past as well, which makes the n(⁴⁴Ca)/n(⁴⁰Ca) ratio useful when quantifying the oceanic calcium cycle [182], [183]. The isotope-amount ratio n(⁴⁴Ca)/n(⁴⁰Ca) has been used to trace sources of calcium in soil and river water [184]. The isotope-amount ratio n(⁴⁴Ca)/n(⁴⁰Ca) ratio of calcium carbonate may serve as a paleothermometer to determine seawater temperatures in the past, making use of the temperature-dependent isotopic fractionation between ⁴⁰Ca and ⁴⁴Ca [185], [186].

Fig. 4.20.1:

Variation in atomic weight with isotopic composition of selected calcium-bearing materials (modified from [17]).

The radioactive isotope⁴⁵Ca (half-life of 163 days) is used to study calcium behavior in soils, detergents, water-purification systems, and glassy materials. ⁴⁵Ca is introduced into a system and monitored to measure various types of calcium responses within the system and to investigate how calcium of one matrix may interact with another (i.e. calcium of soil mixing with that of fertilizers). ⁴⁵Ca has been used to investigate the transport of contaminants in groundwater through the unsaturated zone [187].

4.20.2 Calcium isotopes in medicine

Stable isotopes of calcium (⁴²Ca, ⁴⁴Ca, ⁴⁶Ca, and ⁴⁸Ca) and radioisotopes of calcium (⁴⁵Ca and ⁴⁷Ca, with a half-life of 109 h) can be used for tracing calcium uptake, utilization, and excretion in the body. For example, most of our knowledge on the efficiency by which calcium is absorbed in the intestine (bioavailability) comes from studies in which calcium in the diet was labeled with stable or radioactive isotopes. In such studies, the isotope-labeled food is ingested and fecal matter tested for the presence and quantity of unabsorbed isotope. When coupling oral ingestion of food labeled with one calcium isotope with an intravenous injection of a second calcium isotope, this technique can be used as a means to measure calcium absorption within the body by measuring excretion of both tracers in the urine. In a similar fashion, dietary absorption of magnesium and zinc can be studied [184], [188].

Stable and radioactive isotopes are used in biomedical research and clinical practice to study disorders associated with calcium metabolism, in particular in relation to bone health and calcium accumulation in body tissues (vascular calcification, kidney stone formation). Stable isotope tracers have been used successfully to study bone calcium balance during space-flight and in-bed-rest studies. A long-living calcium radioisotope (⁴¹Ca), with a half-life of 9.9×10⁴ years, has been used successfully for labeling of bone calcium to measure bone calcium turnover via urinary excretion of the tracer [189].

4.21.1 Scandium isotopes in biology

Radioactive ⁴⁶Sc is used as a non-absorbed isotopic reference material for determining digestibility, absorption in the gut, and secretion sites for nutrients associated with feed residues in ruminating animals (animals that chew their food repeatedly for an extended period of time) [190].

4.21.2 Scandium isotopes in Earth/planetary science

The radioactive isotope⁴⁶Sc has been used for sediment labeling to determine the transportation of sediments by water flow in rivers, estuaries, harbors, and seas. The half-life of ⁴⁶Sc is about 84 days and when released into an estuary with similar grain density and grain size, a gamma spectrometer (instrument for measuring the intensity of gamma radiation versus the energy of each photon) can be used to measure the intensities of ⁴⁶Sc in the sediments and the movement of the sediments can be determined [191], [192], [193].

4.21.3 Scandium isotopes in industry

⁴⁶Sc is a beta emitter and has been used as a tracer in oil refinery crackers for crude oil (converting crude oil into gasoline and other lower-molecular weight hydrocarbon fractions). Its beta radiation enables the substance to be tracked as the oil travels [194]. Due to its easily traceable properties, coastal engineers use ⁴⁶Sc to develop dredging strategies and to design navigation channels based on silt movement [192].

4.21.4 Scandium isotopes in medicine

⁴⁶Sc is used in isotope-carrying antibodies for bonding with tumor-associated cell surface antigens (substances that causes the production of an antibody when introduced into the body, e.g. toxins, bacteria, and viruses). ⁴⁶Sc is added to DTPA-derivatized (process by which a compound is chemically changed, producing a new compound that has properties more amenable to a particular analytical method) monoclonal antibodies and has been shown to target tumor cells, specifically in vivo, where it accumulates to high levels in the tumor (Fig. 4.21.1) [195], [196].

Fig. 4.21.1:

Comparison of biodistribution of ⁴⁶Sc citrate and ⁴⁶Sc-labeled caDTPA-antibody conjugates in healthy mice (circles) and leukemic mice (diamonds) 1 h after injection in tail vein (modified from [195], [196]).

4.22.1 Titanium isotopes in Earth/planetary science

The isotope-amount ration(⁵⁰Ti)/n(⁴⁶Ti) is used to study the early history of the Solar System. The value of the ratio can help determine whether the Solar System was created from a well-homogenized source [197], [198]. For example, variations in titanium isotopic compositions of various groups of meteorites can be observed (Fig. 4.22.1) [199].

Fig. 4.22.1:

Cross plot of the isotope-amount ration(⁵⁰Ti)/n(⁴⁷Ti) and the isotope-amount ratio n(⁴⁶Ti)/n(⁴⁷Ti) of selected groups of meteorites (modified from [199]), assuming measured n(⁵⁰Ti)/n(⁴⁷Ti) and n(⁴⁶Ti)/n(⁴⁷Ti) isotope-amount ratios of 0.697 19 and 1.109 18, respectively [200]. Normal titanium isotopic compositions were observed in standards, but ⁴⁶Ti and ⁵⁰Ti isotope anomalies were resolved among different meteorite groups.

4.22.2 Titanium isotopes in industry

The isotope-amount ratio n(⁴⁸Ti)/n(⁴⁹Ti) has been used in Isotope Ratio Method (IRM) analysis (initial titanium ratio/final titanium ratio) to estimate the energy production of nuclear reactors. This ratio can also be used to confirm that a reactor is being used for non-proliferation purposes (purposes other than to assist in the formation of nuclear weapon grade materials) [201].

4.23.1 Vanadium isotopes in Earth/planetary science

The isotopic abundances of ⁵⁰V and ⁵¹V have been used as an indicator of planetary core formation processes (Fig. 4.23.1). Vanadium is greatly depleted in the Earth’s mantle compared with that in chondritic meteorites (chondrites). It is assumed that the deficit of vanadium in the Earth’s crust is accounted for by its partitioning into the core [202]. The ratios of ⁵⁰V and ⁵¹V have been used as a test of the X-wind model, which accounts for a portion of the extinct radioactive nuclides present in the early Solar System by radiation from the young Sun [202]. ⁵¹V is depleted in meteorites compared to Earth [203].

Fig. 4.23.1:

Variation in the isotope-amount ration(⁵¹V)/n(⁵⁰V) of selected meteorites and that of bulk silicate Earth (modified from [203]), assuming a measured n(⁵¹V)/n(⁵⁰V) isotope-amount ratio of 399.5 [204].

4.23.2 Vanadium isotopes in industry

⁵¹V is used in solid state Nuclear Magnetic Resonance (NMR) to provide information to material scientists about surface species of vanadium oxide catalysts (substances that increase the rate of chemical reactions without themselves undergoing any permanent chemical change), their interaction with the supporting material, and their reactions during catalytic processes [205].

4.24.1 Chromium isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of chromium possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of chromium in natural terrestrial materials (Fig. 4.24.1).

Fig. 4.24.1:

Variation in atomic weight with isotopic composition of selected chromium-bearing materials (modified from [17]).

SiC grains are formed in very high-temperature events that occurred before the formation of the Solar System. The chemical and isotopic composition of certain elements in these grains, such as chromium, provides insights into the origin of the Solar System. The ⁵⁴Cr nucleus is only produced by supernovae. Excess amounts of this isotope in the SiC grains (relative to terrestrial isotopic composition) in primitive meteorites suggest a heterogeneous distribution of ⁵⁴Cr in the early Solar System and different sources of material to our Solar System [206]. The early solar nebula was divided into two components. One contained chromium depleted in the lighter isotopes and the other contained heavier chromium isotopes. Isotopic studies indicate these components formed a homogeneous mixture in the early Earth, but they separated during partitioning of the Earth’s core (Fig. 4.24.1) [207], [208].

Mobility and toxicity of chromium metal depend largely on the oxidation state of the element. Isotopes of chromium are fractionated by reduction-oxidation (redox) chemical reactions. The isotopic composition has been used to trace the origin of the element in the environment and provide information on reduction-oxidation chemical processes [209].

4.24.2 Chromium isotopes in medicine

Stable isotopes of chromium are used to investigate the metabolism of chromium (III), which is an essential nutrient. Chromium stable isotopes (⁵³Cr and ⁵⁴Cr) have been administered to patients and the relative metabolic activity of each isotope is measured to study insulin function in patients suffering from diabetes (a disease in which the body is unable to produce any or enough insulin, and/or is not able to properly use the insulin that it does produce, resulting in elevated levels of glucose in the blood) [210]. ⁵¹Cr and ⁵³Cr have been used to label red blood cells to determine blood volume and life-time of red blood cells in the body [210].

4.25.1 Manganese isotopes in Earth/planetary science

Radioactive ⁵⁴Mn (half-life of 312 days) has been used as a tracer to study migration of heavy metals in effluents (flowing out) from mining waste [109], [110].

4.25.2 Manganese isotopes in geochronology

The radioactive isotope⁵³Mn is formed by the interaction of protons, produced by cosmic rays, on iron in rocks. The accumulation of ⁵³Mn, having a half-life of 3.7×10⁶ years, at the Earth’s surface enables determination of exposure ages of landforms to cosmic rays and quantification of erosion rates. For example, Schaefer et al. [211] measured 13 samples from nine dolerite (igneous rock containing plagioclase, pyroxene, and olivine) surfaces in the Dry Valleys, Antarctica. They found that the terrestrial ⁵³Mn concentrations correlate well with cosmic-ray-produced ³He and ²¹Ne concentrations in the same samples (Fig. 4.25.1), which suggests that ⁵³Mn is produced continuously in place and retained over millions of years without loss. Their results suggest that ⁵³Mn concentrations in rocks can be used to monitor Earth-surface processes on time scales exceeding 10×10⁶ years.

Fig. 4.25.1:

Cross plot of cosmic-ray produced radioactive ⁵³Mn and ³He from 13 igneous-rock samples collected from land surface at the Dry Valleys, Antarctica (modified from [211]). The correlation between ⁵³Mn and ³He indicates that ⁵³Mn is produced continuously in place and has been used to monitor Earth-surface processes.

4.25.3 Manganese isotopes in medicine

⁵¹Mn, ⁵²Mn and ^52mMn (with half-lives of 46 min, 5.6 days, and 21 min, respectively) are radioactive isotopes that emit positrons that are used in positron emission tomography (PET) imaging [212], [213]. The m in the superscript of ^52mMn indicates a metastable state of the isotope.

4.26.1 Iron isotopes in biology

Natural iron enriched in its least abundant stable isotopes, ⁵⁷Fe and ⁵⁸Fe, are used as a tracer in human studies to assess absorption, excretion, distribution, and utilization of iron in basic and applied research [108], [109], [110], [214], [215], [216]. The two radioisotopes, ⁵⁵Fe and ⁵⁹Fe, have sufficiently long half-lives of 2.75 years and 44.5 days, respectively, to be used as tracers, but potential health and environmental hazards limit their use to diagnostic applications in patient care (i.e. disorders of blood and of iron metabolism) [110], [215], [216].

4.26.2 Iron isotopes in Earth/planetary science

⁶⁰Fe is an extinct radionuclide with a half-life of 2.6×10⁶ years that has fully decayed to ⁶⁰Ni since formation of the Solar System. The distribution of the product (radiogenic) ⁶⁰Ni in extraterrestrial material, such as meteorites, has been used to gain insight into the early history of the Solar System [216]. Because molecules, atoms, and ions of the stable isotopes of iron possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of iron in natural terrestrial materials (Fig. 4.26.1). Small variations in stable iron isotopic compositions caused by physical and chemical isotopic fractionation processes have been used to study mass transfer processes in nature and chemical equilibria [17], [216], [217].

Fig. 4.26.1:

Variation in atomic weight with isotopic composition of selected iron-bearing materials (modified from [17]).

4.26.3 Iron isotopes in industry

⁵⁵Fe is a beta emitting nuclide that serves as an electron source together with ⁶³Ni (with a half-life of 99 years) in electron-capture detectors. Electron-capture detectors are used as thickness gauges or as detectors for organic analytes in gas chromatography [218].

4.26.4 Iron isotopes in medicine

⁵²Fe, with a half-life of 8.3 h, emits positrons and is used in positron emission tomography (PET) studies. It can be produced in a cyclotron from stable ⁵⁰Cr by alpha particle capture [99], [219], [220].

4.26.5 Iron isotopes used as a source of radioactive isotope(s)

Stable ⁵⁶Fe is used for production of radioactive ⁵⁵Co (with a half-life of about 18 h), as an emitter of positrons for PET applications using the reaction ⁵⁶Fe (p, 2n) ⁵⁵Co [221], [222].

4.27.1 Cobalt isotopes in industry

⁶⁰Co (with a half-life of 5.27 years) is used to irradiate food sources as a method of preserving food (Fig. 4.27.1). The gamma radiation from ⁶⁰Co kills bacteria and other organisms that cause disease and spoilage of food (see Fig. 4.27.1). The use of radioactive compounds for preserving food is not always viewed positively. Some individuals are concerned that harmful compounds will be produced during the irradiation process. However, there is no evidence to support the claim that irradiation is dangerous for food preservation [108]. Many medical products today are sterilized using gamma rays from a ⁶⁰Co source. This technique of sterilization is generally much cheaper and more effective than steam-heat sterilization because it is a cold process. For example, it can be performed on packaged items, such as disposable syringes. This sterilization technique is applicable to a wide range of heat-sensitive items, such as powders, ointments, and solutions, as well as biological preparations, such as bone, nerve, skin, etc., used in tissue grafts [108].

Fig. 4.27.1:

Plants growing in the gamma greenhouse at Brookhaven National Laboratory. The plants are arranged in concentric rings around the radioactive ⁶⁰Co source, which is in the pipe extending into the floor (circa 1959) [223]. (Photo Source: Life Sciences Foundation (LSF) Magazine).

⁶⁰Co is also used in industrial radiography to detect structural flaws in metal parts. The radiation can penetrate metals and the X-ray pattern produced by the radiating material can provide information on its strength, composition, and other properties [108]. Because of the above property, ⁶⁰Co is also used in leveling devices and thickness gauges used to test welds and castings [108].

4.27.2 Cobalt isotopes in medicine

⁶⁰Co is a radioactive metal isotope that is used in cancer treatments by radiotherapy. When ⁶⁰Co undergoes radioactive decay, high-energy gamma rays (energies of 1.17 MeV and 1.33 MeV) are emitted and have been used in brachytherapy to treat various types of cancer. Brachytherapy (brachy is Greek meaning “short distance”) is a method of radiation treatment in which sealed sources are used to deliver a radiation dose at a distance of up to a few centimeters by surface, intracavitary (insertion of the radioactive isotope in a body cavity), or interstitial (between cells) application [75]. ⁶⁰Co is used as a source of high-energy ionizing gamma radiation that can be directed to cancer cells from a device outside the body (external radiotherapy).

⁶⁰Co (and sometimes ⁵⁷Co and ⁵⁸Co, with half-lives of 0.75 year and 71 days, respectively) is the key component of the Schilling test, which is a method for determining whether a patient’s body is making and using vitamin B12 properly. The cobalt isotope is used to label cobalt in vitamin B12 to monitor how the body processes this essential vitamin [224].

⁵⁷Co delivers the smallest radiation dose of all the cobalt isotopes. As a result, it has been used in the past for imaging and estimating organ size and location and in evaluating tumors of the head and neck [75], [99], [225], [226], [227].

4.28.1 Nickel isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of nickel possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of nickel in terrestrial silicate rocks (Fig. 4.28.1) [228].

Fig. 4.28.1:

Variation in isotope-amount ration(⁶⁰Ni)/n(⁵⁸Ni) of terrestrial nickel-bearing silicate rocks (modified from [228]), assuming a measured n(⁶⁰Ni)/n(⁵⁸Ni) isotope-amount ratio of 0.385 198 [229].

4.28.2 Nickel isotopes in geochronology

Anomalies in ⁶⁰Ni abundance caused by decay of now extinct ⁶⁰Fe have been used to study the early history of our Solar System (see section 4.26.2). ⁵⁹Ni is a cosmogenic radionuclide with a half-life of 7.6×10⁴ years. Decay of ⁵⁹Ni has been used to assess the terrestrial age of meteorites and to determine abundances of extraterrestrial dust in ice and sediment [230].

4.28.3 Nickel isotopes in industry

⁶³Ni (with a half-life of 99 years) is produced from stable ⁶²Ni and is a beta-emitting radionuclide that serves as an electron source together with ⁵⁵Fe in electron-capture detectors. Electron-capture detectors are used as thickness gauges or as detectors for organic analytes in gas chromatography (Fig. 4.28.2) [108]. ⁶³Ni is also used to ionize substances in ion mobility spectrometry–the basis of the instrument used in airports to screen passengers for drugs and bombs [231]. ⁶³Ni is also used as a fluorescence-inducing source in elemental analysis by X-ray fluorescence spectroscopy and in miniaturized long-lived nuclear batteries [108]. Until the mid-1980s, nuclear batteries were used in pacemakers, but then they were replaced by long-lasting lithium batteries [232].

Fig. 4.28.2:

Shimadzu GC-8A Gas Chromatograph (GC) with an Electron-Capture Detector (ECD). (Image Source: The Reston Chlorofluorocarbon Laboratory, U.S. Geological Survey) [233], [234].

4.28.4 Nickel isotopes used as a source of radioactive isotope(s)

⁶¹Ni is used as a radiation target for production of the radioactive isotope⁶¹Cu (with a half-life of 3.3 h), which emits positrons for positron emission tomography (PET) applications using the ⁶¹Ni (p, n) ⁶¹Cu reaction. ⁶⁴Ni is used as a radiation target for production of ⁶⁴Cu (with a half-life of 12.7 h), which is used in radioimmunotherapy by attaching it to an antibody for delivery of cytotoxic radiation (toxic to living cells) to a target cell via the ⁶⁴Ni (p, n) ⁶⁴Cu reaction [235]. ⁶⁰Ni is used for the production of ⁵⁷Co (with a half-life of 0.75 year), which is used as a reference source for gamma cameras that are used in nuclear medicinevia the ⁶⁰Ni (p, ⁴He) ⁵⁷Co reaction [235].

4.29.1 Copper isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of copper possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of copper in natural terrestrial materials (Fig. 4.29.1). ⁶³Cu and ⁶⁵Cu have been used to study copper isotope science of supergene (formed by descending solutions) copper minerals for potential use as an indicator of the paleohydraulic (ancient hydraulic) gradient, and for potential to provide a vector toward unrecognized copper source regions [236]. Copper isotope ratios of iron oxides and supergene copper sulfides in surface samples or fossil leached caps of ore deposits are being used in prospecting to rank prospects and focus on drilling areas that have the greatest potential for mature enrichment profiles [236].

Fig. 4.29.1:

Variation in atomic weight with isotopic composition of selected copper-bearing materials (modified from [17]).

4.29.2 Copper isotopes in forensic science and anthropology

The copper isotope-amount ration(⁶⁵Cu)/n(⁶³Cu) along with the silver isotope-amount ratio n(¹⁰⁹Ag)/n(¹⁰⁷Ag) and lead isotope-amount ratios n(²⁰⁶Pb)/n(²⁰⁴Pb), n(²⁰⁷Pb)/n(²⁰⁴Pb), and n(²⁰⁸Pb)/n(²⁰⁴Pb) have been used to determine the origin of European coins and the flow of goods in the historical world market. Metals from Peru and Mexico and those from European mining sites have distinct isotopic signatures that enable the origin of the metal to be determined based on the isotopic compositions of silver, copper, and lead in the coins. Silver from mines in Mexico and Peru in the 16^th century was used to mint coins but did not influence the European coin market until the 18^th century [237].

4.29.3 Copper isotopes in medicine

The radiopharmaceutical⁶²Cu-PTSM, which contains radioactive ⁶²Cu (with a half-life of 9.7 min), is used as a tracer in positron emission tomography (PET) to quantify myocardial perfusion (heart blood-flow measurements) [238], [239]. The radioisotope⁶⁴Cu (with a half-life of 12.7 h) is used for PET imaging and radiotherapy to diagnose, understand, and monitor disease (Fig. 4.29.2) [238], [240]. The stable isotope ⁶⁵Cu has been used as a tracer to study copper absorption, utilization, and excretion in humans [241], [242].

Fig. 4.29.2:

An illustration of a small-animal positron emission tomography (PET) system that uses the ⁶⁴Cu radioisotope to generate a reconstructed image of the animal in a noninvasive manner. (Reprinted with permission from Monica, S. & Anderson, C.J., 2009 [240]. Copyright © 2009 American Chemical Society).

4.30.1 Zinc isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of zinc possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of zinc in natural terrestrial materials (Fig. 4.30.1). Stable zinc isotopes have been used as tracers to investigate biogeochemical and chemical processes in environmental contamination sites [243]. The isotope-amount ration(⁶⁶Zn)/n(⁶⁴Zn) can be used as an environmental tracer for detecting the pathways of anthropogenic zinc [244], [245], [246].

Fig. 4.30.1:

Variation in the isotope-amount ration(⁶⁶Zn)/n(⁶⁴Zn) of selected zinc-bearing materials (modified from [247], assuming a n(⁶⁶Zn)/n(⁶⁴Zn) value of 0.57372 for a Johnson–Mattey zinc solution [248]).

4.30.2 Zinc isotopes in medicine

Oral tracers of enriched ⁶⁷Zn and intravenously injected stable isotopic tracers with enriched ⁷⁰Zn are used simultaneously to determine the fraction of dietary zinc absorbed in humans, maintaining the amount or concentration of a nutrient or biomolecule in organs and body fluids. For example, zinc-isotope tracers can be administered to humans to determine if zinc absorption in their bodies may be impaired by ingestion of certain foods, food components, or dietary supplements. One such study conducted with Peruvian women showed that prenatal iron supplements affected the absorption of zinc during pregnancy. Another isotope tracer study investigated zinc deficiency in children with Crohn’s disease (an inflammatory disease of the intestines, especially the colon and ileum) [249], [250]. Zinc radioisotopes (e.g.⁶⁵Zn, with a half-life of 244 days) can also be used for determining zinc absorption in humans, but they are now used rarely because of radiation hazards [251], [252]. ZnO nanoparticles enriched with ⁶⁷Zn have been used as biological/environmental nanotoxicity tracers [253].

4.30.3 Zinc isotopes used as a source of radioactive isotope(s)

The ⁶⁸Zn (p, 2p) ⁶⁷Cu (with a half-life of 62 h) reaction in which targets with zinc enriched in ⁶⁸Zn are irradiated and the neutron induced reaction ⁶⁷Zn (n, p) ⁶⁷Cu are both processes for producing ⁶⁷Cu for radiotherapy [254]. Irradiation of ⁶⁴Zn with a deuteron (the nucleus of ²H, consisting of a proton and a neutron) in a cyclotron will produce the radioisotope ⁶⁴Cu (with a half-life of 12.7 h), which can be used for therapeutic applications and diagnosis with positron emission tomography (PET) via the ⁶⁴Zn (d, 2p) ⁶⁴Cu reaction [255].

4.31.1 Gallium isotopes in medicine

⁶⁸Ga (with a half-life of 68 min) is a radioactive isotope that emits positrons, which are used to produce high-resolution imaging with positron emission tomography (PET). Unlike ¹⁸F, which is most commonly used, ⁶⁸Ga is more easily produced using a cost-effective generator with the parent radionuclide⁶⁸Ge (with a half-life of 271 days) (Fig. 4.31.1). Once produced, ⁶⁸Ga easily couples to biomolecules (most commonly peptides) that target G-protein coupled receptors, which are over-expressed on human tumor cells. The labeled protein acts as a radioactive tracer for cancer diagnostics. PET images are often coupled with CT images to get a more complete picture of the body [256], [257], [258], [259], [260], [261], [262]. Radiopharmaceutical⁶⁷Ga (with a half-life of 78 h) is a gamma-emitting isotope used in scintigraphy for medical imaging [263], [264], [265].

Fig. 4.31.1:

Gallium-68 generator used to provide medical therapy with the positron-emitting radionuclide⁶⁸Ga. The parent radionuclide, ⁶⁸Ge, has a half-life of 271 days and has been used as the source of ⁶⁸Ga, which has a half-life of only 68 min. Image kindly provided by Dr. Anatolii Razbash, Cyclotron Co. Ltd., Obninsk, Russia.

4.32.1 Germanium isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of germanium possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of germanium in terrestrial materials (Fig. 4.32.1).

Fig. 4.32.1:

Variation in the isotope-amount ration(⁷⁴Ge)/n(⁷⁰Ge) of selected geranium-bearing rocks and marine precipitates (modified from [266]), assuming a measured n(⁷⁴Ge)/n(⁷⁰Ge) isotope-amount ratio of 1.7794 [267].

4.32.2 Germanium isotopes in medicine

⁶⁸Ge is used to calibrate positron emission tomography (PET) scanners, which have been used for medical diagnostic procedures [268].

4.32.3 Germanium isotopes used as a source of radioactive isotope(s)

⁷²Ge and ⁷⁴Ge are used to produce the radioactive isotopes⁷²As and ⁷⁴As, with half-lives of 26 h and 17.8 days, respectively. The arsenic nuclei can attach to tumors and the decay of these isotopes is used to image the location of cancerous tumors in vivovia the ⁷²Ge (n, p) ⁷²As reaction and the ⁷⁴Ge (n, p) ⁷⁴As reaction [269]. ⁷⁰Ge, ⁷²Ge, and ⁷⁴Ge have all been used to produce the medical radioisotope⁷³Se via the ⁷⁰Ge (⁴He, n) ⁷³Se reaction, via the ⁷²Ge (⁴He, 3n) ⁷³Se reaction and via the reaction ⁷⁴Ge (⁴He, 5n) ⁷³Se, respectively [269].

4.33.1 Arsenic isotopes in biology

⁷³As and ⁷⁶As (with half-lives of 80.3 days and 1.1 days, respectively) are important radioactive tracers used in environmental and biomedical studies to quantify arsenic uptake [270]. ⁷⁴As (with a half-life of 17.8 days) has been used to investigate the biotransformation (modification of a chemical compound by an organism) of arsenate by mammals. In one study rabbits were injected with ⁷⁴As-labeled arsenate. After a given amount of time, blood and blood products were sampled and tested for the presence and quantity of labeled arsenate metabolites [270]. Inhalation of dust or smoke containing ⁷⁴As is thought to be a causal agent of lung cancer. In one study [271], the “absorption rate from the bronchial tree (a respiratory tract, which conducts air into the lungs) was rapid for the first several days and then tapered off slowly. In three patients an average of 45 percent of the inhaled arsenic was eliminated in the urine in 10 days and about 0.5 percent in the stools. The remainder must be assumed to have been deposited in the body, exhaled, and/or eliminated in body secretions and excreta over a long period of time.” See Fig. 4.33.1.

Fig. 4.33.1:

Combined urine and fecal elimination of inhaled ⁷⁴As over a 10-day period. The ratio of urine to fecal elimination was approximately 30 to 1 (modified from [271]).

4.33.2 Arsenic isotopes in medicine

⁷²As (with a half-life of 26 h) and ⁷⁴As are useful in molecular imaging because they are radioactive isotopes that emit positrons that can be designed to bind to monoclonal antibodies (moAb), which accumulate in tumors and then ⁷²As- or ⁷⁴As-labeled ligands will bind to the moAbs. Once the ⁷²As- or ⁷⁴As-labeled ligand binds to the moAb, positron emission tomography (PET) is used to visualize the exact location of the tumor [272]. A specific example of using radiolabeled antibodies for better imaging of tumors is the combination of ⁷⁴As with bavituximab, which is an antibody that binds strongly to unique lipids on the surface of tumors. When a thiol group is introduced to bavituximab, arsenic is able to bind covalently, creating a simple and elegant radio-label for targeting cancerous tumors [269].

4.34.1 Selenium isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of selenium possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of selenium in natural terrestrial materials (Fig. 4.34.1).

Fig. 4.34.1:

Variation in the isotope-amount ration(⁸²Se)/n(⁷⁶Se) of selected selenium-bearing materials (modified from [273]).

4.34.2 Selenium isotopes in industry

⁷⁵Se (with a half-life of 120 days) is used for X-ray radiography of welds to visualize welds and ensure that each weld is appropriate for its purpose [274].

4.34.3 Selenium isotopes in medicine

⁷⁵Se-selenomethionine (organic compound that combines to form proteins, found in Brazil nuts and soybeans) has been used to study the production of digestive enzymes (biological catalysts that accelerates chemical reactions) [275]. Selenium stable isotopes are used in metabolic studies to monitor selenium intake and output [276], [277].

4.34.4 Selenium isotopes used as a source of radioactive isotope(s)

⁷⁷Se and ⁷⁸Se are used to produce the therapeutic radioisotope⁷⁷Br via the ⁷⁷Se (n, p) ⁷⁷Br and the ⁷⁸Se (n, 2p) ⁷⁷Br reactions, respectively. ⁸⁰Se is used to produce ^80mBr via the reaction ⁸⁰Se (n, p) ^80mBr. The m the superscript of ^80mBr indicates a metastable state of the isotope.

4.35.1 Bromine isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of bromine possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of bromine in natural terrestrial materials (Fig. 4.35.1). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [13], [278]. ⁷⁹Br has been used as a groundwater tracer (Fig. 4.35.2). Introduction of a solution spiked with ⁷⁹Br to groundwater and measurement of the change in the isotope-amount ration(⁷⁹Br)/n(⁸¹Br) over time has been used to monitor tracer breakthrough and to calculate bromide travel time [279].

Fig. 4.35.1:

Variation in atomic weight with isotopic composition of selected bromine-bearing materials (modified from [13]).

Fig. 4.35.2:

Depiction of a hypothetical subsurface/groundwater tracer test with ⁷⁹Br. The tracer cloud identifies the location of the dissolved ⁷⁹Br spike. ⁷⁹Br concentration in water of the tracer cloud is compared to ⁷⁹Br concentration in groundwater samples in the neighboring sample site. (Diagram Source: U.S. Geological Survey, 2009) [280].

4.35.2 Bromine isotopes in medicine

⁷⁷Br (with a half-life of 57 h) is used to label radiopharmaceuticals that bind to estrogen receptors for tumor imaging. ⁷⁵Br (with a half-life of 97 min) is being used with positron emission tomography (PET) imaging [281].

4.35.3 Bromine isotopes used as a source of radioactive isotope(s)

⁷⁹Br is used in the proton cyclotron to produce ⁷⁷Kr, which decays to ⁷⁷Br via the reaction ⁷⁹Br (p, 3n) ⁷⁷Kr, which decays into ⁷⁷Br [282].

4.36.1 Krypton isotopes in forensic science and anthropology

⁸⁵Kr (with a half-life of 10.7 years) has been used in atmospheric monitoring programs to track the effect of atomic facilities on the surrounding environment. ⁸⁵Kr is co-generated with plutonium in the fuel elements of nuclear fission reactors and can be monitored at short distances (i.e. 1 to 5 km) from an area of clandestine plutonium separation from spent fuel from the nuclear reactor. The differences in ⁸⁵Kr levels in the atmosphere have been used to estimate the amount of plutonium separated at weekly intervals. The production of plutonium for nuclear weapons and the output from commercial reprocessing plants have released large amounts of ⁸⁵Kr into the atmosphere [283].

4.36.2 Krypton isotopes in geochronology

⁸⁵Kr has minimal natural production in the Earth, but its concentration in the atmosphere has increased steadily because of human activities related to the nuclear industry. ⁸⁵Kr enters oceans, lakes, and groundwater through equilibration of the water with air. ⁸⁵Kr is produced terrestrially as a fission product of nuclear reactors and released into the atmosphere with the noble gases. It is also produced in the atmosphere via the cosmic ray neutron-activation reaction, ⁸⁴Kr (n, γ) ⁸⁵Kr. Thus, the ⁸⁵Kr specific activity can be used to determine the time since water was isolated from the atmosphere (Fig. 4.36.1). This approach provides a valuable addition to the use of tritium (³H) as an indicator of ocean circulation and groundwater age on decadal (a period of 10 consecutive years) time scales [284], [285].

Fig. 4.36.1:

⁸¹Kr has been used to date the groundwater being discharged from springs and wells. The photo shows collection of a ⁸¹Kr sample from an artesian well in Farafra Oasis, Egypt [289]. (Photo Source: N. C. Sturchio, University of Delaware, Delaware, USA).

Krypton stable isotopes react in the upper atmosphere by cosmic-ray-induced spallation and neutron activation to produce radioactive ⁸¹Kr, with a half-life of approximately 2.1×10⁵ years. In the atmosphere, ⁸¹Kr is chemically inert and has a long residence time; because of these characteristics, it is expected that ⁸¹Kr has a relatively constant and well-constrained atmospheric source. Natural cosmogenic⁸¹Kr is incorporated from air into infiltrating groundwater and has been used to determine the age of groundwater over time scales ranging to over 10⁶ years [286], [287], [288], [289].

4.36.3 Krypton isotopes in industry

⁸⁵Kr has been used as the illumination element of indicator lights of appliances and can be combined with phosphors to create materials that glow in the dark. Light is created when radiation from ⁸⁵Kr strikes the phosphor [98]. ⁸⁵Kr can be used to detect container leaks by placing the radioactive gas inside a container and measuring (with a radiation detecting device) the amount of radioactive ⁸⁵Kr that escapes. Because the gas is inert, Kr will not react with anything else in the container [98].

4.36.4 Krypton isotopes in medicine

A patient can inhale gaseous radioactive ⁸⁵Kr, which is then absorbed in the bloodstream, enabling the blood flow of the patient to be studied. Movement of the ⁸⁵Kr can be tracked with a radiation detector to reveal pathways followed by the blood and to quantify blood velocity [99], [284], [290].

4.37.1 Rubidium isotopes in biology

Due to biological similarities between rubidium and potassium, the radionuclide⁸⁶Rb (with a half-life of 18.7 days) is used as a tracer in biological or medical investigations for applications where the half-life of the radioactive-tracer⁴²K (half-life of 0.5 day) is too short [110]. ⁸⁶Rb (with a half-life of 18.7 days) has been used measure the metabolism in small vertebrates (Fig. 4.37.1), such as dunnarts (furry, narrow-footed marsupials about the size of a mouse) [291]. The advantage of this technique over the standard doubly labelled water method, using water enriched in ²H and ¹⁸O, include lower equipment requirements, lower technical expertise, and longer time spans over which measurements can be made. This technique could be very useful for measuring the metabolism of amphibians and insects.

Fig. 4.37.1:

Exponential decay of ⁸⁶Rb for Sminthopsis macroura (striped-faced dunnart; an Australian marsupial that weighs between 15 and 25 g; turquoise crosses) and Sminthopsis ooldea (an Australian marsupial called the Troughton’s dunnart that weighs between 10 and 18 g; green diamonds) in thermoneutrality (after [291]). The solid lines are the best fit of the fraction of initial enrichment remaining, taking into account both radioactive decay and biological elimination of ⁸⁶Rb.

4.37.2 Rubidium isotopes in geochronology

⁸⁷Rb (with a half-life of 4.97×10¹⁰ years) is a long-lived radioisotope that is transformed into ⁸⁷Sr by emission of a beta-minus particle (an electron) and an antineutrino. From the abundance of ⁸⁷Sr and the Rb/Sr amount ratio in a rock, its age of crystallization can be calculated. Rb/Sr dating is one of the most widely employed techniques for dating geological samples [292].

4.37.3 Rubidium isotopes in medicine

⁸²Rb (with a half-life of 75 s) acts similarly to potassium and is used for imaging of the heart to better assess heart muscle function as a radioactive analog to potassium [293], [294]. ⁸²Rb is being considered as an alternative to highly-enriched uranium for producing medically important radioisotopes [293].

4.38.1 Strontium isotopes in Earth/planetary science

Stable isotopic fractionation of strontium is small because the relative differences between the masses of strontium stable isotopes are small (mass numbers are 86, 87, and 88 for the most abundant stable isotopes). Also, strontium is not subject to reduction-oxidation reactions in normal terrestrial environments, which would cause isotopic fractionation to be more evident. Nevertheless, current studies are exploring potential applications of stable strontium isotopic fractionation; for example, it has been used as a proxy for temperature during coral growth and for insights into the diets of ancient populations [295], [296].

The relative abundance of natural radiogenic⁸⁷Sr in seawater is related to the relative rates of processes that add or remove strontium in the ocean (seafloor spreading, mid-ocean-ridge hydrothermal activity, and continental weathering). Over geologic time, these processes have fluctuated and the isotope-amount ration(⁸⁷Sr)/n(⁸⁶Sr) has changed systematically. By measuring the n(⁸⁷Sr)/n(⁸⁶Sr) ratio in marine fossils of known age, it is possible to identify when such environmental changes occurred. Conversely, it is possible to estimate the ages of marine deposits by comparing measured n(⁸⁷Sr)/n(⁸⁶Sr) ratios with the global time chart; this process is known as strontium isotope stratigraphy [297].

4.38.2 Strontium isotopes in forensic science and anthropology

The isotope-amount ratio n(⁸⁷Sr)/n(⁸⁶Sr) is highly variable in rocks, minerals, soils, and waters, and it can be transmitted to plants (Fig. 4.38.1), animals, and manufactured materials. Measurements of n(⁸⁷Sr)/n(⁸⁶Sr) ratios are used for forensic applications in food authentication (determining where food came from), archaeology, crime-scene investigation, and human migration [298], [299].

Fig. 4.38.1:

Variation in strontium isotope-amount ratios of twenty species of exotic wood from thirteen countries (modified from [300]).

4.38.3 Strontium isotopes in geochronology

The ⁸⁷Rb-⁸⁷Sr dating technique utilizes the fact that ⁸⁷Sr is a product of radioactive ⁸⁷Rb decay (half-life of 4.97×10¹⁰ years) and is a useful tool for determining ages of rocks and minerals spanning the age of the Earth (Fig. 4.38.2) [301].

Fig. 4.38.2:

Cross plot of n(⁸⁷Sr)/n(⁸⁶Sr) isotope-amount ratio and n(⁸⁷Rb)/n(⁸⁶Sr) amount ratio of sphalerites (zinc sulfide mineral) from the Kipushi base metal deposit, Democratic Republic of Congo (modified from [302]). ⁸⁷Sr is produced by decay of radioactive ⁸⁷Rb. Rock containing higher amounts of ⁸⁷Rb will over time produce higher amounts of ⁸⁷Sr, for example sample KI 1270/128 R. Rock containing lower amounts of ⁸⁷Rb will over time produce smaller amounts of ⁸⁷Sr, for example sample KI 1270/113 R. Assuming all the sphalerites in this figure were formed at the same time, one can determine the age of formation of the sulfides from the slope of the line through the data points, here (451.1 ± 6.0)×10⁶ a, and this line is called an isochron.

4.39.1 Yttrium isotopes in medicine

Carbon nanotubes (CNT), which are nano-scaled carbon tubes, are being examined in nanobiotechnology research studies because it has been discovered that CNTs labeled with ⁸⁶Y (with a half-life of 0.6 day) are soluble when they are injected into mice. This discovery was made after mice were given an intravenous or intraperitoneal (directly into a body cavity) injection with the ⁸⁶Y CNT and then were examined using positron emission tomography (PET) scans to observe whether the ⁸⁶Y had been flushed from their systems. The PET scan determined that accumulation of ⁸⁶Y occurred in the liver, kidney, and spleen with very rapid blood clearance. This has broad implications for developing drug treatments [303]. Radiomicrosphere therapy (RT) that uses ⁹⁰Y (with a half-life of 64 h) microspheres is a proven therapy that helps treat hepatic (liver) cancer (Fig. 4.39.1) [304]. ⁹⁰Y is also used in radiosynovectomy to reduce joint pain [305].

Fig. 4.39.1:

Ultrapure ⁹⁰Y. (Photo Source: Pacific Northwest National Laboratory) [306].

4.40.1 Zirconium isotopes in industry

Zirconium enriched in ⁹⁰Zr has been proposed for the cladding (covering) of reactor fuel elements (Fig. 4.40.1) because it has a lower neutron absorption cross section than natural abundances of zirconium and is well suited for coverage of metal parts without absorbing neutrons [307].

Fig. 4.40.1:

The cores of nuclear reactors have fuel pins that are typically made of uranium-oxide. To keep fission products from escaping into the coolant, these pins are surrounded by zirconium cladding. (Modified from [308]).

4.41.1 Niobium isotopes in biology

⁹⁵Nb (with a half-life of 35 days) and ⁹⁵Nb-oxalates have been used to study the absorption, retention and distribution of niobium in the body [309], [310].

4.41.2 Niobium isotopes in Earth/planetary science

Nuclear physicists are trying to study the generation of new isotopes and their elements in stars (astrophysical nucleosynthesis) via the rapid neutron capture process (r-process). Physicists at the Radioactive Isotope Beam Facility (RIBF) of the RIKEN Nishina Center for Accelerator-Based Science in Wako, Japan, have begun creating and studying highly neutron-rich isotopes that are thought to only be produced by the r-process. The data for many neutron-rich isotopes is incomplete, and the RIKEN team is filling in key missing information that is needed to simulate the r-process (including information on the half-lives of the neutron-rich isotopes). So far, the half-lives of 38 neutron-rich isotopes have been measured from krypton to technetium, including ¹¹¹Nb and ¹¹²Nb. When the missing information has been obtained, physicists will have a better understanding of the r-process and how elements are created [311], [312].

4.41.3 Niobium isotopes in medicine

⁹⁵Nb and ^95mNb (with a half-life of 3.6 days) have been used in tumor research and tumor imaging studies (Fig. 4.41.1) [313], [314], [315]. The m in the superscript of ^95mNb indicates a metastable state of the isotope.

Fig. 4.41.1:

Tumor/Non-tumor ratios of ⁹⁵Nb-bevacizumab at 4, 24, 48 and 168 h post injection (modified from [315]). Bevacizumab, sold under the trade name Avastin, is a drug that slows the growth of new blood vessels and was approved by the U.S. Food and Drug Administration for selected metastatic cancers, including colon cancer. This in vivo biodistribution study (a distribution of compounds within a biological system or organism) shows increased tumor uptake of ⁹⁵Nb-bevacizumab and a satisfactory tumor/blood ratio.

4.42.1 Molybdenum isotopes in Earth/planetary science

Molybdenites display a variation in isotopic composition (Fig. 4.42.1) [316]. The isotopic composition of molybdenum in ocean sediments depends on oxygen levels in the ocean. When oxygen levels are high, the lighter isotopes of molybdenum are scavenged by iron and manganese oxides into sediments. However, when oxygen levels are low, the mechanism for molybdenum removal becomes more efficient and more of the heavier isotopes of molybdenum are found in iron and manganese oxides. Thus, the molybdenum isotopic composition of these sediments can be used as a proxy for oxygen levels in the paleo oceans (history of the oceans in the geological past) to gain insights into mechanisms that may have been responsible for mass-extinction events in the Earth’s history [317].

Fig. 4.42.1:

Cross plot of n(⁹⁸Mo)/n(⁹⁵Mo) isotope-amount ratio and n(⁹⁷Mo)/n(⁹⁵Mo) isotope-amount ratio of selected molybdenum-bearing materials (modified from [316]), assuming a measured n(⁹⁸Mo)/n(⁹⁵Mo) isotope-amount ratio of 1.530 40 and a measured n(⁹⁷Mo)/n(⁹⁵Mo) isotope-amount ratio of 0.603 67 [318].

4.42.2 Molybdenum isotopes in industry

Depleted ⁹⁵Mo has been used in the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (Tennessee, USA). The use of U-10Mo fuel elements (90 percent uranium, 10 percent molybdenum) would allow the conversion from high-enrichment uranium (HEU) fuel, 92 percent, to low-enrichment uranium (LEU) fuel, below 20 percent, for nuclear non-proliferation purposes [319].

4.42.3 Molybdenum isotopes used as a source of radioactive isotope(s)

⁹⁵Mo is used to produce medical radioisotope⁹⁷Ru via the ⁹⁵Mo (⁴He, 2n) ⁹⁷Ru reaction. The isotope ⁹⁹Mo is commercially produced by the fission of ²³⁵U and is the parent radionuclide of ^99mTc, which is the most widely used radiopharmaceutical in the world. The much longer half-life of ⁹⁹Mo (about 66 h) enables the radionuclide to be transported more easily than the short-lived (6 h half-life) ^99mTc. The n(⁹⁹Mo)/n(^99mTc) amount-ratio generator was originally developed at Brookhaven National Laboratory (Fig. 4.42.2) in the early 1960s and is now a patented system [320].

Fig. 4.42.2:

Pictured above is Brookhaven National Laboratory where the n(⁹⁹Mo)/n(^99mTc) amount-ratio generator was originally developed in the early 1960s. (Picture Source: Brookhaven National Laboratory) [321].

4.43.1 Technetium isotopes in medicine

^99mTc is an isomer of ⁹⁹Tc with a half-life of approximately 6 h that is used to label peptides for morphologic (the form and structure of an organism) and dynamic modeling of renal (kidney), hepatic (liver), bone, and cardiac imaging [320], [322]. ^99mTc radiopharmaceuticals absorb to a variety of tumors. These tumors can be imaged using single-photon emission computed tomography (SPECT) coupled with non-invasive computed tomography (CT scan), which provides a high level of functional and anatomical information in a three-dimensional image (Fig. 4.43.1) [323], [324]. Medronate is a radioactive pharmaceutical, which has been used to find, treat, or study certain diseases or body functions. ^99mTc-labeled medronate (^99mTc-MDP) is used in a diagnostic test to detect metastases from prostate, lung or thyroid cancer, making use of a gamma camera to record the distribution of ^99mTc-MDP within the body. A two-dimensional image of the affected areas is produced.

Fig. 4.43.1:

Single-photon emission computed tomography (SPECT CT) machine. (Image Source National Institute of Allergy and Infectious Diseases (NIAID) and National Institutes of Health) [325].

4.44.1 Ruthenium isotopes in Earth/planetary science

¹⁰⁰Ru is the product of a rare (and hence very long-lived) nuclear decay process from the double beta decay of ¹⁰⁰Mo. A careful measurement of the half-life for this decay, which is 7.1×10¹⁸ years, can be used to place an upper limit on the mass of the electron neutrino, which is a neutral and weakly interacting subatomic particle first postulated by Wolfgang Pauli in 1930 [326].

Ruthenium and molybdenum share many similarities. They both have seven isotopes (96, 98, 99, 100, 101, 102, and 104 for ruthenium and 92, 94, 95, 96, 97, 98, and 100 for molybdenum), and their isotopes are formed by the same nucleosynthesisp-processes, r-processes, and s-processes, namely, p, r, s and r, s only, s and r, s and r, and r, respectively. The molybdenum and ruthenium isotopic composition of most meteorites lie along a mixing line (Fig. 4.44.1). The ruthenium and molybdenum of silicates in the Earth also lie on this line, which supports the hypothesis that the Earth accreted homogeneously. That is, the feeding zone of the Earth did not change substantially over time as both the bulk of the Earth and the late veneer accreted from material having the same ruthenium-molybdenum isotopic reservoir [327].

Fig. 4.44.1:

Cross plot of n(¹⁰⁰Ru)/n(¹⁰¹Ru) isotope-amount ratio [328], [329] and n(⁹²Mo)/n(⁹⁶Mo) isotope-amount ratio [330], [331] of selected meteorite groups (modified from [327]), assuming a measured n(¹⁰⁰Ru)/n(¹⁰¹Ru) isotope-amount ratio of 0.738 48 [332] and a measured n(⁹²Mo)/n(⁹⁶Mo) isotope-amount ratio of 0.878 61 [318].

4.44.2 Ruthenium isotopes in medicine

¹⁰⁶Ru plaque brachytherapy has been used for eye preservation and tumor control of uveal (the middle layer of the wall of the eye) melanoma [333]. The half-life of ¹⁰⁶Ru is 373 days.

4.44.3 Ruthenium isotopes used as a source of radioactive isotope(s)

⁹⁶Ru is used to produce radioisotopes⁹⁴Ru (with a half-life of 52 min) and ⁹⁵Ru (with half-life of about 1.64 h) via the reactions ⁹⁶Ru (n, 3n) ⁹⁴Ru and ⁹⁶Ru (n, 2n) ⁹⁵Ru, respectively (Fig. 4.44.2) [334], [335]. ¹⁰⁴Ru is used to produce the radioisotope ¹⁰⁵Rh (with a half-life of about 35 h) via the reaction ¹⁰⁴Ru (p, γ) ¹⁰⁵Rh. ¹⁰⁵Rh has been used in the treatment of bone pain [334].

Fig. 4.44.2:

The 88-Inch cyclotron was used to produce both light and heavy ions, including ⁹⁴Ru and ⁹⁵Ru. (Photo Source: Lawrence Berkeley National Laboratory) [336].

4.45.1 Rhodium isotopes in medicine

The beta particles of ¹⁰⁵Rh (with a half-life of about 35 h) are used in target radiotherapy to kill cancer cells or cause cancer cell sterilization [334]. The gamma rays from ¹⁰⁵Rh enable in vivo tracking during radiotherapy [334]. ¹⁰⁵Rh has been used in the treatment of bone pain (Fig. 4.45.1) [334], [337].

Fig. 4.45.1:

Bone cancer cells that have been pap stained and magnified to 400 times. The beta particles and gamma rays of ¹⁰⁵Rh are used, respectively, in radiotherapy to kill cancer cells and for in vivo tracking during radiotherapy [339]. (Photo Source: National Cancer Institute at the National Institutes of Health).

Ocular brachytherapy currently is performed using ¹²⁵I (with a half-life of about 59 days) or ¹⁰⁶Rh (with a half-life of about 30 s) seeds [338]. Brachytherapy can allow a good spatial dose distribution over the ocular tumor with lower radiation on adjacent tissues. In the case of irradiation of the eyeball with ¹⁰⁶Rh, 80 percent of the dose has been absorbed within a depth of 5.2 mm and 90 percent has been absorbed within 7.2 mm (Fig. 4.45.2). This limits the application of ¹⁰⁶Rh; however, when ¹⁰⁶Rh can be used, the radiation dose can be lower, which is preferred.

Fig. 4.45.2:

Variation in absorbed dose of ¹⁰⁶Rh as a function of tissue depth in ocular brachytherapy (modified from [338]).

4.46.1 Palladium isotopes in Earth/planetary science

Small palladium nucleosynthetic anomalies in isotopic composition (related to s-process variability) were identified in type IVB iron meteorites [340]. These nucleosynthetic isotope anomalies may represent spatial and/or temporal heterogeneity in the early solar nebula or may be due to chemical processing within the solar nebula [327], [341]. Palladium and molybdenum isotopic compositions on selected iron meteorites are correlated (Fig. 4.46.1). One possible conclusion is that “a common presolar carrier must have been thermally processed on which the more volatile (a measure of the tendency of a substance to vaporize) Pd was lost and homogenized in the solar nebula, resulting in the deviation from the s-process” variability [342]. Because these palladium (and other element) anomalies are persistent throughout the measured iron meteorites, the thermal processing must have occurred prior to the formation of the parent body that produced iron meteorites [342].

Fig. 4.46.1:

Cross plot of n(¹⁰⁴Pd)/n(¹⁰⁵Pd) and n(⁹⁷Mo)/n(⁹⁶Mo) isotope-amount ratios of selected meteorites (modified from [342]), assuming a measured n(¹⁰⁴Pd)/n(¹⁰⁵Pd) isotope-amount ratio of 0.498 88 in terrestrial material [343] and a measured n(⁹⁷Mo)/n(⁹⁶Mo) isotope-amount ratio of 0.574 70 in terrestrial material [318].

4.46.2 Palladium isotopes in geochronology

The isotope-amount ration(¹⁰⁷Pd)/n(¹⁰⁷Ag) is used in geochronology to help date major thermal events in the Solar System. Although ¹⁰⁷Ag is naturally occurring, ¹⁰⁷Ag is also the daughter product of the beta decay of ¹⁰⁷Pd. If both excess ¹⁰⁷Ag and ¹⁰⁷Pd (with a half-life of 6.5×10⁶ years) are present in a sample of extraterrestrial origin, then the material would have formed sometime after ¹⁰⁷Pd decayed. The n(¹⁰⁷Pd)/n(¹⁰⁷Ag) amount ratio can be measured to help determine when the ¹⁰⁷Pd decay process began and how much time has elapsed since the material was formed [344], [345], [346], [347], [348].

4.46.3 Palladium isotopes in medicine

Seeds of the radioactive isotope¹⁰³Pd are internally placed in the body to fight prostate and other cancers locally. ¹⁰³Pd has a half-life of 16.99 days and releases energy at about 80 X-rays and 186 Auger electrons per 100 decays of ¹⁰³Pd. Therefore, this makes this isotope an ideal candidate for internal radiotherapy for the treatment of cancers [349].

The radioisotope¹⁰⁹Pd (with a half-life of 13.5 h) can be used as a form of cancer therapy. For example, ¹⁰⁹Pd-labeled porphyrins or porphyrin-like substances are used as diagnostic and therapeutic techniques to help locate and address areas of tumorous growth. Porphyrins accumulate in tumors of the body and when radiolabeled porphyrins are introduced to the body, the X-rays and energy released can help determine the location and even treat the cancerous tumors [350].

4.46.4 Palladium isotopes used as a source of radioactive isotope(s)

¹⁰⁴Pd is the major target used for cyclotron production of the medically important radioactive isotope ¹⁰³Pd via the reaction ¹⁰⁴Pd (p, p n) ¹⁰³Pd [349].

4.47.1 Silver isotopes in Earth/planetary science

The measurement of relative amounts of ¹⁰⁷Ag and ¹⁰⁹Ag is used to study the processes responsible for the isotopic fractionation of silver isotopes in ore deposits, which depends on the specific minerals and environmental conditions. This is currently an area of active research and it is thought that the relative amounts of the isotopes of silver are altered during the formation of the ore [351], [352].

4.47.2 Silver isotopes in forensic science and anthropology

Silver isotope-amount ratiosn(¹⁰⁷Ag)/n(¹⁰⁹Ag) along with isotope-amount ratios of copper n(⁶⁵Cu)/n(⁶³Cu), and isotope-amount ratios of lead (n(²⁰⁶Pb)/n(²⁰⁴Pb), n(²⁰⁷Pb)/n(²⁰⁴Pb) and n(²⁰⁸Pb)/n(²⁰⁴Pb)) have been used to determine origins of European coins and information on the flow of goods in the world market over time (Fig. 4.47.1). Metals from Peru and Mexico and those from European mining have distinct isotopic signatures that enable the origin of the metal to be determined by examining the isotopic compositions of silver, copper, and lead in the coins. Abundant silver sources, mined in Mexico and Peru in the 16^th century, were used to mint coins, but they were not a major influence in the European coin market until the 18^th century (Fig. 4.47.1) [237].

Fig. 4.47.1:

Cross plot of lead model age and n(¹⁰⁹Ag)/n(¹⁰⁷Ag) isotope-amount ratio of selected coins (modified from [237]) assuming a n(¹⁰⁹Ag)/n(¹⁰⁷Ag) value of 0.929 04 for the isotopic reference material SRM 981a silver nitrate [237]. The isotopic signatures of the silver, copper, and lead in the metals used to make Spanish coins have been used to trace (see tracer) the origin of the metals to help determine the flow of metal in the global market during the 16^thcentury. The Mexico coins show little variation in the silver mole fraction, although the observed range of isotopic variation is about 30 times the analytical uncertainty. The antique coins have two groups, the oldest on the right and the younger on the left. They are statistically different at the 99-percent confidence level. The Catholic Kings coins are distinct from the rest of the medieval coins.

4.47.3 Silver isotopes in geochronology

The amount ration(¹⁰⁷Pd)/n(¹⁰⁷Ag) is used in geochronology to date major events in the Solar System [344], [345], [346], [347], [348], [353]. Although ¹⁰⁷Ag is naturally occurring, it is also the daughter product by beta decay of ¹⁰⁷Pd. If both excess ¹⁰⁷Ag and ¹⁰⁷Pd are present in a sample of extraterrestrial origin, then the material would have formed sometime after ¹⁰⁷Pd decayed (i.e. sometime after the 6.5-million-year half-life of ¹⁰⁷Pd). The n(¹⁰⁷Pd)/n(¹⁰⁷Ag) amount ratio can be measured to help determine when the ¹⁰⁷Pd decay process began and determine how much time has elapsed since the material was formed.

4.47.4 Silver isotopes in industry

¹⁰⁷Ag is being studied as a possible target for cyclotron production of ¹⁰³Pd (with a half-life of 17 days) via the ¹⁰⁷Ag (p, α n) ¹⁰³Pd reaction. ¹⁰³Pd releases X-rays and Auger electrons at the rate of about 80 X-rays and 186 Auger electrons per 100 decays of ¹⁰³Pd, which makes this isotope an ideal candidate for internal radiotherapy for the treatment of cancers. The production of this isotope in a no-carrier form (not formed in another solution) is important for its medical uses. By using neutrons, photons, and charged particles to force reactions with isotopes of a higher mass number than 103, ¹⁰³Pd will occur in a fraction of those reactions. The most common methods of ¹⁰³Pd production use targets of rhodium or other isotopes of palladium. However, ¹⁰⁷Ag has also been studied as a feasible option [349], [354]. ¹⁰⁹Ag is used to produce the gamma reference source ^110mAg to help calibrate gamma detectors [349], [354].

4.48.1 Cadmium isotopes in biology

Metal accumulation is a threat to our world’s water systems and wildlife. As a way to measure the influence of heavy metals on wildlife utilizing mass spectrometric techniques, some researchers use animal food enriched in specific cadmium isotopes. These experiments work by exposing the animals to a diet enriched in ¹⁰⁶Cd and/or other stable isotopes of metals (for example, ⁶⁵Cu and/or ⁶²Ni) for a period of time. Depending on the purpose of the experiment, the residence time of the food in the gut is determined and isotopic compositions of the gut and/or feces are measured viainductively coupled plasma mass spectrometry (ICP-MS). This information is used to measure bio-uptake (absorption and incorporation of a substance by living tissue) and accumulation rates of metals in an exposed animal [355], [356].

4.48.2 Cadmium isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of cadmium possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are small but measureable variations in the isotopic abundances of dissolved cadmium in ocean water, which are a consequence of isotopic fractionation associated with biological uptake (Fig. 4.48.1) [357], [358], [359].

Fig. 4.48.1:

Variations in the isotope-amount ration(¹¹⁴Cd)/n(¹¹⁰Cd) of dissolved ocean cadmium as a function of latitude south for Zero Meridian surface water samples (modified from Xue et al. [359], with data from Abouchami et al. [358] assuming a n(¹¹⁴Cd)/n(¹¹⁰Cd) value of isotopic reference material SRM 3108 of 2.304 07 [360]).

4.48.3 Cadmium isotopes used as a source of radioactive isotope(s)

¹¹²Cd is used to produce the diagnostic radioisotope¹¹¹In (with a half-life of 2.8 days) via the reaction ¹¹²Cd (p, 2n) ¹¹¹In [94].

4.49.1 Indium isotopes in medicine

¹¹¹In (with a half-life of 2.8 days) is used in indium leukocyte imaging (Fig. 4.49.1), in which white blood cells that are abundant at sites of infection are labeled with ¹¹¹In to help locate the source of the infection [361], [362], [363].

Fig. 4.49.1:

Radionuclide imaging of infection. Leukocytes are white blood cells in the body that protect the body from infection and can be dyed with ¹¹¹In to locate the site of an infection in the body. (Image Source: Love, Charito MD and Palestro, Christopher J. MD., 2004) [363].

4.49.2 Indium isotopes used as a source of radioactive isotope(s)

¹¹³In is used to produce ¹¹³Sn (with a half-life of 115 days) via the reaction ¹¹³In (p, n) ¹¹³Sn, and ¹¹³In is used to produce the radioisotope¹¹⁰In (with a half-life of 1.15 h) [364], [365].

4.50.1 Tin isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of tin possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable variations in the isotopic abundances of tin in natural terrestrial materials (Fig. 4.50.1) [366].

Fig. 4.50.1:

Variation of the isotope amount ration(¹²⁴Sn)/n(¹²⁰Sn) of selected cassiterite samples from China (modified after [366]).

4.50.2 Tin isotopes in medicine

^117mSn (with a half-life of 14 days) DTPA is routinely used for diagnostic bone imaging and for the treatment of bone pain caused by the spread of cancer to bones. The m in the superscript of ^117mSn indicates a metastable state of the isotope. By using ^117mSn DTPA, marrow toxicity can be reduced, and the therapeutic efficacy of using radionuclides is maintained [367]. ^117mSn is a promising radionuclide for therapeutic applications because the radionuclide decays in a way that causes less damage to healthy tissues and bone marrow than other available treatments. These properties of ^117mSn make it useful for the treatment of inflammatory synovial disease (i.e. rheumatoid arthritis) [368].

4.50.3 Tin isotopes used as a source of radioactive isotope(s)

¹¹²Sn is used to produce the radioisotope¹¹³Sn (with a half-life of 115 days) via the reaction ¹¹²Sn (n, γ) ¹¹³Sn. This is used for n(¹¹³Sn)/n(^113mIn) generators for the elution (extracting one material from another) of ^113mIn (with a half-life of 1.66 h) as chloride for blood pool imaging. The m the superscript of ^113mIn indicates a metastable state of the isotope. ^117mSn is a medical radioisotope that can be produced using ¹¹⁶Sn and ¹¹⁷Sn [369].

4.51.1 Antimony isotopes in Earth/planetary science

Molecules, atoms, and ions of the stable isotopes of antimony possess slightly different physical and chemical properties, and they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are measureable substantial variations in the isotopic abundances of antimony in natural terrestrial materials (Fig. 4.51.1) [370]. The stable isotopes ¹²¹Sb and ¹²³Sb have been used to measure movement of sediments and rocks originating from locations high in antimony. ¹²¹Sb and ¹²³Sb move with the sediments and have been used as tracers in areas low in antimony to determine the originating location of certain metal/metalloid contaminants in streams [371], [372], [373].

Fig. 4.51.1:

Variation in isotope-amount ration(¹²³Sb)/n(¹²¹Sb) of antimony in terrestrial materials (modified from [370]), assuming a measured isotope-amount ratio n(¹²³Sb)/n(¹²¹Sb) of 0.747 85 [374].

4.51.2 Antimony isotopes in industry

In the 1950s, ¹²⁴Sb and ¹²⁵Sb (with half-lives of 60 days and about 1000 days, respectively) were used commercially as tracers. They were injected into oil pipelines as a way to detect the residence time and flow rate of the substance through the pipeline. The presence of these isotopes could be detected by means of a Geiger counter held above the pipeline. If the pipeline had a leak, the tracer would escape and its contamination and movement could be detected in the soil. ¹²⁴Sb and ¹²⁵Sb are now both treated as environmental contaminants [375].

4.51.3 Antimony isotopes used as a source of radioactive isotope(s)

¹²³Sb is used to produce ¹²⁴I (with a half-life of 100 h), which is used in radioimmunotherapy and also in positron emission tomography. It can be produced from the ¹²³Sb (³He, 2n) ¹²⁴I reaction [376]. ¹²¹Sb and ¹²³Sb can both be used for the production of ¹²³I (with a half-life of 13.2 h) via³He and alpha particle-induced reactions with ¹²¹Sb and ¹²³Sb, although the most common production route is via¹²⁴Xe or ¹²³Te [377].

4.52.1 Tellurium isotopes in Earth/planetary science

Tellurium isotopes are a mixture of r-process, s-process, and p-process nucleosynthesis products, making them useful for studying the contribution of stellar products to the molecular cloud from which the Sun and planets were formed (Fig. 4.52.1) [378], [379], [380].

Fig. 4.52.1:

Variation in isotope-amount ration(¹³⁰Te)/n(¹²⁸Te) of tellurium in selected meteorites and terrestrial materials (modified from [380]), assuming a measured isotope-amount ratio n(¹³⁰Te)/n(¹²⁸Te) of 1.066 65 [381]. Based on these data, Fehr et al. [380] conclude that the regions of the solar disk that were sampled during accretion of meteorite parent bodies were well mixed and homogeneous on a large scale, with respect to tellurium isotopes.

4.52.2 Tellurium isotopes in geochronology

The double beta decay of ¹³⁰Te (with a half-life of 7×10²⁰ years) has been used for the determination of gas-retention ages of tellurium minerals [382].

4.52.3 Tellurium isotopes used as a source of radioactive isotope(s)

¹²⁰Te is used for the production of ^120gI, where “g” indicates ground state, via the ¹²⁰Te (p, n) ^120gI reaction, which is used as a positron emission tomography (PET) and beta-emitting isotope [383], [384]. ^120gI has a half-life of 1.36 h. ¹²²Te is used in the production of the radioisotope¹²²I (with a half-life of 3.6 min) via the reaction ¹²²Te (p, n) ¹²²I, which is used in gamma imaging [385]. ¹²³Te is used for the production of radioactive ¹²³I (with a half-life of 13.2 h) via the ¹²³Te (p, n) ¹²³I reaction, which is used in thyroid imaging [386] and for in vivo medical studies using single-photon emission computed tomography (SPECT) [386]. ¹²⁴Te is used for the production of both ¹²³I and the PET isotope ¹²⁴I via the ¹²⁴Te (p, 2n) ¹²³I and ¹²⁴Te (p, n) ¹²⁴I reactions, respectively [386], [387], [388], [389]. The half-life of ¹²⁴I is 100 h.

4.53.1 Iodine isotopes in forensic science and anthropology

¹³¹I (with a half-life of about 8 days) and ¹²⁹I are both fission products; ¹²⁹I is a long-lived fission product with a half-life of 1.7×10⁷ years that can be helpful in the detection of the movement of radiation after a radioactive event, such as occurred at the Japanese reactors at Fukushima. In nuclear reactors and weapons tests, uranium and plutonium undergo fission processes in which one of the fission products is the long-lived isotope¹²⁹I. This isotope has been used as a groundwater tracer to determine evidence of nuclear fission, and it can also be tracked in rainwater as evidence of a fission event in the air (weapons explosion; Fig. 4.53.1) [390], [391], [392].

Fig. 4.53.1:

Global distribution of ¹²⁹I in water samples before the Fukushima disaster in Japan. Scale spans 1×10⁵ atoms ¹²⁹I/L to 1×10¹¹ atoms ¹²⁹I/L on a logarithmic (lg₁₀) scale. (Image Source: Snyder, G., A. Aldahan, and G. Possnert, 2010) [392].

4.53.2 Iodine isotopes in geochronology

Natural cosmogenic¹²⁹I enters groundwater and other terrestrial environments from the atmosphere and then decays to ¹²⁹Xe. The isotope-amount ration(¹²⁹I)/n(¹²⁷I) can be used as a clock to estimate time since cosmogenic ¹²⁹I entered the system. The amount of product ¹²⁹Xe in such cases is too small to measure; however, excess quantities of ¹²⁹Xe can be found in meteorites and other very old samples that contained extinct primordial¹²⁹I. Younger water bodies also can be differentiated from older water bodies by determining the amount of anthropogenic¹²⁹I released since the 1960s from sources such as nuclear bomb tests [393], [394].

4.53.3 Iodine isotopes in medicine

¹²⁵I, which has a half-life of about 59 days, is used encapsulated in radiotherapy to target and treat sites of cancerous tumors [395]. ^120gI (with a half-life of 1.36 h), where the “g” indicates ground state, and ¹²⁴I (with a half-life of 100 h) are radioactive isotopes that emit positrons and they are used in quantitative, diagnostic imaging of the body using positron emission tomography (PET) [383], [384], [385], [387], [388], [389]. ¹²³I and ¹³¹I (with half-lives of 0.55 day and 8 days, respectively) are used with single-photon emission computed spectroscopy (SPECT) for basic three-dimensional imaging [386], [395]. Radioactive iodine isotopes are produced from radioactive tellurium isotope (see Section 4.52.3).

4.54.1 Xenon isotopes in forensic science and anthropology

Radiogenic xenon isotopes are produced by nuclear reactions in atomic bombs and nuclear reactors. For example, ¹³¹Xe, ¹³³Xe, and ¹³⁵Xe are some of the fission products of ²³⁵U and ²³⁹Pu, and finding these isotopes would be evidence of a nuclear bomb reaction. Measurements of xenon isotopes (e.g. in the atmosphere or the subsurface) have been used to identify contamination from these sources, for example, to detect faults in nuclear reactors or to monitor compliance with nuclear test bans (Fig. 4.54.1) [396].

Fig. 4.54.1:

The mean weekly ¹³³Xe time series in ground-level air at Freiburg, Germany, between 1977 and 2009. The record indicates persistent low levels of anthropogenic¹³³Xe that are generally attributable to normal acceptable releases from nuclear power plants with variability related in part to multiple sources and changing wind patterns. A major spike occurred in 1986 during the Chernobyl reactor accident in the Ukraine region of the former USSR. The half-life of ¹³³Xe (5.2 days) is sufficiently long for it to escape from its source and be distributed in air near the source, but sufficiently short that long-term background levels are very low. Records such as this also can be used to detect undocumented nuclear explosions. (Modified from [396]).

4.54.2 Xenon isotopes in geochronology

The stable isotopes of xenon hold many clues about the formation of the elements, solar-system history, and Earth processes [29], [101]. For example, ¹²⁹Xe has been used as a detector of “extinct” radionuclides. Some ¹²⁹Xe is radiogenic as a result of being produced by the radioactive decay of ¹²⁹I (half-life of 1.7×10⁷ years). Because the half-life of ¹²⁹I is much smaller than the age of the Earth, primordial¹²⁹I (i.e. that which was present at the beginning of Earth’s history) is essentially gone after it decayed to ¹²⁹Xe over geologic time. This means that radiogenic ¹²⁹Xe could be a marker of the former existence of the “extinct” isotope ¹²⁹I. Because primordial ¹²⁹I was produced largely in supernovae, detection of radiogenic ¹²⁹Xe in meteorites and terrestrial samples also implies that the time elapsed between ¹²⁹I supernova nucleosynthesis and planetary condensation was short compared to the subsequent history of the Solar System. The many isotopes and reaction mechanisms of xenon have contributed numerous insights into Earth processes through the study of “xenology” (xenon isotopic variations used as geodynamic tracers to study the dynamics of the Earth) [397].

4.54.3 Xenon isotopes in medicine

Xenon isotopes are used in numerous ways to investigate the movement of inhaled gases in lungs and other parts of the body. If radioactive isotopes of xenon [¹²⁷Xe (with a half-life of 0.1 year), ¹³³Xe, and hyperpolarized (having non-equilibrium alignment of nuclear spins, suitable for magnetic resonance) ¹²⁹Xe] are inhaled, they can be tracked throughout the body by externally monitoring their decay products using magnetic resonance microscopy [high resolution magnetic resonance imaging (MRI) at microscopic (nanometer) levels] (Fig. 4.54.2). This imaging technique is used to assess how well oxygen is taken up and transported by the blood [398].

Fig. 4.54.2:

Xenon ventilation imaging has progressed greatly since first being used in 1998. One of the first ¹²⁹Xe images (a) has been progressively improved by polarization, gas delivery technology, and magnetic resonance (MR) acquisition strategies (b–d). (Image Source: Driehuys and Hedlund, 2007, © Sage Publications) [398]).

4.54.4 Xenon isotopes used as a source of radioactive isotope(s)

¹²⁴Xe is used in the production of radioisotopes¹²³I and ¹²⁵I (with half-lives of 0.55 day and 59 days, respectively) via the reactions ¹²⁴Xe (n, n p) ¹²³I and ¹²⁴Xe (n, γ) ¹²⁵I, respectively, which are used in diagnostic procedures and cancer treatment, respectively [398].

4.55.1 Caesium isotopes in biology

¹³⁷Cs (with a half-life of 30 years) can be used as a tracer in fungal mycelia (an extensive matrix of underground hyphae (stems of growth from a fungus)) to monitor the immobilization of this radioactive caesium isotope. After the nuclear reactor accident at Chernobyl, large quantities of ¹³⁷Cs were released as fission products into the environment. Areas with large fungal populations and fungal mycelia seemed to immobilize the ¹³⁷Cs isotope, which limited the spread of the radioactive isotope [399], [400].

4.55.2 Caesium isotopes in Earth/planetary science

River floodplains are an important site for storing suspended sediments and contaminants transferred from upstream catchments. ¹³⁷Cs measurements of floodplain sediments provide a technique for estimating overbank sediment deposition, and it can provide information on spatial patterns of sediment deposition (Fig. 4.55.1) [401], [402], [403].

Fig. 4.55.1:

Topography and excess ¹³⁷Cs inventory as a function of surface sediment size, less than 63 μm (modified from Walling and He [403]).

4.55.3 Caesium isotopes in geochronology

Nuclear fission of ²³⁵U (or other fissionable materials) yields ¹³⁷Cs as a product. Although ¹³⁷Cs is not naturally present in the environment, it can be collected from nuclear reactor processing and then used as an environmental tracer. ¹³⁷Cs adheres tightly to porous sediments and will follow the movement of the sediment. By exposing sediments to ¹³⁷Cs and allowing this combination to move dynamically, gamma ray spectrometry can then be used to measure the activity of ¹³⁷Cs and monitor the movement of the radioactive sediments [404], [405], [406].

¹³⁷Cs dating of sediments not older than 60 years is useful in natural and artificial lakes and other environments because of its widespread production and release during atmospheric nuclear weapons testing, which began in the late 1940s, plus subsequent releases, such as during the accident at the Chernobyl nuclear reactor in April 1986. The ¹³⁷Cs concentration profile in a sediment core can be matched with the historical record of ¹³⁷Cs release to determine the approximate age profile of the sediment [406], [407].

4.55.4 Caesium isotopes in industry

High-energy gamma rays from ¹³⁷Cs serve as food irradiation devices to remove bacteria and other harmful microorganisms (living single celled organisms such as virus, algae and fungus) from food. Although ¹³⁷Cs is not used commercially for large-scale food irradiation, it has been proposed that it can be used this way. Gamma rays from the radioactive ¹³⁷Cs destroy the DNA of organisms to enable foods to last longer (i.e. irradiation of fruits and vegetables stops the ripening process) and be contamination free [408], [409].

4.56.1 Barium isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of barium possess slightly different physical and chemical properties, they can be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. von Allmen et al. [410] observed barium isotopic fractionation in the global barium cycle (Fig. 4.56.1).

Fig. 4.56.1:

Variation in isotope-amount ration(¹³⁷Ba)/n(¹³⁰Ba) of selected barium-bearing substances (modified from [410]), assuming a measured isotope-amount ratio n(¹³⁷Ba)/n(¹³⁴Ba) of 4.6470 for mean terrestrial barium [414].

High-precision barium isotope measurements reveal differences of up to 25 parts per million in the isotope-amount ration(¹³⁷Ba)/n(¹³⁶Ba) and 60 parts per million in the n(¹³⁸Ba)/n(¹³⁶Ba) ratio between chondrites and the Earth. These differences probably arose from incomplete mixing of nucleosynthetic material in the solar nebula. Barium isotopes may be the decay products of now-extinct ¹³⁵Cs (with a half-life of about 1.6×10⁶ years), which is thought to be a nucleosynthetic component. Chondritic meteorites have a slight excess of supernova-derived material as compared to Earth, demonstrating that the solar nebula was not perfectly homogenized upon formation (Fig. 4.56.1) [411], [412], [413].

4.57.1 Lanthanum isotopes in Earth/planetary science

Studies have shown that ¹³⁸La (with a half-life of 1.06×10¹¹ years) can be used along with ¹³⁸Ce and ¹³⁶Ce to measure time elapsed from a supernova explosion producing large numbers of neutrinos [415].

4.57.2 Lanthanum isotopes in geochronology

¹³⁸La decays to ¹³⁸Ce and ¹³⁸Ba, respectively, by beta decay with a half-life of 1.06×10¹¹ years and by electron capture with a half-life of 1.56×10¹¹ years. The isotope-amount ration(¹³⁸Ce)/n(¹⁴²Ce) has been used for dating rocks on long time scales (billions of years) and as a chemical tracer in geochemistry [416]. The increase in radiogenic¹³⁸Ba in rocks enriched in rare earth elements, such as allanite, enables one to determine the age of such rocks (Fig. 4.57.1) [417].

Fig. 4.57.1:

Cross plot of the isotope-amount ration(¹³⁸Ba)/n(¹³⁷Ba) and the amount ration(¹³⁸La)/n(¹³⁷Ba) in rocks and minerals from Amîtsoq, West Greenland, which yield an age of 2.408×10⁹ years [417].

4.57.3 Lanthanum isotopes used as a source of radioactive isotope(s)

¹³⁹La is used for the production of the medical radioisotope¹³⁹Ce via the ¹³⁹La (p, n) ¹³⁹Ce reaction [418].

4.58.1 Cerium isotopes in Earth/planetary science

When combined, ¹³⁸La–¹³⁸Ce and ¹⁴⁷Sm–¹⁴³Nd are two decay systems that are useful for studying processes affecting the light-rare-earth elements (lanthanum, cerium, praseodymium, neodymium, and samarium) and the igneous evolution of the Moon and Earth because different igneous materials have different cerium isotopic compositions (Fig. 4.58.1) and can be used in mass balance investigations [419], [420].

Fig. 4.58.1:

Cerium isotope-amount ratios of selected terrestrial and extraterrestrial materials (modified from [421]). Data Sources: B, [421]; MM, [422]; S, [423].

4.58.2 Cerium isotopes in geochronology

¹³⁸Ce is a radiogenic isotope produced by decay of ¹³⁸La, with a half-life of 1.06×10¹¹ years, one of the longest clocks in geochronology. Thus, the isotope-amount ration(¹³⁸Ce)/n(¹⁴²Ce) can be used for dating rocks on long time scales (billions of years) and can also be used as a chemical tracer in geochemical studies.

4.58.3 Cerium isotopes in medicine

¹⁴⁴Ce (with a half-life of 0.78 year) has been used for brachytherapy applications in cells and vessels of the body. The half-life and specific activity of ¹⁴⁴Ce give it a potential advantage over the commonly used isotope¹⁹²Ir of higher dose rate at shorter distances and lower irradiation of organs outside the tumor [424]. ¹⁴⁴Ce enables the treatment of larger arteries as compared with ³²P, another isotope commonly used for this style of radiotherapy.

4.59.1 Praseodymium isotopes in medicine

Because of its relatively short half-life (19.12 h) and decay primarily by beta decay (96.3 percent beta decay and 3.7 percent alpha decay), ¹⁴²Pr has been proposed for two main innovative applications in medicine, namely in microsphere brachytherapy and in eye plaque brachytherapy [425]. ¹⁴²Pr is advantageous because penetration of the beta fraction of the radiation is limited to a few millimeters in tissue, therefore limiting the dose of radiation to the treated site. ¹⁴²Pr may be produced either by fast neutron activation or thermal neutron activation of stable ¹⁴¹Pr.

Research in metal-bearing radiopharmaceuticals is being conducted to determine the most efficient way to produce and process radioactive metals for in vivo tracing. This research has led to the development of a potential radionuclide generator that administers radioactive metal complexes to be observed during positron emission tomography (PET) imaging. A n(¹⁴⁰Nd)/n(¹⁴⁰Pr) amount-ratio radionuclide generator has been designed to administer ¹⁴⁰Pr complexes, such as ¹⁴⁰Pr-DTPA, to be used as a tracer during a PET scan [426]. The half-life of ¹⁴⁰Pr is 3.4 min. The n(¹⁴⁰Nd)/n(¹⁴⁰Pr) ratio radionuclide generators can also be used for administering ¹⁴⁰Pr-phosphonate complexes to identify the development of skeletal metastases. Once the skeletal metastases are found, ¹⁵³Sm-EDTMP can be administered as a radiotherapeutic agent to treat bone cancer (Fig. 4.59.1) [426]. The half-life of ¹⁵³Sm is 1.9 days.

Fig. 4.59.1:

Glass microcapillary for use in brachytherapy investigations [425], composed of a mixture of silica, aluminium, oxygen, and praseodymium-141 and praseodymium-142. The glass was irradiated in a nuclear reactor to produce radioactive ¹⁴²Pr from stable ¹⁴¹Pr. Image kindly provided by Dr. Clara Ferreira (University of Oklahoma, Oklahoma City, Oklahoma, USA).

4.60.1 Neodymium isotopes in geochronology

¹⁴³Nd is a radiogenic isotope produced by decay of ¹⁴⁷Sm, with a half-life of 1.06×10¹¹ years. Thus, the isotope-amount ration(¹⁴³Nd)/n(¹⁴⁴Nd) can be used for dating rocks on long time scales and as a chemical tracer in geochemistry (Fig. 4.60.1) [427], [428]. The very small accumulation of ¹⁴²Nd in billion-year-old metamorphosed rocks from Greenland [from the relatively short-lived (about 68×10⁶ years) alpha decay of ¹⁴⁶Sm] provided evidence that the crust of the Earth formed before the young planet was more than 100×10⁶ years old. This is because only a short amount of time could have elapse to incorporate the ¹⁴⁶Sm parent radionuclide into the ancient Greenland minerals before it decayed [429], [430].

Fig. 4.60.1:

Cross plot of n(¹⁴³Nd)/n(¹⁴⁴Nd) isotope-amount ratio and n(¹⁴⁷Sm)/n(¹⁴⁴Nd) amount ratio for two periods of scheelite (calcium tungstate; ore of tungsten) mineralization (metamorphism) (modified from [428]). ¹⁴³Nd is produced by decay of ¹⁴⁷Sm. Rock containing higher amounts of ¹⁴⁷Sm at time of mineralization will over time produce higher amounts of ¹⁴³Nd (e.g. sample stage 3 K1 shear zone and sample stage 4 K2). Alternatively, rocks containing lower amounts of ¹⁴⁷Sm at time of mineralization will over time produce lower amounts of ¹⁴³Nd (e.g. sample stage 3 K1 gneiss and sample Brl). Samples from an older mineralization event will have proportionally more ¹⁴³Nd because of the longer accumulation time for ¹⁴³Nd; thus, the line through the bluish-fluorescent scheelites with an age of (319 ± 34)×10⁶ a has a substantially higher slope than the line through the whitish-bluish-fluorescent scheelites with an age of (29 ± 17)×10⁶ a. These lines from which age of mineralization (crystallization) can be determined are called isochrons.

4.60.2 Neodymium isotopes used as a source of radioactive isotope(s)

¹⁴⁶Nd has been used in the production of ¹⁴⁷Pm (with a half-life of 2.6 years), via the reaction ¹⁴⁶Nd (n, γ) ¹⁴⁷Nd, which is followed by a subsequent electron decay reaction, ¹⁴⁷Nd→¹⁴⁷Pm+β^- reaction. ¹⁴⁷Pm is a radioactive power-generation source [431].

4.61.1 Promethium isotopes in industry

The beta-particle-emitting isotope¹⁴⁷Pm (with a half-life of 2.68 years) is used in the nuclear fuel industry to measure the thickness of the inner surface layer of graphite in the cladding tube where the nuclear fuel rod is placed in a nuclear fuel reactor (Fig. 4.61.1). The graphite serves as a protective layer against mechanical contact between the nuclear fuel rod and the Zircaloy cladding (fuel-rod holding tube) and as a diffusion barrier against fission products. By placing a layer of ¹⁴⁷Pm along the inner surface of the cladding before the graphite, the long half-life of ¹⁴⁷Pm and constant beta-particle emission provide a reliable and simple technique to measure the thickness of the graphite along the inner surface of the tube (called the beta-ray backscatter technique) [432], [433], [434].

Fig. 4.61.1:

The beta-ray backscatter technique requires a layer of ¹⁴⁷Pm between the cladding and the graphite layer to measure the thickness of the graphite along the inner surface of the cladding tube. (Modified from [308]).

The beta decay property of ¹⁴⁷Pm makes this radioisotope an ideal candidate for nuclear batteries (beta voltaics). Long-lived power supplies for remote and sometimes hostile environmental conditions are needed for space and sea missions, and nuclear batteries can uniquely serve this role. A nuclear battery using beta voltaics can have an energy density (quantity of energy per unit mass) near a thousand watt-h per kilogram with 21 percent efficiency, which is much greater than the best chemical batteries [435].

4.62.1 Samarium isotopes in Earth/planetary science

One possible origin for the Moon is from debris ejected by an indirect giant impact of Earth by an astronomical body the size of Mars when the Earth was forming [436]. The kinetic energy liberated is thought to have melted a large part of the Moon forming a lunar magma ocean. Samarium isotope measurement results [437], along with measurements of isotopes of hafnium, tungsten, and neodymium[438], suggest that lunar magma formed about 70×10⁶ years after the Solar System formed and had crystallized by about 215×10⁶ years after formation. ¹⁴⁷Sm (with a half-life of 1.06×10¹¹ years) is used to study the formation of potassium, rare earth elements, and phosphorus-rich rocks [439].

4.62.2 Samarium isotopes in geochronology

¹⁴⁷Sm is used for determining formation ages of igneous and metamorphic rocks via analysis of the minerals which compose them, such as those shown in Fig. 4.62.1 [440], [441], [442].

Fig. 4.62.1:

Cross plot of n(¹⁴³Nd)/n(¹⁴⁴Nd) isotope-amount ratio and n(¹⁴⁷Sm)/n(¹⁴⁴Nd) amount ratio of carbonate and fluorocarbonates at the Bayan Obo rare-earth-element-niobium-iron deposit in Inner Mongolia, China (modified from [442]). ¹⁴³Nd is produced by decay of ¹⁴⁷Sm. Rock containing higher amounts of ¹⁴⁷Sm at time of mineralization will over time produce higher amounts of ¹⁴³Nd (e.g. fluorite samples). Alternatively, rocks containing lower amounts of ¹⁴⁷Sm at time of mineralization will over time produce lower amounts of ¹⁴³Nd (e.g. huanghoite samples). Samples from an older mineralization event will have proportionally more ¹⁴³Nd because of the longer accumulation time for ¹⁴³Nd; thus, the slope of the line through the samples above correlates to the time since mineralization (formation), and such a line is called an isochron.

4.62.3 Samarium isotopes in medicine

The radioisotope¹⁵³Sm (with a half-life of 1.9 days) is used in medicine to treat the severe pain associated with cancer that has spread to bones (Fig. 4.62.2) [443], [444], [445].

Fig. 4.62.2:

Targeting of bone metastases with ¹⁵³Sm-EDTMP in a prostate cancer patient. ANT indicates the anterior view of the patient; POST indicates the posterior view of the patient; arrow represents uptake in the pubic bone of the patient. (Image Source: Pandit-Taskar, Batraki, and Divgi, 2004) [445].

4.62.4 Samarium isotopes used as a source of radioactive isotope(s)

¹⁴⁷Sm bombarded with ⁴⁰Ca produces the radioisotope ¹⁸²Pb [446].

4.63.1 Europium isotopes in geochronology

For more than 40 years, weapons-grade plutonium was manufactured by the Krasnoyarsk Mining and Chemical Combine in the now closed town of Krasnoyarsk Krai, Russia, using single-pass uranium-graphite production reactors [447]. Water from the Yenisei River was used for heat removal from the reactor core. Radioactively contaminated water was discharged into the Yenisei River and was a primary source of contamination of bottom sediments and floodland for hundreds of kilometers down gradient from the Krasnoyarsk Mining and Chemical Combine. In 2002, radioactive contamination of the bottom sediments and floodlands was composed primarily of ¹³⁷Cs, ¹⁵²Eu, ¹⁵⁴Eu, and ⁶⁰Co [447]. The decrease in the isotope-amount ration(¹⁵⁴Eu)/n(¹⁵²Eu) down the depth profiles (Fig. 4.63.1) enables one to determine the age of bottom sediments and floodlands of the Yenisei River and calculate their average formation rates [447].

Fig. 4.63.1:

Variation in the isotope-amount ration(¹⁵⁴Eu)/n(¹⁵²Eu) along the vertical profile of floodland sediments at the tail end of Atamanovskii Island, Russia (modified from [447]).

4.63.2 Europium isotopes in industry

Europium isotopes have been used in nuclear-control applications because they are good neutron absorbers [448]. ¹⁵²Eu (with a half-life of 13.5 years), which is produced by ¹⁵¹Eu via the neutron capture reaction ¹⁵¹Eu (n, γ) ¹⁵²Eu, and ¹⁵⁴Eu (with a half-life of 8.59 years) are used as reference sources for calibration in gamma ray spectroscopy (Fig. 4.63.2) [449].

Fig. 4.63.2:

¹⁵²Eu is used as a reference source for calibrating gamma-ray spectrometer systems like the one pictured here. (Photo Source: Snyder and Duval, 2003. U.S. Geological Survey Open-File Report 03-029) [450].

4.63.3 Europium isotopes used as a source of radioactive isotope(s)

Reactions on ¹⁵³Eu can produce the therapeutic radionuclide¹⁵³Sm (with a half-life of about 1.9 days) via fast neutron irradiation ¹⁵³Eu (n, p) ¹⁵³Sm [451].

4.64.1 Gadolinium isotopes in Earth/planetary science

The lunar surface is continuously exposed to cosmic radiation, and the interaction between planetary material and cosmic rays produces secondary neutrons. The neutron flux can be investigated using the large neutron capture cross sections of ¹⁴⁹Sm, ¹⁵⁵Gd, and ¹⁵⁷Gd. For example, ¹⁵⁷Gd will absorb neutrons and be converted to ¹⁵⁸Gd. On a cross plot of n(¹⁵⁸Gd)/n(¹⁶⁰Gd) isotope-amount ratio and n(¹⁵⁷Gd)/n(¹⁶⁰Gd) isotope-amount ratio (Fig. 4.64.1), values will move from the lower right corner to the upper left corner of the cross plot with increasing time or increasing flux.

Fig. 4.64.1:

Cross plot of n(¹⁵⁸Gd)/n(¹⁶⁰Gd) and n(¹⁵⁷Gd)/n(¹⁶⁰Gd) isotope-amount ratios of samples from Apollo lunar sites A-12 and A-15 (modified from [452]).

4.64.2 Gadolinium isotopes in medicine

The addition of ¹⁵⁷Gd to Neutron Capture Therapy (NCT) has been shown to be more effective at targeting tumors than the previous method of using only ¹⁰B for the treatment (Fig. 4.64.2) [453]. ¹⁵³Gd (with a half-life of 0.66 years) is used in the production of photon line sources (an optical source that emits one or more spectrally narrow lines as opposed to a continuous spectrum) to manufacture ¹⁵³Gd line sources [454]. ¹⁵³Gd is also used as a photon source of the dual-photon absorptiometry (DPA) technique that is used to measure bone mineral content (BMC). Studies for this technique have been conducted in horses and humans [455], [456].

Fig. 4.64.2:

Patient undergoing neutron therapy. The red lasers cross to target the patient’s tumor. A beam of neutrons is fired at the target to stop the growth and eradicate the tumor. (Photo Source: Reidar Hahn, Fermilab Visual Media Services Photo Database, Fermi National Accelerator Laboratory) [457].

4.65.1 Terbium isotopes in medicine

¹⁴⁹Tb (with a half-life of 4.1 h) is being used in targeted radiotherapy using alpha particles for labeling radioimmunoconjugates in cancer treatments [458], [459]. ¹⁶¹Tb (with a half-life of 6.9 days) attached to a bioconjugate (two covalently linked molecules, one or more of which is a biomolecule), is being used in cancer therapy as a targeted radiation treatment of cancer cells [459], [460]. ¹⁶¹Tb is being used for imaging as it allows for on-line monitoring of its distribution using gamma cameras [460]. ¹⁴⁹Tb is produced by the reaction ¹⁴²Nd(¹²C,5n)¹⁴⁹Dy, which is followed by a subsequent positron decay reaction ¹⁴⁹Dy→¹⁴⁹Tb+β⁺. It can also be produced by the reaction ¹⁴¹Pr(¹²C,4n)¹⁴⁹Tb; beam geometry is important for satisfactory yield of ¹⁴⁹Tb (Fig. 4.65.1) [461].

Fig. 4.65.1:

¹⁴⁹Tb is produced from the reaction ¹⁴²Nd(¹²C,5n)¹⁴⁹Dy, which is followed by a subsequent positron decay reaction ¹⁴⁹Dy → ¹⁴⁹Tb + β⁺. A ten-fold increase in production is achieved by optimal beam geometry (modified from [461]).

4.66.1 Dysprosium isotopes in industry

The isotopes of dysprosium are highly magnetic and have been the subject of physics research involving interactions of isotopes and the structure of lattice supersolids (spatially ordered material with superfluid properties, i.e. zero viscosity). The Magneto-Optical Trapping (MOT) chamber is used for slowing atoms (isotopes) to study the physics of neutral atoms by using a laser light to cool atoms (“Doppler cooling”) and magnetic quadrupole fields to slow and “trap” the neutral atoms (Fig. 4.66.1) [462], [463].

Fig. 4.66.1:

Magneto-Optical Trapping (MOT) of isotopes of dysprosium. (Used with permission from: Prof. Benjamin Lev, Stanford University) [466].

¹⁶⁴Dy has a large neutron absorption cross section, so dysprosium is used for control rods [464]. ¹⁶¹Dy has been a key isotope for studying the Mössbauer Effect, which is the resonance and absorption of gamma ray emissions on nearby atoms in a solid state [465].

4.66.2 Dysprosium isotopes in medicine

¹⁶⁵Dy (with a half-life of 140 min) is commonly used in arthritis therapy (radiosynovectomy). Rheumatic inflammation of the membranes of joints is often treated by the injection of ¹⁶⁵Dy-ferric oxide directly into the joint space of the knee. Leakage from the joint has been shown to be minimal [467].

4.66.3 Dysprosium isotopes used as a source of radioactive isotope(s)

¹⁶⁴Dy is used to produce ¹⁶⁶Dy (with a half-life of 3.4 days) via double neutron capture [468], [469], [470]. ¹⁶⁶Dy, which decays to ¹⁶⁶Ho, is used in cancer and arthritis therapy [468], [471].

4.67.1 Holmium isotopes in medicine

Radiosynovectomy with ¹⁶⁶Ho-radiopharmaceutical agents can be used for treatment of arthritis. The half-life of ¹⁶⁶Ho is 1.1 days. ¹⁶⁶Ho ferric hydroxide macroaggregate ([¹⁶⁶Ho] FHMA) radiosynovectomy is being used because FHMA minimizes extra-articular (outside a joint) leakage of the radioisotope [472], [473]. ¹⁶⁶Ho has been used for radioimmunotherapy (RIT) with labeled antibodies [474]. The ¹⁶⁶Ho-chitosan complex (a linear polysaccharide, which is a long-chain molecule like cellulose that is used by the body for energy storage) is being used for hepatic (liver) cancer therapy [475]. ¹⁶⁶Ho-labeled radiopharmaceuticals have been used for alleviating pain from bone metastases [443], [473], [476].

¹⁶⁶Ho microspheres have been used for intra-arterial radioembolization (treatment where radioactive particles are delivered to a tumor through the bloodstream) of liver metastases (Fig. 4.67.1) [475]. ¹⁶⁶Ho is paramagnetic and emits both beta and gamma radiation, which makes it ideal for radioembolization. These properties also enable the distribution of ¹⁶⁶Ho microspheres to be visualized with magnetic resonance imaging and single-photon emission computed tomography (SPECT) [475].

Fig. 4.67.1:

Schematic overview of the administration system for ¹⁶⁶Ho-RE (radioembolization) (reprinted with permission. Copyright © 2010 Smits et al.; BioMed Central Ltd.) [475]. The administration system consists of the following components: iodine contrast agent (Visipaque^®, GE Healthcare) (1), saline solution (2), 20-ml syringe (Luer-Lock) (3), three-stopcock manifold (4), one-way valve (5), inlet line (6), administration vial containing the ¹⁶⁶Ho-poly(L-lactic acid) microspheres (7), outlet line (8), flushing line (9), Y-connector (10) and catheter (11).

The ¹⁶⁶Ho-Patch is a specially designed radioactive skin patch that is used for external radiation of superficial skin cancers and Bowen’s disease in areas that are sensitive and difficult to treat by methods that are more destructive and have poor cosmetic results (i.e. areas of the face) [477], [478].

4.68.1 Erbium isotopes in biology

Radiolabeled¹⁷¹Er (with a half-life of 7.5 h) tablets have been used to study bowel movements of individuals using external scintigraphy. Such tablets have an enteric coating and contain small amounts of stable erbium oxide (¹⁷⁰Er) initially. The tablets are then irradiated at a low neutron flux to produce radioactively labeled ¹⁷¹Er tablets, via the ¹⁷⁰Er (n, γ) ¹⁷¹Er reaction. This method is a noninvasive approach for determining gastric emptying rates and visualizing segments of the digestive system in an individual [479], [480].

4.68.2 Erbium isotopes in medicine

¹⁶⁹Er (with a half-life of 9.4 days) is used in radiosynovectomy, which is a regularly practiced radiotherapy, on rheumatoid arthritis patients whose condition is resistant to standard methods of treatment (Fig. 4.68.1). Rheumatoid arthritis is a chronic, inflammatory, autoimmune disease of the joint capsule (synovial sac), which is lined with a thin membrane called the synovium, of an individual’s moveable joints (synovial joints). In radiosynovectomy, the radiopharmaceutical called ¹⁶⁹Er- citrate colloid, which contains colloidal particles that are labeled with β-emitting ¹⁶⁹Er, is directly injected into the synovial cavity (the cavity between the bones in a moveable joint inside of the synovium) of the affected joint. These radioactive-colloid particles are then phagocytized (engulfed) by macrophage-like synoviocytes as well as other phagocytizing inflammatory cells in the patient’s synovium. Necrosis (tissue death) and the inhabitation of cell proliferation (increase in number of cells) result from the radiation of the synovium and therefore, temporarily halts synovitis (which is the condition of when the synovium thickens with inflammation) and improves synovial joint function [481], [482], [483], [484].

Fig. 4.68.1:

Normal joint (top) vs. joint affected by rheumatoid arthritis (bottom) (modified from [485]).

4.69.1 Thulium isotopes in industry

¹⁷⁰Tm (with a half-life of about 130 days) is used in the petrochemical industry for industrial radiography to test welds in pipes and tanks [486].

4.69.2 Thulium isotopes in medicine

¹⁶⁷Tm (with a half-life of 9.2 days) is useful for tumor and bone studies [487]. Stable ¹⁶⁹Tm can be bombarded in a nuclear reactor to create ¹⁷⁰Tm, via the ¹⁶⁹Tm (n, γ) ¹⁷⁰Tm reaction, which emits X-rays and has been used in portable X-ray equipment as a radiation source [488]. ¹⁷⁰Tm has been used in high-dose-rate (HDR) brachytherapy [489] and for use in radiosynovectomy of medium sized joints (Fig. 4.69.1) [490].

Fig. 4.69.1:

Wholebody single-photon emission computed spectroscopy (colored layer in image) and CT scan of Beagle dog recorded after 4 h of administration of ¹⁷⁰Tm-labeled microparticles [490]. Radioactive-colloid accumulation is displayed in the right knee joint. The Beagle did not suffer any health impairment. Image kindly provided by Dr. Andras Polyak (Dept. of Nuclear Medicine, Hannover Medical School, Germany).

4.70.1 Ytterbium isotopes in industry

¹⁶⁹Yb (with a half-life of 32 days) emits gamma rays and can be used to create a radiographic image of an object without the use of electricity. A capsule containing ¹⁶⁹Yb is placed on one side of the object being screened and photographic film is placed on the other. The result will indicate flaws in metal casting or welded joints [491], [492]. Gamma cameras use ¹⁶⁹Yb as a radiation source (Fig. 4.70.1). Gamma cameras are used to locate sealed radioactive sources and hot spots in historical waste. Images of the gamma ray intensity are made and then the 2-D distribution is superimposed on a picture or video image [493], [494].

Fig. 4.70.1:

Gamma cameras are typically used to identify radioactive holdup (material that does not come out of a process as product or waste). The picture to the left is of a tank and the picture to the right shows the radioactivity in the tank. (Photo Source: International Atomic Energy Agency, 2008) [493].

¹⁷¹Yb has been used for making an atomic clock by making use of a ytterbium optical lattice (formed by the interference of counter-propagating laser beams) (Fig. 4.70.2) [495], [496], [497].

Fig. 4.70.2:

The insides of the National Institute of Standards and Technology’s (NIST) optical atomic clock. The red rings are magnetic coils and the red laser beam is an optical lattice. The intersecting violet lasers cool the ytterbium atoms. (Image Source: National Institute of Standards and Technology, 2006) [497].

4.70.2 Ytterbium isotopes in medicine

In the treatment of prostate cancer with brachytherapy seed implants, ¹⁶⁹Yb has been suggested as an alternative to using ¹²⁵I and ¹⁰³Pd [498], [499].

4.70.3 Ytterbium isotopes used as a source of radioactive isotope(s)

The radioisotope¹⁶⁹Yb is manufactured using ¹⁶⁸Yb via the reaction ¹⁶⁸Yb (n, γ) ¹⁶⁹Yb.

4.71.1 Lutetium isotopes in biology

¹⁷⁶Lu (with a half-life of 3.73×10¹⁰ years) is used in labeling experiments to quantify absolute protein abundance (absolute quantities of proteins in a cell) and examine the extent of synthesis of proteins under specific biological conditions [500]. ¹⁷⁵Lu has been used as a yield tracer in inductively coupled plasma mass spectrometry (ICP-MS) determination of plutonium in urine [500].

4.71.2 Lutetium isotopes in medicine

¹⁷⁷Lu (with a half-life of 160 h) has potential for use as an isotope for radioimmunotherapy for the treatment of small, soft tumors and for imaging purposes (Fig. 4.71.1) [501].

Fig. 4.71.1:

Bone scan (on left) and ¹⁷⁷Lu scan (on right) done 10 days apart on a patient with prostate cancer (PC), which metastasized to his bones, and who is being treated with the experimental drug ¹⁷⁷lutetium-labeled J591 (¹⁷⁷Lu-J591). The primary areas of uptake of this drug in the body are in PC metastases, which appear as small dark spots in both sets of scans, and in the liver (the large dark spot in the ¹⁷⁷Lu scans). The location and number of metastases is clearer in the ¹⁷⁷Lu scan than in the bone scan. (Image Source: Bander, Milowsky, Nanus, Kostakoglu, Vallabhajosula and Goldsmith, 2005, © American Society of Clinical Oncology. All rights reserved.) [501].

4.72.1 Hafnium isotopes in geochronology

Some ¹⁷⁶Hf is radiogenic as a result of it being formed as a product of beta decay of radioactive ¹⁷⁶Lu (half-life of 3.73×10¹⁰ years) [301]. Thus, relations between the isotope-amount ratiosn(¹⁷⁶Hf)/n(¹⁷⁷Hf) and n(¹⁷⁶Hf)/n(¹⁷⁶Lu) have been used to determine the ages of minerals and rocks. Because of the long half-life of ¹⁷⁶Lu, these ratios have been used in geochronology studies that document some of the oldest rocks in the Solar System and on Earth (Fig. 4.72.1).

Fig. 4.72.1:

Separation of the Earth into layers (crust, mantle, inner core, and outer core) was largely caused by gravitational differentiation (separating different constituents at temperature where materials are liquid or plastic, owing to differences in density) early in Earth’s history. (Image Source: University of Wisconsin-Madison Space Science and Engineering Center) [505].

Hafnium isotopic compositions of terrestrial materials evolved differently depending on the relative rates of ¹⁷⁶Hf production. Geologists can use calculated lutetium-hafnium ages and the initial isotope-amount ratio n(¹⁷⁶Hf)/n(¹⁷⁷Hf) along with other isotopic data from the oldest rocks in the Earth to infer that the Earth’s crust differentiated within the first few hundred million years after condensation of the oldest solid matter in the Solar System [502].

Radioactive ¹⁸²Hf decays to ¹⁸²W with a half-life of 8.9×10⁶ years, which is much less than the age of meteorites and the Earth. Therefore, measurements of the amounts of hafnium and tungsten isotopes in meteorites and terrestrial samples reveal the earlier presence of ¹⁸²Hf. As a result, this provides information about chemical differentiation and evolution of the early Solar System [503], [504].

4.73.1 Tantalum isotopes in medicine

¹⁷⁸ Ta (with a half-life of 9.3 min) is used in medical studies, such as first-pass radionuclide angiography of mice, to better understand cardiovascular disease. Radionuclide angiography uses a pinhole lens fitted to a high-speed multiwire proportional camera and a n(¹⁷⁸W)/n(¹⁷⁸Ta) amount-ratio generator for minimally invasive quantification of murine ventricular (heart) functions (Fig. 4.73.1) [506], [507]. The multiwire gamma camera has a ¹⁷⁸Ta generator incorporated in its housing, and it provides portable and laboratory ventricular function assessments for cardiovascular patients [507], [508]. Intravenous injections of ¹⁷⁸Ta are used in gated equilibrium blood pool imaging [509]. ¹⁸³Ta (with a half-life of 5.1 days) has potential for use in radionuclide pharmaceuticals and as a tracer for toxicity studies of ecosystems [510].

Fig. 4.73.1:

Multiwire gamma camera containing a ¹⁷⁸Ta generator. [Photographer: Ami Iskandrian, M.D. University of Alabama at Birmingham (used with permission)] [511].

4.73.2 Tantalum isotopes used as a source of radioactive isotope(s)

¹⁸¹Ta is used to produce ¹⁷⁸W, which decays to ¹⁷⁸Ta via the reaction ¹⁸¹Ta (p, 4 n) ¹⁷⁸W, which is followed by a subsequent electron capture decay reaction of ¹⁷⁸W to finally yield ¹⁷⁸Ta. ¹⁷⁸Ta is important for medical studies as noted in Section 4.73.1.

4.74.1 Tungsten isotopes in Earth/planetary science

¹⁸²W is the stable product of the decay of ¹⁸²Hf, which has a half-life of 8.9×10⁶ years. Although ¹⁸²Hf was present at the dawn of the Solar System, this isotope has long since decayed. During the formation of the planets, including Earth, the elements hafnium and tungsten were partitioned into silicate minerals (rock forming minerals with silicon-oxygen bonds that constitute more than 90 percent of the Earth’s crust) and metal phases, respectively. The measurement of excessive amounts of ¹⁸²W, arising from the decay of ¹⁸²Hf that accumulated in silicate minerals, has been used to estimate the time that elapsed between the formation of the Solar System and accretion of the planets (Fig. 4.74.1) [512], [513].

Fig. 4.74.1:

Core formation scenarios. ¹⁸²Hf is produced during the end stages of a supernova explosion and decays to ¹⁸²W. The Early Core Scenario shows that when a core forms relatively early after a supernova explosion, a small amount of ¹⁸²Hf will be present in the mantle that will produce a significant amount of ¹⁸²W. The Late Core Scenario shows that ¹⁸²Hf was produced and decays to ¹⁸²W prior to the formation of the metallic core. Once the metallic core begins to form, it will attract tungsten because it is strongly attracted to metals. Almost all of the ¹⁸²W is partitioned into the metallic core and only a small amount will be left in the mantle. (Diagram Source: Steven Earle, Vancouver Island University) [514].

4.74.2 Tungsten isotopes used as a source of radioactive isotope(s)

Tungsten-rhenium generators use ¹⁸⁸W, which is produced from ¹⁸⁶W, via the following double neutron capture reaction ¹⁸⁶W (n, γ) ¹⁸⁷W (n, γ) ¹⁸⁸W.

4.75.1 Rhenium isotopes in geochronology

The rhenium-osmium dating method is of special interest for the dating of rhenium-bearing ores, gold deposits, copper-nickel deposits, and meteorites. This method is based on the beta-decay of ¹⁸⁷Re (having a half-life of 41.6×10⁹ years) to ¹⁸⁷Os, an example of which appears in Fig. 4.75.1 [515].

Fig. 4.75.1:

Cross plot of n(¹⁸⁷Os)/n(¹⁸⁸Os) isotope-amount ratio and n(¹⁸⁷Re)/n(¹⁸⁸Os) amount ratio of marine and non-marine organic-rich sediments and coals from Egypt (modified from [515]). The Maghara area (about 200 km east of Cairo) has middle-Jurassic coal beds (about 165×10⁶ years old, identified by the green dotted line). The age of marine black shales in the Red Sea area was previously estimated as 74×10⁶ years old, identified by the blue dotted line. ¹⁸⁷Os is produced by decay of ¹⁸⁷Re. Samples from an older formation will have proportionally more ¹⁸⁷Os because of the longer accumulation time for ¹⁸⁷Os; thus, the slope of the line for the Maghara coals (turquoise triangles), having an age of 165×10⁶ years, is substantially higher than the slope of the Red Sea specimens. Note the analytical challenges in obtaining isotope-amount values that plot near the 165- and 74-million-year isochrons. Many of the values plot between the isochrons.

4.75.2 Rhenium isotopes in medicine

¹⁸⁶Re (with a half-life of 89 h) is a beta-emitting radioisotope that is used for cancer treatment, in particular for pain relief in bone cancer and in rheumatoid arthritis (see radiosynovectomy). It is produced from the stable isotope¹⁸⁵Re via the ¹⁸⁵Re (n, γ) ¹⁸⁶Re reaction [188]. ¹⁸⁶Re is also used for radiolabeling of cancer therapeutic agents [188]. ¹⁸⁸Re (with a half-life of 17 h) is used to irradiate coronary arteries with beta particles during insertion of an angioplasty balloon (a tiny balloon that is inserted into an artery and inflated to flatten plaque build-up and improve blood flow) and in palliative therapy, particularly for bone metastases. The beta irradiation can decrease scar tissue formation after the overstretching of arteries by angioplasty.

4.76.1 Osmium isotopes in Earth/planetary science

The isotope-amount ration(¹⁸⁷Os)/n(¹⁸⁶Os) in rocks can be transferred to fluids, such as magmas, groundwaters, rivers, and oceans. Variations in the inherited n(¹⁸⁷Os)/n(¹⁸⁶Os) ratios can provide a useful tracer for fluid sources and migration paths, including different layers of the Earth [301], [504], [516], [517]. Meteorites and meteorite dust impacting the Earth have different osmium isotopic compositions than terrestrial rocks and sediments. As a result, n(¹⁸⁷Os)/n(¹⁸⁶Os)-ratio studies provide evidence of continuing extraterrestrial additions to the Earth over geologic time, as well as providing a method for prospecting in the sedimentary record for large meteorite impact events that may have affected life on Earth [518].

4.76.2 Osmium isotopes in geochronology

Some ¹⁸⁷Os is radiogenic as a result of being formed by the beta decay of radioactive ¹⁸⁷Re, which has a half-life of 4.16×10¹⁰ years. Variations in the isotope-amount ratio n(¹⁸⁷Os)/n(¹⁸⁶Os) and amount ration(¹⁸⁷Re)/n(¹⁸⁶Os) are used for geochronology; for example, variations in these ratios have been used to determine the ages of the Earth, Moon, and meteorites [301]. Kirk et al. [519] measured rhenium-osmium isotopic abundances in gold and pyrites from conglomerates of the Central Rand Group of South Africa (Fig. 4.76.1), which have produced over 48 000 metric tons of gold and have accounted for 40 percent of the world’s total historic production [520]. The gold and rounded pyrites from the conglomerates yield an age of ~3.0×10⁹ years. Kirk et al. find that this age is much older than that of the conglomerate, and they conclude that the gold is detrital (material wearing away by weathering or erosion) and was not deposited by later hydrothermal fluids.

Fig. 4.76.1:

Cross plot of n(¹⁸⁷Os)/n(¹⁸⁸Os) isotope-amount ratio and n(¹⁸⁷Re)/n(¹⁸⁸Os) amount ratio of gold and pyrite from South Africa’s Witwatersrand Supergoup gold deposits (modified from [519]). The turquoise diamonds are gold-bearing samples from Vaal Reef, and they form an isochron with an age of about 3.0×10⁹ years. The purple filled circles are euhedral pyrites (crystals having a flat surface and sharp angles) from the Venterdorp Contact Reef, and they have a much younger age of (672 ± 510)×10⁶ a.

4.76.3 Osmium isotopes used as a source of radioactive isotope(s)

¹⁹²Os can be used for the production of the medical radioisotope^195mPt via the ¹⁹²Os (α, n) ^195mPt reaction.

4.77.1 Iridium isotopes in industry

Metallic ¹⁹²Ir (with a half-life of 74 days) is used as a radiation source in gamma cameras for non-destructive testing of products for manufacturing flaws, such as aircraft parts, boilers, and pipeline welds (Fig. 4.77.1) [274].

Fig. 4.77.1:

This is a radiograph of a weld using ¹⁹²Ir as the source of radiation. The gamma rays given off by the isotope enable imperfections in the weld to be seen on the radiograph. (Photo Source: Hayward and Currie, 2006) [274].

4.77.2 Iridium isotopes in medicine

Metallic ¹⁹²Ir is used in brachytherapy [188], [521], [522], [523]. ^191mIr (with a half-life of 5 s) is used for blood flow imaging (angiography), especially in pediatric populations [524], [525]. The m in the superscript ^191mIr indicates a metastable state of the isotope.

4.77.3 Iridium isotopes used as a source of radioactive isotope(s)

Iridium consists of two stable isotopes (¹⁹¹Ir and ¹⁹³Ir) from which the radioactive isotopes¹⁹²Ir and ^195mPt (with a half-life of 4 days) can be produced. Both are used in nuclear medicine. The m in the superscript ^195mPt indicates a metastable state of the isotope.

4.78.1 Platinum isotopes in Earth/planetary science

Astrophysicists have confirmed an anomaly in the isotopic composition of platinum in the chemically peculiar HgMn star χ Lupi, where the platinum isotopic composition was shown to be a mixture of ¹⁹⁶Pt and ¹⁹⁸Pt (Fig. 4.78.1) [526].

Fig. 4.78.1:

Observed spectra of the chemically peculiar HgMn star χ Lupi (solid blue line) compared to a synthetic spectra calculated with a mixture of 70 percent ¹⁹⁶Pt and 30 percent ¹⁹⁸Pt (dotted pink line) (modified after [526]).

4.78.2 Platinum isotopes in geochronology

The decay of ¹⁹⁰Pt (with a half-life of 4.9×10¹¹ years) to ¹⁸⁶Os over time has been used for dating rocks and iron meteorites [527].

4.78.3 Platinum isotopes in medicine

^195mPt (with a half-life of 4 days) is used for pharmacokinetic studies of platinum-based anti-tumor agents in cancer diagnosis and cancer therapy [188]. The m in the superscript of ^195mPt indicates a metastable state of the isotope. ^195mPt can be produced from the stable isotopes¹⁹²Os or ¹⁹⁵Pt via the ¹⁹²Os (α, n) ^195mPt reaction and the ¹⁹⁵Pt (n, n′) ^195mPt reaction, respectively.

4.79.1 Gold isotopes in biology

¹⁹⁵Au (with a half-life of about 0.51 year) has been used to study particle movement within the lungs of rats [528]. ¹⁹⁸Au (with a half-life of 2.7 days) was used in a study to model gold cycling in plants. This study demonstrated that gold particles are retained by humates (organic constituents of soil), which contain fulvic acid, humic acid, ulmic acid, and lignin and would therefore be likely to accumulate in mull humus or forest litter [529].

4.79.2 Gold isotopes in medicine

¹⁹⁸Au has several medical uses. It has been used as both a diagnostic tool and a treatment option for cancer [530], [531].

1. As a diagnostic tool, colloidal ¹⁹⁸Au is injected into the affected organ. Normal cells will take up the gold colloid, but tumor cells will not. Therefore, an abscess will show up as a “cold area” on a scan [531].

2. As a treatment option, gold is intended to provide localized irradiation and can be implanted or injected into the affected area. When implanted, the gold “seed” offers an advantage over other materials in that it can be left in place due to its short half-life (2.7 days). As a colloidal injection, ¹⁹⁸Au has been found to produce improvement from a wide variety of cancers [530]. Figure 4.79.1a and 4.79.1b, respectively, show squamous cell carcinoma (cancer) on the lower left eyelid of a cat and the eyelid 6 weeks after implantation of ¹⁹⁸Au seeds [532].

Fig. 4.79.1:

(a) Squamous cell carcinoma on the lower left eyelid of a cat [532]; (b) the lower left eyelid 6 weeks after implantation of ¹⁹⁸Au seeds (Reprinted with permission. Copyright 2001 Hardman and Stanley, 2001, Australian Veterinary Journal, Wiley) [532].

Recent studies have shown the effectiveness of ¹⁹⁸Au nanoparticles and nanodevices in reducing tumor size in mice while minimizing radiation spread to other areas [530], [533], [534]. ¹⁹⁸Au has been studied and successfully used as an anti-inflammatory (a substance or treatment that reduces the body tissues response to harmful stimuli, such as swelling) for improving arthritic conditions [535], [536].

4.80.1 Mercury isotopes in Earth/planetary science

¹⁹⁸Hg, ²⁰⁰Hg, and ²⁰²Hg are stable isotopes of mercury that can be used to study environmental sources and environmental sinks of this element in aquatic and terrestrial ecosystems. For example, in an ecosystem, different stable isotopes of mercury can be added to an upland region for run-off evaluation, to a lake for direct deposition analysis, and to a wetland region for outflow contribution analysis (Fig. 4.80.1). As a result, it is possible to determine the entry points of mercury into an ecosystem and determine how the inputs of mercury affect the accumulation of this element in local fish populations. An international consortium of scientists is conducting an experiment called METAALICUS (Mercury Experiment To Assess Atmospheric Loading In Canada and the U.S.). This experiment includes determination of whether mercury contamination in fish is old or new mercury. Tracer studies were performed in northwestern Ontario at the Experimental Lakes Area of the Department of Fisheries and Oceans Canada [537].

Fig. 4.80.1:

²⁰²Hg was added to small watersheds to study the fate of mercury from atmospheric deposition in pristine (in its original condition) lakes as part of the METAALICUS study. (Photo Source: Toxic Substances Hydrology Program, U.S. Geological Survey) [537].

4.80.2 Mercury isotopes used as a source of radioactive isotope(s)

²⁰²Hg is used to produce radioactive ²⁰³Hg (with a half-life of 46.6 days) via the ²⁰²Hg (n, γ) ²⁰³Hg reaction, which is used in gamma radiation calibration and medical tests.

4.81.1 Thallium isotopes in Earth/planetary science

Because molecules, atoms, and ions of the stable isotopes of thallium possess slightly different physical and chemical properties, they commonly will be fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. There are substantial variations in the isotopic abundances of thallium in natural terrestrial materials (Fig. 4.81.1). These variations are useful in investigating the origin of substances and studying environmental, hydrological, and geological processes [538]. The isotope-amount ration(²⁰⁵Tl)/n(²⁰³Tl) has been used to study how trace metals are transported and distributed in hydrothermal fluids [538]. The n(²⁰⁵Tl)/n(²⁰³Tl) ratio has also been used to study the cycling, distribution, and behavior of thallium in the marine environment [538].

Fig. 4.81.1:

Variation in atomic weight with isotopic composition of selected thallium-bearing materials (modified from [13], [17]).

4.81.2 Thallium isotopes in medicine

²⁰¹Tl scintigraphy is used to detect coronary artery disease [539]. Imaging of ²⁰¹Tl (with a half-life of 3 days), can be used for exercise perfusion tests of the myocardium (muscular tissue of the heart), which determine damage to the heart caused by a heart attack or by heart disease (Fig. 4.81.2) [539].

Fig. 4.81.2:

During exercise myocardium perfusion tests, a patient will exercise on a treadmill until they have reached their maximal exercise point (determined by heart rate and age). Once they reach their maximal exercise point, a radionuclide (usually of thallium or technetium) is injected into the intravenous line in their arm, and the patient continues to exercise for a few more minutes. Once the radionuclide reaches the heart, the patient lies flat on a table and a gamma camera takes pictures of the heart for about 30 min. The areas of decreased blood flow or damaged tissue will be illuminated by the radionuclide. (Image source: National Heart Lung and Blood Institute, Diseases and Conditions Index, National Institutes of Health) [540].

4.81.3 Thallium isotopes used as a source of radioactive isotope(s)

²⁰³Tl is used in the production of ²⁰¹Tl via the ²⁰³Tl (p, 3 n) ²⁰¹Pb reaction, which is followed by a subsequent electron capture decay reaction of ²⁰¹Pb to finally yield ²⁰¹Tl. ²⁰⁵Tl is used as an alternative target in the production of ²⁰¹Tl.

4.82.1 Lead isotopes in Earth/planetary science

The study of lead isotopic compositions is used to model the distribution of pollution in water and on land (Fig. 4.82.1). For example, in one study of Lake Härsvatten in Sweden, the isotope-amount ration(²⁰⁶Pb)/n(²⁰⁷Pb) measured at different sediment depths in different areas throughout the lake showed patterns of accumulation of lead pollution. In some cases, these patterns could be related to sediment distribution patterns. Another study used ²¹⁰Pb (with a half-life of 22.6 years) dating methods to study the vertical accretion of sediments in canals and wetland areas in Louisiana over the last 80 to 100 years [541], [542].

Fig. 4.82.1:

Suspended atmospheric dust over California; it is likely that this dust originated in Asia based on lead-isotope studies. (Photo Source: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE, NASA Earth Observatory, 2001) [551], [558].

Three of the stable isotopes of lead (²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb) are produced by the radioactive decay of isotopes of uranium and thorium (²³⁸U, ²³⁵U, and ²³²Th, respectively) and are largely unaffected by environmental and metallurgical processes. Therefore, by examining various isotope-amount ratios of lead isotopes, it is possible to approximate the age of a material. It is also possible to use this information to trace the origins of an object or material [543], [544], [545], [546].

4.82.2 Lead isotopes in forensic science and anthropology

Different geographic regions may have characteristic terrestrial lead isotopic compositions because of variations in the ages and chemical composition of the rocks and minerals in the local environment. Therefore, lead produced at a particular location can have a unique lead isotopic composition and it is possible to trace the history and origins of pollutants by measuring the relative amounts of the four stable isotopes of lead (²⁰⁸Pb, ²⁰⁷Pb, ²⁰⁶Pb, and ²⁰⁴Pb) (Fig. 4.82.2) [547], [548]. Using isotopic abundance data, the source of this toxic metal can be identified as it moves through air and water and eventually to living systems [547], [549]. Scientists have analyzed lead in air pollution in California and found that it originated from Asia. Airborne particles from China have relatively higher amounts of ²⁰⁸Pb, which distinguishes the lead isotopic signature between airborne particles from Asia and North America. This knowledge could have implications in understanding the mixing of particles in the atmosphere and how pollutants are transported over vast distances [547], [549], [550], [551]. Mapping the distribution of lead pollution by studying ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb also allows the identification of those human activities that contribute the highest amounts of lead to the environment [547], [549], [552].

Fig. 4.82.2:

Cross plot of n(²⁰⁶Pb)/n(²⁰⁴Pb) and n(²⁰⁶Pb)/n(²⁰⁷Pb) isotope-amount ratios of lead in selected materials (modified from [548]).

The measurement of the isotopic composition of lead in blood can help to determine the source of this toxic element in the body [553]. Lead is stored in bones and teeth. If a person moves to a different geographical region, the isotopic composition of the lead in the teeth is maintained, recording their place of origin. Bone can store lead for long periods of time (about 20 years), and some skeletal lead may be older and have a different isotopic composition than other skeletal lead. These differences reflect exposure to lead of different origins. By studying the isotope-amount ratio n(²⁰⁶Pb)/n(²⁰⁴Pb) and n(²⁰⁷Pb)/n(²⁰⁶Pb) in bone and teeth, it is possible to determine someone’s place of origin. For example, isotopes of lead were analyzed in the teeth and bones of a human mummy, known as the “Iceman”, to help determine his place of origin [554], [555].

²¹⁰Pb is a relatively short-lived radioactive isotope of lead that is constantly produced by the decay of ²²²Rn in the atmosphere. While living, humans naturally incorporate ²¹⁰Pb from the environment into bones and tissues. The amount of ²¹⁰Pb in the body reaches equilibrium such that the ²¹⁰Pb ingested is in equilibrium with the ²¹⁰Pb that decays. When a person dies, this incorporation of ²¹⁰Pb ceases and the relative amount of this isotope in the body decreases. Therefore, measurement of the ²¹⁰Pb activity in a corpse can help determine time of death [556], [557].

Lead isotope-amount ratios n(²⁰⁶Pb)/n(²⁰⁴Pb), n(²⁰⁷Pb)/n(²⁰⁴Pb), and n(²⁰⁸Pb)/n(²⁰⁴Pb)) along with isotope-amount ratio of silver, n(¹⁰⁷Ag)/n(¹⁰⁹Ag), and isotope-amount ratio of copper n(⁶⁵Cu)/n(⁶³Cu) have been used to determine the origin of European coins (see Section 4.29 on copper) and to investigate the flow of goods in the world market over time [237]. Metals from Peru and Mexico and those from European mining have distinct isotopic signatures that enable the origin of the metal to be determined by examining the isotopic compositions of silver, copper, and lead in the coins. Abundant silver sources mined in Mexico and Peru in the 16^th century were used to mint coins, but were not a major influence in the European coin market until the 18^th century [237].

4.82.3 Lead isotopes in geochronology

The three natural radioactive-decay chains beginning with ²³⁸U, ²³⁵U, and ²³²Th each have comparable half-lives that are much longer than the radioactive isotopes that follow until the production of stable isotopes of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, respectively. Therefore, one can measure the relative amounts of the radiogenic isotopes of lead to determine the length of time that has elapsed since uranium and thorium atoms were incorporated into rocks and minerals. Typically, this method is used to date minerals that are tens of millions to billions of years old. The uranium-lead dating method was used to determine some of the first accurate ages of the Earth (about 4.55×10⁹ years) [554], [555], [556].

4.83.1 Bismuth isotopes in medicine

²¹²Bi and ²¹³Bi (with half-lives of 1 h and 0.76 h, respectively) are both used in medicine for radioimmunotherapy as bismuth-labeled monoclonal antibodies to treat cancer cells from melanoma (skin cancer) (Fig. 4.83.1) and ovarian cancer [559]. Figure 4.83.2 compares the biologic effect of ¹³¹I and ²¹³Bi using a specific monoclonal antibody, B-B4, coupled to ²¹³Bi by a chelating agent (a substance that can form multiple bonds to a single metal ion). ²¹³Bi is a mixed alpha and beta emitter with a half-life of 0.76 h. The primary mode of decay is by beta emission to the very short-lived alpha emitter ²¹³Po. The 8.4 MeV alpha particle emitted by ²¹³Po has a path length of 76 μm in human tissue and is responsible for its cytotoxic effects (toxic to living cells). ²¹³Bi is produced from a series of alpha particle decays beginning with ²²⁵Ac, which is a pure alpha emitter with a half-life of 10 days. A schematic of the Institute for Transuranium Elements (ITU) Standard ²²⁵Ac/²¹³Bi Radionuclide Generator is shown in Fig. 4.83.3.

Fig. 4.83.1:

Melanoma (skin cancer) on a patient’s foot. ²¹²Bi and ²¹³Bi are both used as bismuth-labeled monoclonal antibodies to treat cancer cells from melanoma. (Photo Source: Kelly Nelson, National Cancer Institute) [561].

Fig. 4.83.2:

Comparison of biological effectiveness of ²¹³Bi and ¹³¹I when coupled to the specific monoclonal antibody B-B4 (modified after [562]); MBq/L, million becquerel per litre.

Fig. 4.83.3:

Schematic of the Institute for Transuranium Elements (ITU) Standard ²²⁵Ac/²¹³Bi Radionuclide Generator. Image kindly provided by Dr. Alfred Morgenstern, European Commission, Joint Research Centre – Institute for Transuranium Elements, Karlsruhe, Germany.

²¹²Bi has been used for radioimmunotherapy of leukemia and for targeting the vascular endothelial cells (thin layer of simple squamous cells that forms the interface between circulating blood or lymph and the remainder of the vessel wall) of tumors [560].

4.83.2 Bismuth isotopes used as a source of radioactive isotope(s)

²⁰⁹Bi is bombarded with neutrons in a nuclear reactor to form radioactive ²¹⁰Bi. The ²¹⁰Bi (with a half-life of 5 days) decays via the reaction ²¹⁰Bi→²¹⁰Po+β⁻. The half-life of ²¹⁰Po is 138 days and it is used in static eliminators in machinery [75].

4.84.1 Polonium isotopes in industry

²¹⁰Po (with a half-life of 138 days) is used as static eliminator to remove static electricity in machinery. This is useful in machinery that produces electricity easily, for example, via rolling paper, manufacturing sheet plastics, and spinning synthetic fibers, which all readily produce static [75], [563]. ²¹⁰Po can also make use of its static eliminating properties when used in brushes that function to clean camera lenses and photographic films (Fig. 4.84.1) [75]. ²¹⁰Po has been used to manufacture atomic weapons. When combined with beryllium, polonium can act as a neutron-producing initiator. However, because of its short half-life, ²¹⁰Po is no longer used in this manner [75].

Fig. 4.84.1:

Staticmaster™ Alpha Ionizing Brushes for cleaning optical surfaces and photographic films have a soft, non-abrasive brush and a ²¹⁰Po cartridge. (Photo Source: Reston Stable Isotope Laboratory of the U.S. Geological Survey) [564].

4.85.1 Astatine isotopes in medicine

²¹¹At (with a half-life of 7.2 h) is known to accumulate in the thyroid and occasionally is the preferred treatment for hyperthyroidism and thyroid cancer because the particles emitted from ²¹¹At provide more energy than radiolabeled iodine, the other treatment method (Fig. 4.85.1). However, astatine has shown a tendency to induce tumors, so its use is limited [565]. The ²¹¹At-labeled di-carborane (cluster of boron, carbon, and hydrogen atoms) ligand known as the Venus Flytrap Cluster (VFC) has been used as a robust pharmaceutical in radiotherapy treatment [566].

Fig. 4.85.1:

²¹¹At treats hyperthyroidism and thyroid cancer. (Image Source: © 2012 Terese Winslow LLC, U.S. Govt. has specified rights) [567].

4.86.1 Radon isotopes in Earth/planetary science

Both ²²⁰Rn and ²²²Rn (with half-lives of 56 s and 3.8 days, respectively) are used to study underground environmental and atmospheric gaseous-transport processes [568], [569], [570]. The interaction of radon with streams and rivers enables it to be used as a tracer in groundwater studies (Fig. 4.86.1). ²²²Rn has a short residence time in streams and river channels, which leads to radon loss. As a result, if an area of a stream or river has a high concentration of radon, it suggests that there are local groundwater inputs [568], [569], [570]. In a deep (100 m) contaminated aquifer at a refinery site in Mexico, where the contaminated source was too deep to be directly accessible for sampling, Schubert et al. [571] collected groundwater samples from a few wells available at the site. They used the partitioning of the natural tracer ²²²Rn between uncontaminated groundwater and the NAPL (non-aqueous phase-liquid, such as oil, gasoline, and petroleum) source zone, and they were able to approximately identify the location of the NAPL source zone. As noted in Section 4.88.1, ²²²Rn has been used to quantify submarine groundwater discharge [572].

Fig. 4.86.1:

Air-water equilibrator, which strips radon out of water and into the gas phase so it can be used as a groundwater tracer. (Photo Source: John Crusius, U.S. Geological Survey) [573].

4.86.2 Radon isotopes in geochronology

²²²Rn has been used as a tool to date groundwater in combination with other isotopes or elemental ratios (i.e. helium/radon and xenon/radon amount ratios) [568], [574].

4.88.1 Radium isotopes in Earth/planetary science

The radioactive isotopes²²³Ra (with a half-life of 275 h), ²²⁴Ra (with a half-life of 88 h), ²²⁶Ra (with a half-life of 1600 years), and ²²⁸Ra (with a half-life of 5.75 years) are used as tracers to determine water flow rates. They are ideal environmental tracers because they behave conservatively once released into a water mass (meaning only mixing and decay processes affect their distribution) [578]. The activity ratios A(²²⁴Ra)/A(²²³Ra), A(²²³Ra)/A(²²⁶Ra), A(²²⁴Ra)/A(²²⁸Ra), and A(²²⁸Ra)/A(²²⁶Ra) have been used in lake studies to monitor and detect water inflow and mixing, to determine sources of inflowing water, and to monitor introduced water masses as they move within a body of water (i.e. a lake) [578], [579]. For example, submarine groundwater discharge is an important pathway that transports dissolved substances from aquifers below a seabed to the coastal ocean. Submarine groundwater discharge can be difficult to quantify because it is both spatially and temporally variable. As a result, its relative importance in coastal ocean chemical budgets is commonly poorly known. Peterson et al. [572] used an hourly time series of measurements of multiple radium isotopes²²³Ra, ²²⁴Ra, and ²²⁶Ra to quantify submarine groundwater discharge. They also used ²²²Rn (with a half-life of 3.8 days) measurements to independently quantify submarine groundwater discharge.

4.88.2 Radium isotopes in geochronology

²²⁶Ra and ²²⁸Ra can be used for dating materials up to a few thousand years in age because the half-lives of ²²⁶Ra and ²²⁸Ra are 1600 years and 5.75 years, respectively, even though the long-lived ²²⁶Ra is found in nature as a result of its continuous production by the decay of ²³⁸U. For example, long-lived ²²⁶Ra has been used to date a limestone cave in central Switzerland, corals in the Indian Ocean, and Pleistocene gravel terraces [580]. The activity ratio A(²²⁴Ra)/A(²²³Ra) is a potential age calculator for old lake water because the low ²²³Ra and ²²⁴Ra activities in old lake water are relatively unaffected by mixing [579].

4.88.3 Radium isotopes in medicine

²²⁶Ra is used in brachytherapy (Fig. 4.88.1), which is a method of localized treatment of various types of cancer. A sealed implant (such as a rod, seed, or needle) containing the radioactive isotope ²²⁶Ra is inserted into or near a patient’s tumor to apply a high dose of radiation to the tumor. The sealed implant is inserted by a physician or by an automated device (called a remote afterloader), and it is removed from the patient once the tumor is destroyed [75], [581].

Fig. 4.88.1:

Brachytherapy seeds shown with a penny (19-mm diameter) for scale (modified from [582]).

4.89.1 Actinium isotopes in Earth/planetary science

²²⁷Ac (with a half-life of 21.77 years) has been used as a tracer of deep-sea mixing in the oceans. By determining concentrations of ²²⁷Ac in a water column, scientists can study the rates and patterns of mixing and other vertical exchange processes [583]. As a product of the ²³⁵U decay chain, ²²⁷Ac and other radioisotopes have been used to determine information about the movement of fluids in mid-oceanic ridges and basaltic melts [584], [585].

4.89.2 Actinium isotopes in medicine

²²⁵Ac (with a half-life of 10 days) can be used in cancer treatments (Fig. 4.89.1). The isotope is attached to a chelating agent (a substance that can form multiple bonds to a single metal ion) and delivered to the problem site. The emissions of alpha particles from actinium and its daughter products cause tumor death [586]. ²²⁵Ac in a series of alpha decays produces ²¹³Bi (with a half-life of 0.76 h), which is also used for radioimmunotherapy [587].

Fig. 4.89.1:

The Medical Actinium for Therapeutic Treatment (MATT) is a separations process that recovers ²²⁵Ac from unused nuclear fuel so the isotope can be used in cancer treatment and research. (Image Source: Idaho National Laboratory) [588].

4.89.3 Actinium isotopes used as a source of radioactive isotope(s)

²²⁵Ac, which is a pure alpha emitter, is used to produce ²¹³Bi with an ²²⁵Ac/²¹³Bi radionuclide generator (Fig. 4.89.2). ²¹³Bi is a mixed alpha and beta emitter. The primary mode of decay is by beta emission to the very short-lived, alpha emitter ²¹³Po. The 8.4 MeV alpha particle emitted by ²¹³Po has a path length of 76 μm in human tissue and is responsible for its cytotoxic effects.

Fig. 4.89.2:

Institute for Transuranium Elements (ITU) Standard ²²⁵Ac/²¹³Bi Radionuclide Generator. Image kindly provided by Dr. Alfred Morgenstern, European Commission, Joint Research Centre – Institute for Transuranium Elements, Karlsruhe, Germany.

4.90.1 Thorium isotopes in Earth/planetary science

²³⁴Th (with a half-life of 24 days) has been used as a tracer for estimating the flux of organic carbon in the ocean (Fig. 4.90.1) [589], [590]. ²³⁴Th has been used for estimating the residence time of suspended particulate matter (SPM) in water columns [589].

Fig. 4.90.1:

Particulate, dissolved and total ²³⁴Th in water column profiles in the Ross Sea, Southern Ocean (modified from [590]).

4.90.2 Thorium isotopes in geochronology

The decay of ²³²Th (with a half-life of 1.40×10¹⁰ years) to ²⁰⁸Pb is used to date rocks based on the accumulation of the stable daughter product²⁰⁸Pb. The half-lives of the isotopes between the parent radionuclide²³²Th and stable endpoint ²⁰⁸Pb all have much shorter half-lives than thorium. Therefore, the amount of ²⁰⁸Pb that accumulates in a sample is determined primarily by the amount of ²³²Th parent radionuclide present when the mineral was formed and the time that has elapsed since the mineral solidified [591].

Another dating method, the ²³⁰Th/²³⁴U method, is based on the hypothesis that the sample contains uranium, but no ²³⁰Th at the time of its formation. Then, the age of the specimen is determined mainly by the amount of ²³⁰Th in the specimen. Reliable ages with this method range from several thousand to approximately 350 thousand years [292].

4.90.3 Thorium isotopes in industry

The most precise time and frequency measurements are performed with optical atomic clocks that use as a frequency standard the optical frequency generated as electrons change energy levels. It has been proposed that a nuclear clock, using a nuclear transition could outperform an electron transition. ^229mTh, with a half-life of 13.9 h, has been confirmed as a possible candidate for a nuclear clock [592]. The m in ^229mTh indicates a metastable state of the isotope. The further development of a nuclear frequency standard will require more precise determinations of the energy and half-life of the isomer.

4.91.1 Protactinium isotopes in Earth/planetary science

²³¹Pa (with a half-life of 3.25×10⁴ years) and ²³⁰Th (with a half-life of 7.56×10⁴ years) are produced in seawater by radioactive decay of ²³⁵U and ²³⁴U. The amount ratio of radioactive production of ²³¹Pa and ²³⁰Th, n(²³¹Pa)/n(²³⁰Th), is 0.093. ²³⁰Th is removed from seawater in settling particulates more efficiently than ²³¹Pa, while ²³¹Pa tends to be transported farther in ocean currents. Therefore, the amount ratio n(²³¹Pa)/n(²³⁰Th) in settling particulates tends to be less than the production ratio of 0.093 unless the water mass is stationary and allows both products to settle out. Thus, sedimentary records of excess n(²³¹Pa)/n(²³⁰Th) amount ratios can provide information for changes in the relative magnitude of major ocean circulation (Fig. 4.91.1) [593], [594].

Fig. 4.91.1:

Diagram of ²³¹Pa-²³⁰Th fractionation (preferential separation) in the oceans. NADW is North Atlantic Deep Water. (Diagram Source: Henderson and Anderson, 2003) [595].

4.91.2 Protactinium isotopes in geochronology

²³¹Pa is a natural radiogenic isotope produced by alpha decay of ²³⁵U to ²³¹Th, followed by beta emission to form ²³¹Pa. Although its behavior in the environment as a transient member of the U-series decay chain may be complex, measurements and modeling of ²³¹Pa in relation to the isotopes of uranium and thorium have been used in a variety of geochronologic applications on time scales of 10³ to 10⁵ years [596], [597]. Studies include movement of water masses and particles in the oceans, rates of magma melting and movement beneath volcanoes, and ages of carbonate mineral deposits, including corals, in relation to climate change.

4.92.1 Uranium isotopes in Earth/planetary science

²³⁴U (with a half-life of 2.432×10⁵ years) is a daughter product of ²³⁸U (with a half-life of 4.47×10¹⁰ years) and makes up only 0.0054 percent of the total uranium today. During the decay of the parent radionuclide²³⁸U nucleus (first to ²³⁴Th (with a half-life of 24 days) by alpha decay, then to ²³⁴Pa (with a half-life of 6.7 h) by beta-minus, and finally to ²³⁴U by beta-minus), the energy released will damage the chemical and physical bonds holding the ²³⁴U product nuclei in a mineral. As a result, ²³⁴U may be leached more easily from water or rock samples than ²³⁸U and the isotope-amount ration(²³⁴U)/n(²³⁸U) will vary depending on the extent of water-rock interaction [598].

4.92.2 Uranium isotopes in geochronology

The three natural radioactive decay chains beginning with ²³⁸U, ²³⁵U, and ²³²Th each have comparable half-lives that are much longer than the radioactive isotopes that follow until the production of stable isotopes of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, respectively. When undisturbed, the activities of daughter isotopes in each decay chain are equal to their parents and one can measure the accumulation of the stable isotopes of lead to date the time that has elapsed since a mineral became a closed system (a system that does not exchange matter with its surroundings). Rocks formed hundreds of millions to billions of years ago can be dated using this technique [591]. If a mineral is disturbed at some point during the decay and isotopes in the decay chain are preferentially removed from the system, the equilibria in a decay sequence will be disturbed. For example, one can measure the excess of ²³⁰Th (with a half-life of 7.56×10⁴ years) relative to the ²³⁴U parent radionuclide to date carbonates (speleothems or corals) that are less than 5×10⁵ years old [591].

4.92.3 Uranium isotopes in industry

Nuclei of ²³⁵U are split when bombarded by thermal neutrons. The process is known as nuclear fission and can release tremendous amounts of energy per uranium nucleus. The nucleus that splits will release additional neutrons that, if slowed down sufficiently, can cause subsequent fission events. When properly controlled, ²³⁵U fission can be used to generate heat to drive steam turbines, which in turn produces electricity (Fig. 4.92.1). If the fission process is not controlled, then a rapid and explosive release of energy will occur, similar to that of nuclear weapons [599]. Uranium depleted in ²³⁵U by fission in nuclear reactors (and hence greatly enriched in ²³⁸U compared to “natural” uranium) is used in the manufacture of DUCRETE concrete (Fig. 4.92.2). The incorporation of the large ²³⁸U nuclei makes this material an effective absorber of neutrons and gamma rays, and DUCRETE concrete is used to reduce fluxes of neutrons and high-energy photons. The alpha particles produced by the decay of ²³⁸U are effectively absorbed by the concrete and do not pose a health risk. DUCRETE is being proposed as a suitable material for the storage of radioactive waste [600], [601].

Fig. 4.92.1:

Boiling water reactor fuel can be ²³⁵U enriched as uranium dioxide, which would be located in the core and is identified as “Core”. (Diagram Source: U.S. Nuclear Regulatory Commission) [602].

Fig. 4.92.2:

DUCRETE cask diagram. DUCRETE is being proposed as a suitable material for the storage of radioactive waste. (Diagram Source: Argonne National Laboratory) [601].

4.93.1 Neptunium isotopes in industry

²³⁷Np (with a half-life of 2.14×10⁶ years) is fissionable, meaning that neptunium can be bombarded with neutrons and, as a result, create more neutrons that are free to interact with nearby material and can be used in fast neutron reactors or in nuclear weapons (Fig. 4.93.1) [75], [603], [604]. ²³⁷Np is used in neutron detection instruments [75].

Fig. 4.93.1:

²³⁷Np is fissionable and is used in fast neutron reactors and in nuclear weapons. (U.S. Air Force photo by Senior Airman Alexandra Longfellow, U.S. Department of Energy) [605].

4.93.2 Neptunium isotopes used as a source of radioactive isotope(s)

²³⁷Np is used in the production of ²³⁸Pu (with a half-life of 87.7 years), which is an emitter of alpha particles used in thermoelectric generators and radioisotope-heater units. When ²³⁷Np captures a neutron, it becomes ²³⁸Np, with a half-life of 2.117 days, which decays to ²³⁸Pu [75].

4.94.1 Plutonium isotopes in industry

²³⁸Pu (with a half-life of 87.7 years) is used in radiothermal generators as a heat source to produce electricity. These radiothermal generators are used to power unmanned spacecraft and interplanetary probes that venture too far from the Sun to use solar power, such as the Cassini Orbiter, the Galileo spacecraft, and the Huygens and Galileo probes [75], [606], [607], [608]. ²³⁸Pu has been used in the Apollo lunar missions as part of a nuclear battery. The SNAP-27 (systems nuclear auxiliary power) system produced approximately 75 W of electrical power at 30 VDC per unit (Fig. 4.94.1). The energy source was a 2.5-kg rod of ²³⁸Pu providing thermal power of approximately 1250 W [609]. ²³⁸Pu is used in pacemakers (Fig. 4.94.2).

²³⁹Pu (with a half-life of 2.41×10⁴ years) is used in nuclear weapons. ²³⁹Pu is easily made in nuclear reactors by bombarding ²³⁸U with neutronsvia the reaction ²³⁸U (n, γ) ²³⁹U and ²³⁹U→²³⁹Pu+β⁻. The ²³⁹Pu made by this reaction can itself be split by neutrons to release energy and is used for energy generation in nuclear reactors [75], [610], [611].

Fig. 4.94.1:

²³⁸Pu is used in the SNAP-27 radiothermal generator as a heat source to produce electricity to power spacecrafts, such as for Apollo missions 12, 14, 15, 16, and 17. (Image Source: NASA) [612].

Fig. 4.94.2:

²³⁸Pu is used in cardiac pacemakers, and they should be disposed of properly upon removal (modified from [613]).

4.95.1 Americium isotopes in industry

²⁴¹Am (with a half-life of 433 days) is used in smoke detectors as an ionization source to detect smoke (Fig. 4.95.1). A small piece of ²⁴¹Am oxide is housed inside ionizing smoke detectors. The americium compound emits alpha particles that strike air molecules in their path, causing them to ionize. The ions carry a current from one plate in the detector to a second plate. Current flows continuously until smoke disrupts the current between the two plates. The alarm sounds when the current is disrupted by smoke [75], [614], [615].

Fig. 4.95.1:

In a smoke-free chamber, the ionized air molecules create a current between the two metal plates having a voltage difference. Current flows continuously until smoke disrupts the current, at which time the alarm sounds. (Diagram Source: US Environmental Protection Agency) [615].

²⁴¹Am is used for the control and measurement of industrial material thickness and product quality. In manufacturing, for example, a small piece of ²⁴¹Am is placed above a conveyer belt and a Geiger counter (used to count alpha particles) is placed below the conveyor belt. A specific quantity of radiation is expected to be measured by the Geiger counter. If the product being manufactured (i.e. glass) is thicker than expected, less radiation will be measured, and the product will be rejected [75]. The gamma radiation of ²⁴¹Am is also used in a variety of gauges. Thickness gauges, fluid-density gauges, aircraft fuel gauges, and distance-sensing devices use the density-measuring capabilities of the emitting gamma rays and radiation detector to function.

When ²⁴¹Am is mixed with beryllium (²⁴¹AmBe), it emits neutrons at a high rate. This high rate of neutron generation is useful in oil-well operations to monitor the rate of oil production, and it can also be used in well logging to log the porosity (fraction of void volume to total volume of a material) of the geologic units along the sides of a borehole. Gamma rays from ²⁴¹Am are also used as portable X-ray machines to determine where new wells should be drilled. When a small pellet of ²⁴¹Am is placed in a sealed titanium capsule, it can serve as a portable source for gamma radiography, which is more penetrating than X-rays, to test various materials for defects, such as invisible cracks or faulty welds in pipelines [75], [614], [616].

4.95.2 Americium isotopes in medicine

Gamma-ray emissions from ²⁴¹Am have been used as a radiation source for medical diagnostic tests. In particular, ²⁴¹Am has helped to provide accurate diagnoses of thyroid function, but this use of americium is now obsolete [617].

4.96.1 Curium isotopes in industry

²⁴⁴Cm and ²⁴²Cm (with half-lives of 18.1 years and 163 days, respectively) are strong alpha emitters (see alpha decay). The alpha emission from these isotopes creates a considerable quantity of heat that makes them useful as alpha particle sources, as well as heat generators in RTGs (radioisotopic thermoelectric generators) [75]. During a number of space missions based in America and Europe, ²⁴⁴Cm was the source used for the alpha particle X-ray spectrometer that was on board vehicles such as the Mars Exploration Rover and the Rosetta/Philae [75], [618]. ²⁴⁴Cm has a large neutron capture to neutron fission cross-section ratio and has been used in a nuclear reactor to produce higher mass radio-isotopes of curium (Fig. 4.96.1) [75], [618].

Fig. 4.96.1:

Schematic drawing of the inside of a reactor core at the Oak Ridge National Laboratory’s High Flux Isotope Reactor facility. ²⁴⁴Cm is used as the target in the flux trap. (Image Source: Oak Ridge National Laboratory) [619].

4.98.1 Californium isotopes in industry

²⁵²Cf is a very active source of neutrons (2.3×10⁶ neutrons per second per microgram) with a half-life of 2.65 years. The energy spectrum of the neutrons is very similar to that of a fission reactor and small amounts of ²⁵²Cf provide an ideal portable source for low neutron flux applications [75], [625], [626]. ²⁵²Cf is used for PGNAA (prompt gamma neutron activation analysis, a method for detecting many chemical elements in samples simultaneously) in the analysis of coal, cement, minerals, weapon components, and chemical munitions [627]. This method provides a quick and non-destructive elemental analysis of a sample. For example, ²⁵²Cf, as the neutron source for PGNAA, is used to detect the presence of antitank mines [625].

Neutron activation analysis (NAA) uses ²⁵²Cf as a portable neutron source to bombard a small sample from the area of interest with neutrons and analyze the radioactive emissions from that bombardment to help identify silver or gold ore [75]. ²⁵²Cf has been used in neutron moisture gauges to locate water [628]. ²⁵²Cf is used in borehole geophysical logging for subsurface PGNAA investigation of waste (Fig. 4.98.1) [629].

Fig. 4.98.1:

Simplified diagram of a well logging operation. The geoprobe (black and silver probe in the borehole/well) contains the ²⁵²Cf neutron source with supporting electronics, and it relays information about the physical properties of the surrounding rock to the control computer at the surface as the probe is lowered or raised. The example graphs next to the cross-section of the rock and borehole could indicate the presence of fluids, porosity, and/or type of rock (Photo Source: Frontier Technology Corporation).

Formation fluid identification uses ²⁵²Cf as a chemical neutron source for elastic/inelastic neutron backscattering and/or neutron activation methods in well-logging to determine water- and oil-bearing layers and other downhole properties of the well bore [629].

4.98.2 Californium isotopes in medicine

²⁵²Cf is sometimes used in boron neutron capture therapy (BNCT) as a source of neutrons that can be delivered close to the region of a tumor [75], [625], [626]. Brachytherapy can use ²⁵²Cf to treat many types of cancer [75], [625], [626].