Uranium natural decay series

Radon is a naturally occurring, chemically inert, radioactive gas. It is colorless, odorless, and tasteless. It is part of the uranium-238 decay series, the direct decay product of radium-226. Radon moves to the earth s surface through tiny openings and cracks in soil and rocks. High concentrations of radon can be found in soils derived from uranium-bearing rocks, such as pitchblende and some... 

Figure 32.3 The three still-existing natural decay series. A. Uranium-238 B. Uranium-235 and C. Thorium-232. (Modified from Holtzman 1969 LWV 1985 UNSCEAR 1988 Kiefer 1990 Rose etal. 1990). Principal decay products occur within the heavy borders outlined.

Radioactive decay usually does not immediately lead to a stable end product, but to other unstable nuclei that form a decay series (Kiefer 1990). The most important examples of unstable nuclei are started by very heavy, naturally occurring nuclei. Because the mass number changes only with a decay, all members of a series can be classified according to their mass numbers (see the uranium-238 decay series in Figure 32.2). A total of three natural decay series — formed at the birth of our planet — are named after their parent isotope Th, and (Figure 32.3). Several shorter decay series also exist. For example, Sr decays with a Tb 1/2 of 28 years by [3 emission to °Y, which in turn disintegrates (P emission) with a Tb 1/2 of 64 h to the stable °Zr (Kiefer 1990). Other examples of known radionuclides since the Earth s origin include " °K and Rb. In hazard assessments, all members of a decay series must be considered. 

Since spontaneous fission is extremely rare in Nature, detection of fission events in natural samples would give a strong hint. Alpha-particle spectra would be less specific, because the energies predicted for superheavy nuclei fell into the range covered by the natural decay series deriving from uranium... 

Table 3-4. Uranium Isotope Decay Series Showing the Decay Products of the Naturally Occurring Isotopes of Uranium...

The laws of radioactive decay are the basis of chronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated rather exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archaeology, geology and mineralogy, and various kinds of chronometers are available. 

In the early stages of dating by nuclear methods, the measurement of He formed by a decay in the natural decay series (9, 6 and 7 He atoms in the uranium series, the thorium series and the actinium series, respectively) has been applied. The preferred method was the U/He method which allows dating of samples with very low concentrations of U of the order of 1 mg/kg. Helium produced by a decay is driven out by heating and measured by sensitive methods, e.g. by MS. However, it is difficult to ensure the prerequisites of dating by the U/He method neither " He nor a-emitting members of the decay series must be lost and no " He atoms must be produced by other processes such as decay of Th and spallation processes in meteorites. 

Because of its long half-life, potassium-40 can be used to date objects up to 1 million years old by determination of the ratio of to jgAr in the sample. The uranium-lead method is based on the natural uranium-238 decay series, which ends with the production of stable lead-206. This method is used for dating uranium-containing minerals several billion years old because this series has an even longer half-life. All the 2 6pb in such minerals is assumed to have come from Because of the very long half-life of 4.5 billion years, the amounts of intermediate nuclei can be neglected. A meteorite that was 4.6 billion years old fell in Mexico in 1969. Results of studies on such materials of extrater-... 

Figure 1 The natural uranium-thorium decay series, colored according to particle reactivity. The arrows represent decay with the changes in atomic number (Z) and number of nucleons (N) indicated. All three series end with a stable lead isotope.

Radon and radium In nature, radon is most often associated with uranium deposits, since some of its isotopes are formed as part of the natural decay series. In isolated samples, radon will reach radioequi-librium in about 3 h, and the total rate of a particle emission will be three times that for radon itself. This is because for each a particle from radon decay, approximately one from Po and one from Po will also be emitted. Since the half-life of °Po (22 years) is long compared to analysis times, its decay can be neglected. However, since analysis times are still... 

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... 

Polonium, completing the elements of Group 16, is radioactive and one of the rarest naturally occurring elements (about 3 x 10 " % of the Earth s crust). The main natural source of polonium is uranium ores, which contain about lO g of Po per ton. The isotope 210-Po, occurring in uranium (and also thorium) minerals as an intermediate in the radioactive decay series, was discovered by M. S. Curie in 1898. 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Moore WS (1992) Radionuclides of the uranium and thorium decay series in the estuarine enviromnent. In Uranium-series Disequilibrium Applications to Earth, Marine and Enviromnental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 396-422 Moore WS (1996) Large groundwater inputs to coastal waters revealed by Tla emichments. Nature 380 612-614... 

Natural lead, a metallic element, is a mixture of the following four isotopes lead-204, lead-206, lead-207, and lead-208. Only lead-204 is a primordial isotope of nonradiogenic origin all the others are radiogenic, each isotope being the end product of one of the radioactive decay series of isotopes of thorium or uranium, namely, uranium-238, uranium-235, and thorium-232 the decay series of the uranium isotopes are listed in Figure 12 ... 

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