Primordial radioisotopes

Radioisotopes may occur in the earth naturally as primordial radioisotopes, formed when the planet was created, or be produced by natural or artificial processes. Most fast decaying primordial radioisotopes have long disappeared from the planet since the earth originated about 4.5 billion years ago, such isotopes have decayed and reached a final, stable form. The relatively few primordial radioisotopes still extant in the earth today, therefore, decay very slowly. Among these are potassium-40 and some isotopes of uranium, such as uranium-235 and uranium-238, which are of use for dating archaeologically related minerals and rocks (see Textboxes 15 and 16). 

Some radioisotopes are continuously being produced by the bombardment of atoms on the surface of the earth or in its atmosphere with extraterrestrial particles or radiation. One of these is carbon-14, also known as radiocarbon, which is widely used for dating archaeological materials (see Textbox 55). Many radioisotopes that are not primordial or are not created by natural processes are now produced artificially using specialized equipment many of the "artificial" isotopes are of use for probing and analyzing materials. 

A number of short-lived radionuchdes also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earhest solar system rocks, but their mean fives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuchdes. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate... 

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. 

Uranium is a radioactive element that is ubiquitous in the environment, with typical crustal and seawater concentrations of 2.7 mg kg and 3.0 pgl, respectively. Uranium-238 and U are primordial isotopes and their decay leads to the formation of series of daughter radioisotopes, amongst which is " U (Figure 1). The natural isotopic ratio of 238u 235u 234u 99.2745 0.7200 0.0055, and... 

What is a radioisotope When a combination of neutrons and protons, which does not already exist in nature, is produced artificially, the atom will be unstable and is called a radioactive isotope or radioisotope. There are also a number of unstable natural isotopes arising from the decay of primordial uranium and thorium. Overall there are some 1800 radioisotopes. At present there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially. 

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