Naturally occurring isotopes radionuclides

Not all of the 300 naturally occurring isotopes are stable. Unstable nuclei (protons and neutrons) spontaneously transform (decay) to achieve stability and are the radioactive materials syn. radionuclides or radioisotopes. Many other radioactive isotopes are made artificially by bombarding atoms with neutrons or charged particles in processes that occur in nuclear energy reactors and particle accelerators. As nuclei decay, they emit one of four types of radiation characteristic of the atom ... 

No radionuclides, other than the naturally occurring isotopes of U and their daughters, were measured in excess of background activities in any of the a or y spectra. The weighted averages and Is uncertainties of empirical data from these counting results were =... 

Nuclear activation analysis (NAA) is a method for qualitatively and quantitatively detg elemental compn by means of nuclear transmutations. The method involves the irradiation or bombardment of samples with nuclear particles or high-energy electromagnetic radiation for the specific purpose of creating radioactive isotopes from the stable or naturally-occurring elements present. From the numbers, types and quantities of radioactive elements or radionuclides, it is possible to deduce information about the elemental compn of the original sample... 

Table 11.2 Naturally occurring radioactive substances, a = years, d = days. Radionuclide Decay Process Half-Life Isotopic Abundance (%) Stable End-Product...

Radioactive decay usually does not immediately lead to a stable end product, but to other unstable nuclei that form a decay series (Kiefer 1990). The most important examples of unstable nuclei are started by very heavy, naturally occurring nuclei. Because the mass number changes only with a decay, all members of a series can be classified according to their mass numbers (see the uranium-238 decay series in Figure 32.2). A total of three natural decay series — formed at the birth of our planet — are named after their parent isotope Th, and (Figure 32.3). Several shorter decay series also exist. For example, Sr decays with a Tb 1/2 of 28 years by [3 emission to °Y, which in turn disintegrates (P emission) with a Tb 1/2 of 64 h to the stable °Zr (Kiefer 1990). Other examples of known radionuclides since the Earth s origin include " °K and Rb. In hazard assessments, all members of a decay series must be considered. 

Numerous radionuclides have been applied to marine sedimentary problems. These are generally grouped into cosmic-ray produced (cos-mogenic) nuclides ( C, °Be, Be, "Al), nuclear bomb-produced (fallout) nuclides ( Cs, plutonium isotopes, 24iAm), and naturally occurring nuclides ultimately derived from the decay of 38u, and 232xh parents. 

From the above it can be concluded that in many instances the introduction of an artificial radionuclide into the environment provides us with a natural tracer experiment. Indeed, this is the basis for the application of deterministic compartmental models, based on tracer kinetics, to radioecology (Whicker and Schultz, 1982). This approach is largely based on the assumption that radionuclide movements will exhibit first order kinetics although the existence of naturally-occurring tracees (stable isotopes) at relatively high abundance may result in more complex concentration-dependent kinetics. Furthermore, nutrient analogues may exert even more complex effects on processes such as radioion absorption across root plasma membranes this will become evident later in the chapter. 

H, C, P, and l. The word isotope comes from Greek, meaning at the same place , a useful way to remember that all isotopes of an element are in the same place in the Periodic Table of elements. While not quite correct, often the words isotope and nuclide are used interchangeably. Thus, radioisotopes may be termed radionuclides. The later refers to an atom with an unstable nucleus that undergoes radioactive decay, and these may be naturally occurring or artificially produced. 

The isotope is a naturally occurring radionuclide with a long half-life T = 1.26 X 10 years) and is present in natural potassium at 0.0118 %. Potassium... 

One prominent and well-known kind of nuclear component is that which is produced by the decay of naturally occurring radionuclides (see Table 1). The best- and longest-known examples are He, produced by alpha decay of the natural isotopes of uranium and Th, and " Ar, produced in one branch of the beta decay of " K. (There are several other natural radionuclides which produce He by alpha decay, but whether because of low parent abundance and/or very slow decay, only in very unusual samples is the production of He not strongly dominated by uranium and thorium.) Since radioactive decay laws are well known, the ratio of daughter to parent isotope(s) in a closed system is a simple function of time, whence this phenomenon has been long and extensively exploited as a geochronometer (e.g., see Chapter 1.16). 

Similarly, 1 is a naturally occurring, cosmo-genic, and hssiogenic isotope T ri = 15.7 Myr Fabryka-Martin et al., 1991 FabrykaMartin, 2000). Like H, and C1, was produced in bomb tests, but in greater abundance above the natural level. While some of the other anthropogenic radionuclides have returned to near pre-bomb levels in the surface environment, the level continues to be elevated due to emissions from... 

Fig. 2.1. Details of the chart of isotopes showing the three decay series which contain between them all naturally occurring alpha-emitting radionuclides.

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