Nuclides decay series

Nuclide Decay series Isotopic Specific Gamma-ray Gamma- Fraction of Standard... 

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. 

Figure 3. Parent daughter disequilibrium will return to equilibrium over a known time scale related to the half-life of the daughter nuclide. To return to within 5% of an activity ratio of 1 requires a time period equal to five times the half-life of the daughter nuclide. Because of the wide variety of half-lives within the U-decay-series, these systems can be used to constrain the time scales of processes from single years up to 1 Ma.

Table 2. Half-lives for the U- and Th- decay series nuclides, with decay modes. 

Figure 3. Systematics of radionuclides along the series. The major and minor fluxes to each nuclide can be readily seen from the arrows showm The behavior of each nuclide can be evaluated by considering the surface and groundwater populations individually, or together as the mobile pool. Nuclides in the decay series within the host rock minerals supply atoms at the surface and in the groundwater by recoil during a decay, so that there are greater abundances in the mobile pool of nuchdes progressively along the series, a decay of nuclides at the surface injects atoms back into the minerals as well as into groundwater.

The Ra isotopes in the other decay series can be evaluated similarly. Ra in the series (Table 1) is the product of the third a decay, and so the effects of near-surface deletion or decay of recoiled precursors must be calculated accordingly. Ra in the series is also the product of the third a decay. Further processes that may be considered where circumstances warrant include nonsteady state conditions or removal by precipitation at rates that are fast compared to the decay rate of the Ra nuclides. 

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... 

The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum mechanically stable and the atom is said to be stable, i.e., not radioactive however, if there are too few or too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Unstable nuclides undergo radioactive transformation, a process in which a neutron or proton converts into the other and a beta particle is emitted, or else an alpha particle is emitted. Each type of decay is typically accompanied by the emission of gamma rays. These unstable atoms are called radionuclides their emissions are called ionizing radiation and the whole property is called radioactivity. Transformation or decay results in the formation of new nuclides some of which may themselves be radionuclides, while others are stable nuclides. This series of transformations is called the decay chain of the radionuclide. The first radionuclide in the chain is called the parent the subsequent products of the transformation are called progeny, daughters, or decay products. 

Nozaki, Y. 1991. The systematics and kinetics of U/Th decay series nuclides in ocean water. Rev. Aquat. Sci. 4 75-105... 

Given a number of nuclides at t = 0, calculate the distribution of nuclides in the U-Th decay series at any time t. 

These equations converge towards those of the fractional melting model for tp Dh and, contrary to McKenzie (1985) equation (29), CUq tends to C0 when porosity partition coefficient are of the same order of magnitude, large variability is achieved in both the solid and the residue, a point which will be returned below. Considerable attention has been recently focussed on this model which may explain the fractionation of some strongly incompatible nuclides in the U decay series (McKenzie, 1985 Williams and Gill, 1989 Beattie, 1993). 

All elements with Z > 83 (Bi) are unstable and belong to chains of radioactive decay, or decay series. Three decay series include all radioactive elements in the Z > 83 part of the chart of nuclides—namely, 4n, 4n + 2, and 4n + 3 (because the decay takes place by a emission with mass decrease of four units, or by jS emission with a negligible mass decrease, all nuclides within a series differ by... 

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

Other conditions being equal, the intermediate species with longer half-lives in a decay series have more opportunities to be fractionated from their parents. Hence, in the decay series of two nuclides °Th and Ra have a greater chance to be fractionated. In the decay series, Pa (half-life 32.8 has the greatest chance to be fractionated. In the Th decay series, all the intermediate species have short half-lives (the longest half-life of Intermediates is 5.75 3T for Ra (A, = 0.1205 3 ) and the disturbance of this decay system does not have much utility. That is, the U-series (including U and U series) disequilibrium is much more often applied. Some examples of disturbed decay chain (i.e., fractionation of the intermediate species) are given below ... 

Because Th/U ratio is low in seawater, it is also low in corals grown from seawater, with Th/ U of 0.8 x 10 to 12 x 10 (Edwards et al., 1986/87). This huge deficiency in Th means that initial °Th (the fifth nuclide in the decay series) in coral may be ignored. By measuring the activity ratio of °Th/ U, it is possible to estimate the age of coral and also to calibrate the C geochronometer (Chapter 5). 

If the decay series is not disturbed, secular equilibrium will be reached after a duration of about 10 times the longest half-life of the all the intermediate nuclides, which means 2.4 Myr for the series, 0.33 Myr for the series, and 60 years for the h series. After reaching secular equilibrium, the series would not contain any information on the history of the system. However, a disturbed series before reaching secular equilibrium (i.e., a disequilibrium decay series)... 

Dating using intermediate nuclides in decay series requires an understanding of the evolution of the concentrations of intermediate nuclides after disturbance. The full evolution becomes increasingly more complicated for an intermediate nuclide that requires more steps from the long-lived parent. Pa is the third... 

For the nuclides in the decay series, and °Th are of particular interest because of their long half-lives. is the fourth nuclide in the decay series (Table 2-2a) with a half-life of 244,000 years, and °Th is the fifth nuclide with a half-life of 75,400 years. Denote as nuclide 1, Th as 2, " Pa as 3, as 4, and °Th as 5. For because (decay constant of is the smallest, and X4 (decay constant of is the second smallest and far smaller than Xz and X4, the following may be easily derived ... 

For the next nuclide in the decay series, °Th, because Xs (decay constant of °Th) is not smaller but larger than X4, the activity evolution equation is more complicated. Starting from the fuU equation (Box 2-6), with X3 X2 Xs X4 X4, the following may be derived ... 

Among intermediate nuclides in decay series, °Th dating is the most widely used because of large fractionation between Th and U in various processes. Two applications are especially notable. One is to date corals, and the second is to determine the age of ocean sediment and sedimentation rate. Because the half-life of °Th is 75,400 years, it is useful in determining ages younger than 0.5 Ma. 

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