Radioactive decay primordial nuclides from

There are essentially three sources of radioactive elements. Primordial nuclides are radioactive elements whose half-lives are comparable to the age of our solar system and were present at the formation of Earth. These nuclides are generally referred to as naturally occurring radioactivity and are derived from the radioactive decay of thorium and uranium. Cosmogenic nuclides are atoms that are constantly being synthesized from the bombardment of planetary surfaces by cosmic particles (primarily protons ejected from the Sun), and are also considered natural in their origin. The third source of radioactive nuclides is termed anthropogenic and results from human activity in the production of nuclear power, nuclear weapons, or through the use of particle accelerators. 

A fourth long radioactive decay series, the neptunium series (Fig. 1.2), is conqxrsed of nuclides having mass numbers which divided by 4 have a remainder of 1 (the 4n + 1 series). The name comes from the longest lived A = 4n + I nuclide heavier than Bi, Np, which is considered as the parmt species it has a half-life of 2.14 X 10° y. Inasmuch as this half-life is considerably shorter than the age of the earth, primordial Np no longer exists on earth, and, therefore, the neptunium series is not found as a natural... 

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. 

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. 

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