Natural decay series

Decay series Decay mode of the mother nuclide Half-life of the mother nuclide [y] Range of dating [y] Application 

Systems for which the loss of members of the decay chains can be neglected and in which the concentration of the mother nuclide can be taken as a measure of the age. For these systems eq. (16.4) can be applied in the forms 

For these systems the Pb/Pb method of dating is applied this will be discussed in more detail in section 16.5. 

A practical application of eqs. (16.6), (16.7) and (16.8) is the calculation of the age of the solar system. MS analysis of meteorites containing negligible amounts of U gives the following values for the isotope ratios of the Pb isotopes 2 Pb Pb = 9.4 and 207pb 204pb 2Q 3 jf these values are assumed to be the initial isotope ratios at the time of formation of the solar system, the age is obtained from the present isotope ratios of the Pb isotopes in the solar system and the ratios of the present abundances of U and Pb, for example by application of eq. (16.6)  

Measurement of the radiation of °Pb is difficult, because of its low energy, but the a activity of °Po is easily measurable by a spectrometry (detection limit s 10 Bq) after attainment of radioactive equilibrium and chemical separation. 

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The picture shows one of the three natural decay series according to which heavy, radioactive nuclei eventually decay to stable lead atoms. 

The three still-existing natural decay series... 

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. 

As discussed previously, many heavy nuclei (A > 150) are unstable with respect to a decay. Some of them also undergo (3 decay. In Chapter 3, we discussed the natural decay series in which heavy nuclei undergo a sequence of (3 and a decays until they form one of the stable isotopes of lead or bismuth, 206 207 208Pb... 

Figure 7.4 Mass parabolas for some members of the An + 3 natural decay series. The main decay path is shown by a solid line while a weak branch is indicated by a dashed line.

The natural decay series starting with 232Th has the sequence a(3(3a. Show why this is the case by plotting the mass parabolas (or portions thereof) for A = 232, 228, and 224. 

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... 

All isotopes of radium are radioactive, the longest-lived isotope being 226Ra (a —1600 years). This isotope is formed in the natural decay series of 238U and was first isolated by Pierre and Marie Curie from pitchblende. Once widely used in radiotherapy, it has largely been supplanted by radioisotopes made in nuclear reactors. 

G. W. A. Newton, History of the Unraveling of the Natural Decay Series, Radiochim. Acta 70/72, 31 (1995)... 

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. 

Table 16.3. Natural decay series applicable for dating.

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. 

Radionuclides of major importance in the geosphere and the biosphere are listed in Table 21.1. Not taken into account are radionuclides with half-lives h/2 < 1 d (in the case of activation products of materials used in nuclear reactors, i/2 < 1 y) and with half-lives ti/2 > lO y, radionuclides with fission yields <0.01%, radioisotopes of elements that are not members of the natural decay series,and radionuclides produced solely for medical or technical applications. The radionuclides are arranged according to their position in the Periodic Table of the elements, in order to facilitate the discussion of their chemical behaviour. Radionuclides with half-lives >10y are underlined, because their behaviour over long periods of time is of special importance. 

All isotopes of radium are radioactive, the longest-lived isotope being 226Ra (a 1600yr). This isotope is formed in the natural decay series of... 

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... 

Elements 85 and 87 fall into the region covered by the natural decay series and could therefore be expected to be fed by rare decay branches. As early as 1914, a particles were observed in carefully purified Ac (Z = 89), which implied the formation of element 87 (Meyer et al. 1914). However, the work of Marguerite Perey in 1939 is credited with the discovery of element 87 - the last discovery of a new element in nature (Perey 1939a, b). She proved that a 21 min P emitter ( 87) growing from Ac had chemical properties akin to cesium, and named the element francium (Fr). Element 85, astatine (At), the heaviest known halogen, was first produced artificially in 1940 as 85 (Ty2 = 7 h) by (a,2n) reaction on ° Bi (Corson et al. 1940a, b) before short-lived isotopes were found also in rare branches of the decay series. 

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. 

Many radioactive materials in the natural decay series and in the nuclear fuel cycle are a emitters for which very little radiation escapes a macroscopic sample. If radiochemistry is used to isolate a particular element, and its mass is small, the solid can be deposited on a metal planchet (e.g., by evaporating a small volume of liquid containing the purified material) and counted with the planchet forming the bottom of the gas chamber such as shown in O Fig. 48.3. Under these conditions, the a detection efficiency can be about 52% and the P detection efficiency about 80%. (Some p particles that are emitted toward the planchet are scattered into the gas volume.)... 

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... 

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