Nuclide uranium decay series

Equation (5.31) can be used to model the uranium decay series (U-series) nuclides in the residual melt that is in chemical equilibrium with the solid during dynamic partial melting ... 

Radionuclides in the uranium decay series serve as useful tracers of particle flux. One type of these tracers consists of a soluble parent nuclide and a particle-reactive daughter. These soluble nuclide-particle-reactive pairs include 234 j 230jjj and The half-life of the... 

The longest lived nuclides of the uranium decay series, which are located at the early part of the series, are shown in Fig. 1. In a closed system, given sufficient time, the decay series will reach a state of secular equilibrium wherein the radioactivity (or activity ) of each member will be the same, although the actual number of atoms present may vary greatly. Activity for a given nuclide is defined by ... 

The uranium decay series consist of a group of nuclides that, when their mass munber is divided by 4, have a remainder of 2 (the 4 + 2 series). The parent of this series is with a natural abundance of 99.3% it undergoes a-decay with a half-life of 4.46 X lO y. The stable end product of the uranium series is ° Pb, which is reached after 8 a- and 6 /3-decay steps. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

FIGURE 17.16 The uranium-238 decay series. The times are the half-lives of the nuclides (see Sei tion 17.7). The unit a, for annum, is the SI abbreviation for year. 

The uranium and thorium decay-series contain radioactive isotopes of many elements (in particular, U, Th, Pa, Ra and Rn). The varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments, while the varied half-lives of the nuclides allows investigation of processes occurring on time scales from days to 10 years. U-series measurements have therefore revolutionized the Earth Sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the Earth. 

Cochran JK (1984) The fates of U and Th decay series nuclides in the estuarine environment. In The Estuary as a Filter. Kennedy VS (ed) Academic Press, London, p 179-220 Cochran JK (1992) The oceanic chemistry of the uranium - and thorium - series nuclides. In Uranium-series Disequilibrium Applications to Earth, Marine and Environmental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 334-395 Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cochran JK, Carey AE, Sholkovitz ER, Surprenant LD (1986) The geochemistry of uranium and thorium in coastal marine-sediments and sediment pore waters. Geochim Cosmochim Acta 50 663-680 Corbett DR, Chanton J, Burnett W, Dillon K, Rutkowski C. (1999) Patterns of groundwater discharge into Florida Bay. Linrnol Oceanogr 44 1045-1055... 

Cochran JK. 1984. The fates of uranium and thorium decay series nuclides in the estuarine environment. Estuary Filter, 179-220. 

FIGURE 17.16 The uranium-238 decay series. The times are the half-lives of the nuclides. 

The final members of the decay series are stable nuclides ° Pb at the end of the thorium family, Pb at the end of the uranium-radium family, Pb at the end of the actinium family, and Bi at the end of the neptunium family. In all four decay series one or more branchings are observed. For instance, Bi decays with a certain probabihty by emission of an a particle into Tl, and with another probability by emission of an electron into Po. os-pj decays by emission of an electron into Pb, and Po by emission of an a particle into the same nuclide (Table 4.1), thus closing the branching. In both branches the sequence of decay alternates either a decay is followed by P decay or p decay is followed by a decay. 

It was named in analogy to uranium after the planet Neptune. The Np isotope with the longest half-life (O/2 2.144 10 y) is Np, the mother nuclide of the (artificial) decay series with A = An + (section 4.1). It is produced in nuclear reactors ... 

Other nuclides in Table 2.1 which are of particular interest are uranium and thorium isotopes and their series. Uranium 238, uranium 235 and thorium 232 decay series are schematically presented in Fig. 2.1. A number of radionuclides are formed during these... 

At equilibrium, the activity ratio between any two members of a decay series is 1.00. However, at and near the Earth s surface, disequilibrium of the various nuclides of the uranium series is found to occur. The disequilibrium is especially pronounced in groundwaters (Cherdyntsev, 1971 Osmond and Cowart, 1976). The fractionation of the nuclides can occur as a result of chemical differences between elements, the fractionation of isotopes of a given element may occur because of preferential leaching of one (because of its radiogenic origin), or by the direct action of recoil during radioactive decay (Osmond and Cowart, 1976). 

Examples of reactions proceeding during stellar nucleosynthesis are shown in Table 1. To illustrate the sequence of events, the decay series of uranium-238 is depicted in this table. Radiogenic nuclides decay by the emission of alpha, beta and gamma radiation or by electron capture into so called daughter nuclides at their half-lives. This half-life ranges from parts of seconds to billions of years. 

Nuclear decay series A series of radioactive decays that lead from a large unstable nuclide, such as uranium-238, to a stable nuclide, such as lead-206. 

Turekian, K. K., Kharkar, D, P., and Thomson, J. (1973). Uranium and thorium decay series nuclide abundances in marine plankton. Final Report to Advanced Project Research Agency, ARPA, Order No. 1793, Contract N(K)014-67-A-0097-0022. 

As the detection technique for radioactivity has been refined, a number of long-lived radionuclides have been discovered in nature. The lightest have been motioned in 5.1. The heavier ones, not belonging to the natural radioactive decay series of uranium and thorium, are listed in Table 5.2. is the nuclide of lowest elemental specific activity ( 0.(XX)1 Bq/g) while the highest are Rb and Re (each —900 Bq/g). As our ability to make reliable measurements of low activities increases, the number of elem ts between potassium and lead with radioactive isotopes in nature can be expected to increase. 

The actinium decay series consists of a group of nuclides whose mass number divided by 4 leaves a remainder of 3 (the 4n + 3 series). This series begins with the uranium isotope which has a half-life of 7.04 X 10 y and a specific activity of 8 X 10 MBq/kg. The stable end product of the series is ° Pb, which is formed after 7 a- and 4 /3-decays. The actinium series includes the most important isotopes of the elements protactinium, actinium, ftancium, and astatine. Inasmuch as U is a conqx>nmt of natural uranium, these elem ts can be isolated in the processing of uranium minerals. The longest-lived protactinium isotope, Pa (ti 3.28 X 10 y) has been isolated on the 100 g scale, and is the main isotope for the study of protactinium chemistry. Ac (t 21.8 y) is the longest-lived actinium isotope. 

Natural thorium consists 100% of the isotope Th which is the parent nuclide of the thorium decay series. The specific radioactivity for thorium is lower than that of uranium, and it is normally treated as a non-radioactive element. For radioactive tracer studies the nuclide Th (ti 24.1 d) is used after separation from natural uranium. 

Natural radioactivity provides tracers in a wide range of characteristic timescales and reactivities, which can be used as tools to study the rate of reaction and transport processes in the ocean. Apart from cosmogenic nuclides and the long-lived radioisotope K-40, the natural radioactivity in the ocean is primarily derived from the decay series of three radionuclides that were produced in the period of nucleosynthesis preceding the birth of our solar system Uranium-238, Thorium-232, and Uranium-235 (a fourth series, including Uranium-233, has already decayed away). The remaining activity of these so-called primordial nuclides in the Earth s crust, and the range of half-lives and reactivities of the elements in their decay schemes, control the present distribution of U-series nuclides in the ocean. 

The uranium disintegration series. iU decays by a series of alpha (a) and beta (p) emissions to the stable nuclide Pb. 

O Figure 21.11 of Chap. 21, Superheavy Elements, gives a clearer picture of the nuclides beyond A 200. There is an abrupt absence of nuclides with moderate much less long half-lives between ° Pb and Th. This is due to shell effects that are not included in the semiempirical equation. Of course shell effects are crucial for stabilizing the several islands of stability among heavy elements, which include the parents of the natural decay series as well as surprisingly stable isotopes of elements well beyond uranium. 

Tono Uranium Deposit. Local variations in these parameters in time and space could lead to a cycle of dissolution and re-precipitation of U02(am), which may be consistent with the isotopic evidence among natural decay-series nuclides noted in Section 2.1 suggesting that U has been locally remobiUzed in the deposit during the past several hundred thousand years. Such mobilization/ re-precipitation of U would be most sensitive to local variations in Pco gy... 

---