Uranium-thorium decay series daughter

The accurate clocks provided by the uranium-thorium decay series enable us to extract rate information from the measurement of radioactive disequilibria in the ocean. Among the wide spectrum of available tracers, a mother-daughter pair with appropriate reactivities and half-lives can be found for a multitude of processes related to particle transport, water mass transport and mixing, and gas exchange (Table 5). 

When thorium emits alpha particles, it disintegrates into other daughter radionuclides (radioactive materials), such as radium-226 and radon-222 (from thorium-230 in the uranium-238 decay series) or radium-228 and thoron (radon-220 from thorium-232 in the thorium decay series). It eventually decays to stable lead-208 or -206, which is not radioactive. More information about the decay of thorium can be found in Chapter 3. The toxicological characteristics of radon, radium, and lead are the subject of separate ATSDR Toxicological profiles. 

If we look at the complete decay scheme, we find 14 daughter nuclides, all of whom have much shorter half-lives than their ultimate parent. The total activity, assuming radioactive equilibrium is established, will be 14 times the activity. (A more complete discussion of gamma spectrometry of the uranium and thorium decay series nuclides can be found in Chapter 16, Section 16.1.2)... 

Uranium and thorium are not stable they decay mainly by alpha-particle emission to nuclides that themselves are radioactive. Natural uranium is composed of three long-lived isotopes, a smaller proportion of and an even smaller proportion of the decay-series daughter of Natural thorium has one single isotope, Th. Each of these nuchdes decays to an unstable daughter leading, in turn, to a whole series of nuclides that terminate in one or other of the stable isotopes of lead. Under normal circumstances, in a natural material, the U/ U ratio will be fixed and aU nuclides in each of the series will be in equilibrium. 

The transmutations of the uranium-238 decay series are charted in Figure 2.7. Locate the parent nuclide, uranium-238, on the chart. As the nucleus of uranium-238 decays, it emits an alpha particle. The mass number of the nuclide, and thus the vertical position on the graph, decreases by four. The atomic number, and thus the horizontal position, decreases by two. The daughter nuclide is an isotope of thorium. 

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) ... 

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. 

Very soon after the radioactivity of thorium and uranium had been discovered it was found that pure samples of both of these elements were only very weakly radioactive. However, such pure samples became more and more radioactive with time until they reached a steady level identical to that in the original samples before pmification. This suggested that the uranium or thorium atoms were transforming or decaying into other radioactive daughter elements and that hitherto undiscovered series of such elements might exist. The search for the radioactive products of uranium by Marie and Pierre Curie led to the characterisation of two new elements, which were named polonium, Po, and radium, Ra. Both elements are far more radioactive than uranium and decay so rapidly that no ore deposits are formed. They exist only because they are formed constantly from naturally occurring uranium. 

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 ° T1, in the Th decay series, and 7 " Bi in the decay series are used as indicators of the amount of their parents in the materials. Other members of each chain will also be present. Apart from these primordial nuclides (those surviving from the formation of the solar system), traces of anthropogenic (human-made) species are seen, such as Co in steels and Cs in molecular sieves. There are obvious reasons for some of these radioactive contents aluminium always has traces of uranium and thorium within it, and it is not unreasonable to expect in molecular sieves. However, relatively large amounts (compared to other materials in the list) of uranium and thorium daughter nuclides in epoxies and printed circuit boards are unexpected. Clearly, for some materials, there is ample scope for reducing the activity the detector sees by selecting a material with a lower activity. In a similar table to that above, Dassie (in a private communication) reported 4.3 Bq of Cs kg of Csl and 33 Bq of kg of Nal, considerably more than in Table 13.6 this latter would be a problem were it to be used for low level Compton suppression systems. 

What happens to those uranium and thorium impurities, and their daughters, dming production Gattuna spectrometry of a typical commercial material, three months after manufacture and four weeks after preparation of the counting sample, gave the results shown in Table 16.5. The immediate observation is that the equilibriums of all of the decay series have been disturbed, but not in an obvious manner. 

A radionuclide, upon undergoing disintegration of a particular type, yields a specified nuclide. The original radionuclide is called the parent and the decay product is called the daughter. The daughter may also be a radionuclide. A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results, is called a radioactive series. Examples of such series are the uranium series and the thorium series. 

---