Elements, isotopic composition radionuclides

The activity of naturally radioactive elements is a measure of their mass. Prerequisites of application of the correlation between mass and activity according to eq. (17.1) are that the isotopic composition of the element to be determined is constant and that interfering radioactive impurities are absent. If the daughter nuelides are also radioactive, radioactive equilibrium must be established or the daughter nuclides must be separated off quantitatively. Interference of radioactive impurities may be avoided by measuring the a or y spectrum of the radionuclide considered. 

Radioactivity is a part of nature—in the process of element formation by nuclear reactions taking place in stars, both stable and radioactive isotopes of elements are formed. The isotopic composition of elements is characterized by properties of nuclear reactions that led to the formation of the elements. Elemental composition of the planet Earth, thought to be about 4.5x 10 years old, although not yet in chemical equilibrium, reflects the composition of the material from which it was formed. Therefore, a number of radionuclides occur in nature, having long half-lives (longer than the age of Earth). In addition there are natural processes which continuously produce new radioisotopes. Recently, human activities have also contributed to the increased concentration of some of the radionuclides. 

Sometimes, cosmic-ray-induced isotope shifts are also a nuisance, because they may compromise the precise determinations of isotopic compositions of elements of interest. This is often a major problem in the case of trapped noble gases (Ott 2002, this volume), but may also require attention when e.g., small excesses or deficits of daughter isotopes of an extinct radionuclide present in the early solar system are to be determined (e g., Leya et al. 2000c). 

Isotope ratios, as measured by mass spectrometry, along with the known amount of the tracer, are used to determine the amount of the analyte in the sample. Isotopically altered elements (sometimes consisting of nearly pure stable isotopes) and radioactive isotopes are commercially available for use as tracers. Nearly every element can be purchased as a solution whose atom concentration is known and traceable to NIST. Standard materials are also available with isotopic composition determined to a high degree of accuracy. Radioactive tracers, such as 233U and 244Pu, are available for tracing actinide elements that do not have stable isotopes. Because many of the samples analyzed in radionuclide determinations already have altered isotopic ratios, many times it is possible to use a natural isotope as the tracer. 

When an element has more than one radioisotope, determinations and data analysis are generally more complex because the isotopes may differ in half-life, especially when a series is involved, e.g., radium, thorium, polonium, radon, actinium, protactinium, and uranium. One possibility is to make measurements after the decay of the short-lived radionuclides, but this may require long waiting times. In favorable cases, it is more convenient to measure the activity of decay products (e.g., radon, thoron ( Rn), actinon ( Rn)), or correct the measurements of the short-lived radioisotopes after determination of the isotopic composition. 

Extraterrestrial materials In some extraterrestrial material such as meteorites, elements may show isotopic compositions that are distinct from all terrestrial material investigated. This is related to decay of radionuclides that may already be extinct, due to half-lives which are very short compared with the age of the solar system of 4.6 x 10 years. Such variations are rare for terrestrial materials, in large part due to preferential sampling of the crust, whereas some extraterrestrial material, such as iron meteorites, resemble the Earth s core, in which parent to daughter element ratios may be much higher than in the cmst. 

Interaction between cosmic rays and terrestrial matter The Earth s atmosphere and, to a lesser extent, its surface are constantly bombarded with cosmic radiation which interacts with terrestrial material, resulting in isotopic variations in some elements. The best known example is the production of C from by (n,p) reaction in the atmosphere, with the neutron involved created by cosmic ray-induced spallation. a radionuclide with a half-life of 5730 years, is oxidized to CO2 and enters the food chain via photosynthesis, thus affecting the isotopic composition of C in all living organisms. 

The chemical compositions of individual chondrules have been determined by neutron activation of extracted samples or by electron microprobe analyses of chondrules in situ. Some, but not all chondrules are depleted in moderately volatile elements. There is a compositional continuum between the olivine-rich and aluminum-rich chondrules. Original concentrations of the short-lived radionuclide 26A1 in chondrules suggest they formed very early, before all of this isotope decayed, but as much as 2-5 million years after the formation of CAIs (see Refractory Inclusions, below). 

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