Saturation analysis, radionuclides

The production of radionuclides in meteoroids that are orbiting the Sun takes place by nuclear spallation and neutron-capture reactions with the atoms of the major elements (Bogard et al. 1995 Leya et al. 2000). The concentration of a particular radionuclide in a stony meteoroid exposed to cosmic rays in Fig. 18.14 initially increases with time until it reaches a state of equilibrium or saturation when its rate of decay is equal to its rate of production. When such a meteoroid enters the atmosphere of the Earth and explodes, the resulting meteorite specimens are assumed to be saturated with respect to the cosmogenic radionuclides they contain. After a meteorite has landed on the surface of the Earth, the production of radionuclides stops because the Earth is protected from cosmic rays by its magnetic field and by the atmosphere. Therefore, the rate of decay of cosmogenic radionuclides decreases with time as each nuclide continues to decay with its characteristic halflife. The terrestrial age of a meteorite specimen collected in Antarctica or anywhere else on the Earth is calculated from the rates of decay of the radionuclides (e.g., C1 or A1) that remain at the time of analysis (Jull 2001). 

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