Natural decay series 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. 

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. 

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 ... 

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... 

The physical properties of uranium and uranium compounds important in the nuclear fuel cycle and defense programs are listed in Table 3-2. The percent occurrence and radioactive properties of naturally occurring isotopes of uranium are listed in Table 3-3. The two decay series for the naturally occurring isotopes of uranium are shown in Table 3-4. 

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

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. 

The major decay paths for the naturally occurring isotopes of uranium and thorium are shown in Table 1. Other actinides of environmental importance include Np, Pu, Pu, and " Am. These have decay series similar to and overlapping those of uranium and thorium. Neptunium-237 (fy2 = 2.14 X 10 yr, a) decays to ° Bi through a chain of intermediates, emitting seven a- and four j3 "-particles. Plutonium-238 (h/2 = 86 yr, a) decays into an intermediate daughter on the decay series. 

Radon (Rn-222) is an odorless and colorless natural radioactive gas. It is produced during the radioactive decay of radium-226, itself a decay product of uranium-238 found in many types of crustal materials, that is, rocks and soils. Rn-222 has a short half-life (3.8 days) and decays into a series of solid particulate products, known as radon progeny or radon daughters, all of which have even shorter half-lives ( 30 min or less). Other isotopes of radon also occur naturally, but due to differences in half-life and dosimetry their health significance is minimal compared to that from exposure to Rn-222. 

The disintegration of a radioactive nucleus is often the beginning of a radioactive decay series, which is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope. Table 23.3 shows the decay series of naturally occurring uranium-238, which involves 14 steps. This decay scheme, known as the uranium decay series, also shows the half-lives of all the products. 

Pa. The only isotope of protactinium with a half-life longer than 1 month is Pa. It is a member of the 4n + 3 decay series of U, occurring in secular equilibrium with natural uranium at a concentration about the same as that of radium. The activity of Pa in natural uranium at secular equilibrium is 0.022 Ci/Mg of 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. 

This series (Figure 16.3) was so called because it was originally thought that actinium-227 was the parent element. However, actinium-227 decays too quickly for the series to persist for any length of time, and eventually uranium-235 was proved to be the true parent element. Uranium-235 is the less common naturally occurring isotope of uranium. The series ends with yet another stable lead isotope, lead-207, and the series formula is 4x + 3. 

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... 

---