Long-lived radioactive decay systems

The P-decay of Rb to Sr, with a half-life of 48.8 billion years (Table 10.2), is one of the dassic radiometric dating systems and has been used in cosmo- and geochronology for almost half a century. Strontium has four stable isotopes and as more than two stable non-radiogenic isotopes are thus available, instrumental mass bias can be corrected by internal normalization (see also Chapter 5). For the correction of the radiogenic Sr/ Sr ratios, Sr/ Sr is most suitable, as this blankets the isotope of interest ( Sr) and features two isotopes with relatively high abundances, which minimizes the analytical uncertainty of the measurements. 

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The long-lived radioactive decay systems commonly used to characterize mantle compositions, their half-lives, and the isotope ratios of the respective radiogenic daughter elements are given in Table 1. The half-lives of Sm, Sr, Hf, Re, and Th are several times greater than the age of the Earth, so that the accumulation of the radiogenic daughter nuclide is nearly linear with time. This is not the case for the shorter-lived... 

Long-lived radioactive decay systems are based on unstable parent nuclides with half-lives that are sufficiently long, such that only part of the inventory that was present at the start of the solar system has since decayed away (Figure 10.2). Consequently, most long-lived radioactive parents feature half-lives that are within one to two orders of magnitude of the Earth s age, with values of about a 1-100 Ga (Table 10.2). Such... 

The decay products of the long-lived radioactive systems are important tools for tracing geological time and Earth processes. The main parent-daughter pairs used for studies in the Earth and Planetary sciences are Rb-Sr, Th-U-Pb, Sm-Nd, Lu-Hf, and Re-Os. Traditionally most practitioners have not focused their careers... 

After successful initial operation of the passive core cooling system, the core temperature will be maintained at an acceptably low value and decay heat will be removed for the extended period of time required by the long-lived radioactivity remaining in the core. 

It has been hoped [20,21] that a method could be developed which would directly detect the radioatoms that are present in nature by an efficient ultra-sensitive mass spectrometer technique which would not itself depend upon the fact that the atoms being investigated are radioactive. The advantage of an efficient mass spectrometer system for long-lived radioisotopes can be seen from the equation for calculating the number of atoms present in a sample from its measured radioactive decay rate ... 

The isotopic compositions of several elements have changed since the formation of the solar system. Radioactive isotopes have decayed, and the abundances of their daughter isotopes have increased over time. For long-lived isotopes of particular importance to radiochronology (flagged with an asterisk), Table 4.2 gives both the present abundance and the abundance at the time the solar system formed (italicized values). 

There are 280 naturally occurring nuclides that make up the 83 stable and long-lived elements. These are all the elements up to Bi with Z = 83, except for unstable Tc (Z = 43) and Pm (Z = 61) that only have short-lived isotopes, but the long-lived Th and U bring the total back to 83. Here long-lived or short-lived is with respect to the half-life of an isotope against radioactive decay and the age of the solar system. Long-lived means then an element is still present in measurable quantities since the solar system formed 4.6 Gyr ago, and radioactive isotopes with half-lives above 0.6 Gyr usually qualify... 

In addition to stable elements, radioactive elements are also produced in stars. The unstable but relatively long-lived isotopes °K, Th, and are the internal heat source that drives volcanic activity and processes related to internal convection in the terrestrial planets. The short-lived transuranium elements such as Rn and Ra that are found on the Earth are all products of U and Th decay. These isotopes are sometimes used as tracers of natural terrestrial processes and cycles. Long-lived isotopes, such as Rb and Sm, are used for the precise dating of geological samples. When the solar system formed, it also contained several short-lived isotopes that have since decayed and are now extinct in natural wstems. These include A1, Pu, Pd, and 1. Al, with a half-life of less than 1 Ma, is particularly important because it is a potentially powerful heat source for planetary bodies and because its existence in the early solar system places tight constraints on the early solar system chronology. 

There are several cases in Table 1 (those with half-lives less than 10 years) where the half-life of the parent isotope is so short that any that was incorporated into a rock at the time of formation of the solar system 4.5 Ga ago will have long since decayed away. However, the noble gas daughter isotopes may still be present, and can be used to deduce the very early history of the solar system. Freshly fallen meteorites do contain some of these short-lived radioactivities, as a result of bombardment by cosmic rays (Wider... 

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. 

A third source for radionuclide generator systems is the recovery of radioactive parents from extinct radioactive decay processes, such as Th, which is recovered from decay products. The Th represents a convenient, long-lived iT i = 7,340 years) source from which Ac is recovered, which is the parent of the Ac/ Bi generator system. 

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