Thorium radioactive decay series

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. 

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 ... 

ISOTOPES There are 41 isotopes of polonium. They range from Po-188 to Po-219. All of them are radioactive with half-lives ranging from a few milliseconds to 102 years, the latter for its most stable isotope Po-209. Polonium is involved with several radioactive decay series, including the actinium series, Po-211 and Po-215 the thorium series, Po-212 and Po-216 and the uranium decay series, Po-210, Po-214, and Po-218. 

Daughters of alpha emitters The recoil method can also be useful for the separation of daughter products produced by a decay of a parent. This technique has been applied to studies of short-lived daughters In the radioactive decay series of uranium, thorium, and actinium (175) ... 

A series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus is called a radioactive decay series. As you can see in Figure 25-11, uranium-238 first decays to thorium-235, which in turn decays to protactinium-234. Decay reactions continue until a stable nucleus, lead-206, is formed. 

In the first steps of its radioactive decay series, thorium-232 decays to radium-228, which then decays to actinium-228. What are the balanced nuclear equations describing these first two decay steps ... 

Th, U and U are the first members of radioactive decay series, forming other radioactive elements with atomic numbers 84-91, which are therefore present in small amounts in thorium and uranium ores. The 238U series is illustrated in Topic A. Fig. 1. Each series ends with a... 

Radium is element number 88, in which all of its isotopes are radioactive hence, what little radium is found on Earth is mostly as a trace element in uranium ores. The most common isotope has a mass number of 226 with a half-life of 1,604 years. The second longest-lived isotope is radium 228, with a half-life of 5.77 years. The other isotopes have much shorter half-lives ranging from microseconds to days. Radium is constantly being formed as part of the radioactive decay series of uranium and thorium. Because it decays so quickly, however, only minute quantities of radium ever exist at any one time. 

It is important to be able to balance the nuclear reaction for each of the steps in a radioactive decay series. For example, the first step in the uranium decay series is the decay of uranium-238 to thorium-234, with the emission of an a particle. Hence, the reaction is... 

Th is the parent of the 4n radioactive decay series shown in Fig. 6.1 and listed in Table 6.3. The last column of Table 6.3 gives the ratio of the number of atoms of each decay product of natural thorium to Th, assuming that the thorium has been undisturbed long enough, around 40 years, for its decay products to reach equilibrium. At equilibrium, the activities of all these radioactive nuclides are equal, except for Po and T1, which are alternative decay products of Bi. 

The two proved the existence of two radioactive decay series one starting with nraninm and the other with thorium. The final product of both series was lead. They predicted that hefium should be the decay product of radium, and they were the first to calculate the tremendous amount of energy that could be evolved during radioactive reactions. 

FlC- 1-2, The three naturally occurring radioactive decay series and the man-made neptunium series. Although (which is the parent to the actinium series) and (which is the parent to the thorium series) have been discovered in nature, die decay series shown here begin with the most abundant Icmg-Uved nuclides. 

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. 

Another common method of dating U-minerals is by considering its content of lead isotopes. Lead has four stable isotopes of which three are end products of radioactive decay series. The fourth lead isotope, Pb, is foimd in lead minerals in about 1.4% isotopic abundance and has no radio-genetic origin. At the time of formation of the earth, all the Pb in nature must have been mixed with unknown amounts of the other lead isotopes. If a lead-containing mineral lacks Pb, it can be assumed that presence of the other lead isotopes together with uranium and/or thorium must be due to their formation in the decay... 

Radon-222 is a direct decay product of radium-226, which is part of the decay series that begins with uranium-238 (see Chapter 3, Figure 3-1). Thorium-230 and thorium-234 are also part of this decay series. Uranium, thorium, and radium are the subjectof other ATSDR Toxicological Profiles. Other isotopes of radon, such as radon-219 and radon-220, are formed in other radioactive decay series. Flowever, radon-219 usually is not considered in the evaluation of radon-induced health effects because it is not abundant in the environment (Radon-219 is part of the decay chain of uranium-235, a relatively rare isotope) and has an extremely short half-life (4 seconds). Radon-220 is also usually not considered when evaluating radon-related health effects. While the average rate of production of radon-220 is about the same as radon-222, the amount of radon-220 entering the environment is much less than that of radon-222 because of the short half-life of radon-220 (56 seconds). All discussions of radon in the text refer to radon-222. 

Protactinium is a radioactive element with over 20 isotopes, of which the one of interest is Pa with a half-life of 32,480 years and a corresponding decay constant of 2.134 x 10 a . It is a daughter of Th in the radioactive decay series. In oceans, its geochemical properties resemble those of thorium since the isotopes are removed from seawater either by adsorption on mineral grains or by incorporation into authigenic minerals. Hence, sediments deposited in oceans may contain not only excess unsupported Th, but also unsupported Pa. If both are removed from seawater equally efficiently, their activity ratios will change with time in a manner controlled by their decay constants. 

Attainment of secular equilibrium in radioactive decay series starting with uranium-238, uranium-235, and thorium-232 depends on none of relevant daughters leaving or entering the systems concerned. Once this equilibrium is established, the decay rates of the daughters in... 

The exponential laws of radioactive-series decay and growth of radionuclides were first formulated by Rutherford and Soddy in 1902, to explain their results (Rutherford and Soddy 1902,1903) on the thorium series of radionuclides. In 1910, Bateman (Bateman 1910) derived generalized mathematical expressions that were used to describe the decay and growth of the naturally occurring actinium, uranium, and thorium series until the discovery of nuclear fission and other new radioactive decay series were found in the 1940s. For the description of half-lives and decay constants, activities and number of radionuclides involved in the decay of two radionuclides, Friedlander et al. (1981) have given a representative overview (see also O Chap. 5 in Vol. 1). 

The numerous radionuclides present in thorium and uranium ores are members of genetic correlated radioactive decay series, which are represented in Fig. 1. In all of these decay series, only a and decay are observed. With emission of an a particle (jHe), the atomic number... 

Most important mineral Some isotopes of astatine are present in uranium and thorium minerals as part of their radioactive decay series. Astatine belongs to the most rare elements of all. Its total amount in the earth s crust is estimated to be less than 30 grams. 

Natural uranium is 99.28% which decays as we have described. However, the natural element also contains 0.72% This isotope starts a second radioactive decay series, which consists of a sequence of alpha and beta decays, ending with lead-207. The third naturally occurring radioactive decay series begins with thorium-232 and ends with lead-208. All three radioactive decay series found naturally end with an isotope of lead. 

The chain of radioactive decay that begins with continues through a number of steps of a and j8 emission until it eventually terminates with a stable isotope of lead— 82 1. The entire scheme is outlin in Figure 25-2. All naturally occurring radioactive nuclides of high atomic number belong to one of three radioactive decay series the uranium series just described, the thorium series, or the actinium series. (The actinium series actually begins with uranium-235, which was once called actino-uranium.)... 

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. 

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