Radioactive decay scheme

Table 5—Radioactive decay scheme data for radionuclides of cerium observed in previous environmental surveillance studies ...

The radioactive decay scheme of the parent nuclide is well known. 

The radioactive decay schemes of radon and thoron are shown in Fig. 1.1. The old generic nomenclature (RaA, ThB etc.) is now superseded by the isotopic designation (218Po, 212Pb etc.), but where necessary for clarity the old designation will be added. 

Batch decay is concerned with the radioactive decay of a given amount of initially pure parent material. The decay products will build up and, if radioactive, will later die away as time progresses. An example is the decay chain resulting from the radioactive disintegration of Pb, which is itself a member of the radioactive decay scheme of Starting with Pb, the... 

Table 3 Radioactive decay schemes that influence the isotopic composition of naturally occurring Os...

Dating sulfide minerals is complicated because they lack radioactive elements such as uranium, thorium, and rubidium. However, some contain potassium, e.g., the mineral rasvumite, KFe2S3. Samples of this mineral were collected from an alkali-rich igneous rock at Coyote Peak near Areata, northern CaKfomia, and they gave a date of 26.5 0.5 Ma, while associated phlogopite mica yielded a total release date from Ar/ Ar measurements of 28.3 0.4 Ma. This confirms that potassium-bearing sulfide minerals can be dated on the basis of the radioactive decay scheme... 

Appendix A Radioactive decay schemes of uranium and thorium (after Friedlander and Kennedy, 1949)... 

The most common type of source for Fe Mossbauer spectroscopy consists of elemental Co incorporated into a host metal lattice such as rhodium or copper. In the case of Sn measurements, " Sn-enriched CaSnOa or BaSnOa is used as a source. Schematic diagrams of the radioactive decay schemes for these two isotopes are shown in Figure 5. In addition to these transitions, internal conversion processes may give rise to emission of radiation of other energies. For example, in the case of Fe, the / = state may decay via the ejection of a X-shell 7.3-keV electron, and the hole created be filled by an L-shell electron, leading to the emission of either a 6.4-keV electron (Auger process) or X-ray in order to conserve energy. 

Even though some of the daughters in natural radioactive decay schemes have very short half-lives, all are present because they are constantly forming as well as decaying. It is likely that only about one gram of radium-226 was present in several tons of uraniiun ore processed by Marie Curie in her discovery of radium in 1898. Nevertheless, she was successful in isolating it. The ore also contained only a fraction of a milligram of polonium, which she was able to detect but not isolate. 

Radioactive decay schemes can be used to determine the ages of rocks and thereby the age of Earth (see Section 25-5). The appearance of certain radioactive substances in the environment can also be explained through radioactive decay series. The nuclides °Po and Pb have been detected in cigarette smoke. These radioactive isotopes are derived from found in trace amounts in the phosphate fertilizers used in tobacco fields. These a-emitting isotopes have been implicated in the link between cigarette smoking and cancer and heart disease. 

The natural radioactive decay scheme of Figure 25-2 suggests the eventual fate of all in nafure—conversion to lead. Naturally occurring uranium minerals always have associated with them some nonradioactive lead formed by radioactive decay. From the mass ratio of Pb to 92U in such a mineral, it is possible to estimate the age of the igneous rock containing the mineral. The age of fhe rock refers to the time elapsed since molten magma solidified to form the rock. One assumption of this method is that the initial radioactive nuclide, the final sfable nuclides, and all the products of a decay series remain in the rock. Another assumption is that any lead present in the rock initially consisted of the several isotopes of lead in their present, naturally occurring abundances. 

This is an important scheme. It is characteristic not only of radioactive decay but also of many chemical systems. In the model shown, the reversion of I to A is taken to be unimportant we shall consider the effect of a back reaction in Section 4.3. The differential equations are... 

Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with great precision immediately opened up the entire field of uranium and thorium decay chain studies. This area of study was formerly the poaching ground for radioactive measurements alone but now became part of the wonderful world of mass spectrometric measurements. (The same transformation took place for radiocarbon from the various radioactive counting schemes to accelerator mass spectrometry.)... 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

The most familiar radioactive element is uranium, which has two naturally occurring isotopes of mass numbers 235 and 238 that decay very slowly. Review the first few steps in the decay of uranium-238, which changes to lead-206 after the emission of 8 alpha and 6 beta particles. The earliest stages of the decay scheme involve only three elements, as shown in Figure 3-2. 

Table 3-2 lists important physical properties of radium and selected radium compounds. Radioactive properties of the four naturally-occurring radium isotopes are listed in Table 3-3. In addition to the naturally occurring isotopes, there are 12 other known isotopes of radium. The principal decay schemes of the uranium and thorium decay series that produce the naturally-occurring radium isotopes are presented in Figure 3-1.

The two fission-produced radio-strontium isotopes of interest in environmental samples are 90Sr and 89Sr. Sr-90 has a fission yield of 5.8 %, a half-life is 28.78 a, and the radioactive daughter 90Y with a half life of 2.67 d, to which it decays by beta-particle emission. Sr-89 has a fission yield of 4.7%, a half life of 50.52 d, and decays to the stable daughter89 Y. The decay schemes given in Figure 13.1 show that these two radio-strontium isotopes for practical purposes can only be measured by beta-particle counting. 

In order to determine the half-life, the decay scheme, and other nuclear characteristics of a radioactive nuclide, it is important to use a sample of very high radiochemical purity. In addition in the measurement of nuclear reaction cross sections, fission yields and in activation analysis, the amounts of the radioactive nuclide produced must be determined. Thus It Is also necessary to determine the yield of... 

There are very few known chemical reactions which fit precisely into this scheme of two consecutive first-order reactions, but all of the well-known schemes of radioactive decay which involve two or more consecutive steps are characterized precisely by this type of behavior. 

The situation becomes more complicated if the nuclides I and 2 in the reaction scheme (8.41) are not only produced by radioactive decay, but also by binuclear reactions ... 

The signamres of long-lived radioactive isotope decay schemes provide some of the most unequivocal evidence for chemical evolution of the mantle. Excellent reviews of the application... 

The preceding equations hold for decay schemes that involve the simple transformation of one isotope to another. Three types of decay schemes useful for geochronology are special cases that yield somewhat more complicated age equations. Branched decay leads to the production of two different daughter isotopes from a single radioactive isotope. A familiar example is the spontaneous conversion of to both " °Ar (through electron capture) and " °Ca (through /3 decay). In such cases, the age equations must be modified by factors that correct for the fact that not all decays result in the production of the daughter isotope of interest. For the — " Ar... 

Of all the different geochronological decay schemes that have been employed in cmstal studies the U-Pb system is unique in that there are two radioactive parent isotopes ( U and U) that decay to two different daughter isotopes ( ° Pb and ° Pb). Because of this unique dual decay scheme the U-Pb system has long been of considerable interest as a geochronological technique. However, because of the extreme... 

All absolute dating methods that have proven dependable are based on radioactive decay. Virtually all of the methods depend on the methodical decrease in the amount of the radioactive nuclide and the growth of the corresponding daughter product. The one exception is in the area of radiation-induced damage in solids, which is the basis of thermoluminescence or ESR dating. This latter scheme will be dealt with later in the chapter. Here the basic principles of canonical radioactive geochronometry as applied to marine deposits is reviewed. 

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