Nuclides intermediate

The half-life of Th is 1.41 X 10 years. None of the nuclides intermediate between Th and Pb has a half-life greater than a few years. The ratio of Pb to Th in a typical uranium deposit is 0.0483. The estimated age of the earth is 10 years. What is the half-life of U Assume that no Th and Pb were present in this deposit when the earth was formed. 

Parent Half-fife, yr Stable daughter Natural abundance of daughter, % Intermediate nuclides... 

The previous section showed that if the decay chain remains undisturbed for a period of approximately 6 times the longest half-lived intermediate nuclide then the chain will be in a state of secular equilibrium (i.e., equal activities for all the nuclides). The key to the utility of the U-series is that several natural processes are capable of disrupting this state of equilibrium. 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

Since Ra and " Ra are both produced by recoil from the host mineral, it might be assumed that the production rates are equal. However, the relative recoil rates can be adjusted by considering that the parent nuclides near the mineral surface may not be in secular equilibrium due to ejection losses i.e., the activity of Th may be lower than that of Th due to recoil into groundwater of the intermediate nuclide Ra. Krisnaswami et al. (1982) calculated that the recoil rate of " Ra is 70% that of Ra if radionuclides are depleted along the decay chain in this way. 

In addition to the assumptions of initial conditions, the validity of U-Pb methodology relies on closed system behavior of U, Pb and intermediate nuclides in the decay chain. Concordance between the two U-series decay chains is most likely to be compromised by Rn loss because Rn is the only gas in the decay chains and has a high diffusivity. Radon-222 in the decay chain has a half life of 3.8 days. This is much longer than the half-life of Rn (3.96 s) in the decay chain. Therefore, partial loss of Rn will give rise to an apparent age younger than the true age, whereas the 207p /235p ... 

Nuclide—A species of atom characterized by the constitution of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content or, alternatively, by the atomic number (Z), mass number A (N+Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable time. Thus, nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not so considered. 

Thus the 13 C neutron source (with a little assistance from 22Ne) in thermally pulsing low- and intermediate-mass stars is well established as the chief source of the main component of s-process nuclides in the Solar System. It is not quite clear, however, whether the r0 parameter is something unique, or just the average over a more-or-less broad distribution of values nor is it clear why a similar s-process pattern is seen in stars that are metal-deficient by factors of up to 100 (see Pagel Tautvaisiene 1997). 

Alpha particles are composed of two protons and two neutrons. Thus they have Z = 2, N = 2, and A = 4 and correspond to a helium nucleus He. The emission of a particles thus produces a decrease of 4 units in A. An unstable nuclide undergoing a decay may emit a particles of various energy and thus directly reach the ground level of the stable product. Alternatively, as in )3 emission, an intermediate excited state is reached, followed by y emission. Figure 11.7 shows, for example, the decay process of ioTh., which may directly attain the ground level of by emission of a particles of energy 5.421 MeV or intermediate excited states by emission of a particles of lower energy, followed by y emission. 

Because in nature the ground state of a stable nuclide is often attained by decay chains involving intermediate species decaying at different rates, it is worth evaluating the implications of the relative magnitudes of the various decay constants on the isotopic composition of the element. 

The above condition of equal activity of all radioactive nuclides in a decay chain (except for branch decays) is known as secular equilibrium. More detailed solutions for the concentration evolution of intermediate species can be found in Box 2-6. 

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 ... 

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... 

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. 

First the U- Pb system is discussed. undergoes 8 a-decays and 6 p-decays to finally reach the stable nuclide ° Pb (see Table 2-2a). The half-life of is 4.468 X 10 years, and the decay constant is 1.55125 x 10 ° yr . Assuming the changes in the concentrations of the intermediate nuclides (such as... 

Equation 5-53 assumes that every decayed nuclide becomes ° Pb. This is approximate because of intermediate nuclides in the decay chain. Based on the discussion in Section 2.2.1.1, if (i) a relative precision of 1% in age is required, (ii) both and are incorporated with equal activity, and (iii) the activities of intermediate nuclides lie between 0 and 2 times the activity of then for ages... 

There are several lines of evidence that nucleosynthesis takes place in stars. The compositions of the outer envelopes of evolved low- and intermediate-mass stars show enhancements of the products of nuclear reactions (hydrogen and helium burning and s-process nucleosynthesis, as defined below). The ejecta of supemovae (stellar explosions) are highly enriched in short-lived radioactive nuclides that can only have been produced either just before or during the explosion. At the other extreme, low-mass stars in globular clusters, which apparently formed shortly after the universe formed, are deficient in metals (elements heavier than hydrogen and helium) because they formed before heavy elements were synthesized. 

Americium-241, a radioisotope used in smoke detectors, decays by a series of 12 reactions involving sequential loss of a, a, (3, a, a, (3, a, a, a, (3, a, and f3 particles. Identify each intermediate nuclide and the final stable product nucleus. 

Schlenker RA. 1986. Comparison of intake and committed dose equivalent permitted by radiation protection systems based on annual dose equivalent and committed dose equivalent for a nuclide of intermediate effective half-life. Health Phys 51 207-213. 

Sorption of polyvalent ions increases very rapidly with pH on oxides and hydrous oxides Because natural materials generally contain oxides or at least adsorbing groups that behave like oxides, sorption of polyvalent nuclides at intermediate pH will tend to follow behavior typical of oxides Thus, a strong dependence of sorption on salt concentration may not be observed This lack of dependence on salt concentration does not preclude ion exchange as a mechanism however, since this type of behavior follows directly from equilibrium equations ... 

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