Cosmogenic isotopes radioactivities

In the second case were utilized cosmogenic radioactive isotopes, which form in the upper atmosphere the effect of cosmic irradiation. This method is based on the assumption that the content of these nuclides in the atmosphere is stable as a consequence of equilibrium between rates of their formation and decay. These get into the subsurface with atmospheric precipitation, and their content consistently declines with time according to equation (2.358). The age in this case is determined as a function of the ratio of cosmogenic isotopes radioactivity value in the atmosphere and in the sample instead of and in equation (2.358) ... 

Radioactive decay enables a very conditional determination of surface origin water existence duration in the geologic medium. Such estimates are based on measuring either the excess of radiogenic stable isotopes or deficit of radioactive cosmogenic isotopes in water (Kazemi, 2006). 

By contrast, environmentally relevant doses are far smaller. Environmental e osure is received from radioactive minerals and their decay products, cosmic rays and cosmogenic isotopes (such as tritium), and medical X-rays. There is also occupational exposure that is variable but, on the average, small. The average annual dose received by a U.S. resident is on the order of 3 mSv, equivalent to 3 nKjy of low LET radiation (23). Thus, die acute dose in a typical laboratory experiment is about three orders of magnitude greater than die annual dose from background sources. 

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. 

Cosmogenic Nuclides Isotopes of elements produced by the action of cosmic rays (often radioactive). 

The most famous cosmogenic radionuclide is 14C (t1/2 = 5730 a), which is produced by the interaction of cosmic ray neutrons via an (n,p) reaction with nitrogen [14N(n, p)14C], whereas the radioactive decay of 14C takes place by (3 decay to form the stable 14N isotope. 14C is the most important cosmogenic radionuclide for dating (see Section 9.7.5) in archaeology and can be analyzed using isotope sensitive accelerator mass spectrometry. Extremely small isotope ratios 14C/12C = 12 in nature can be measured by means of AMS.28... 

Another isotopic manifestation is the so-called Cosmogenic Radioactivity which is produced in meteorites owing to reactions of their stable atoms with the cosmic rays that bombard the meteorites. Owing to this, meteorites have many live short-lived radioactivities within them when they fall to ground, and measuring their amounts determines when the meteorite fell and how deep within it was the location of the sample prior to the collision that released it into space (see Extinct radioactivity and Meteorites). 

Voshage H. and Hintenberger H. (1963) The cosmic-ray exposure ages of iron meteorites as derived from the isotopic composition of potassium and the production rates of cosmogenic nuclides in the past. In Radioactive Dating, International Atomic Energy Agency, Vienna, pp. 367-379. 

There are nine known radioactive isotopes and six are listed in Table 2. Sulfur-35 has the longest half-life and is produced by cosmogenic synthesis in the upper atmosphere cosmogenic S-35 (Tanaka and Turekian, 1991) is sufficiently long lived to be useful in determining overall removal and transformation rates of SO2 from the atmosphere and an estimated dry deposition flux to total flux ratio is —0.20 in the eastern US (Turekian and Tanaka, 1992). 

As apparent from the properties listed in Table 1, ocean chemists have a variety of tracers at hand, covering a range of residence times and chemical behaviors. Those tracers varying due to radioactive decay have distinct isotopic compositions in their various source materials (Table 2). This makes them particularly useful both as water mass tracers, and to reconstruct the flux from these various sources into the oceans. It may be surprising to And the cosmogenic nuclide °Be in this list of otherwise radiogenic tracers. The reason is that Be behaves very similarly to the other tracers in that the ratio °Be/ Be is distinct in different water masses. Given that °Be is the only tracer of which the flux into the oceans is known, t can be calculated precisely from its water column concentration. Further, the continent-derived isotope Be is the only tracer of which the flux into the oceans can be calculated from the °Be/ Be ratio. 

The table summarizes our knowledge about the essentiality, toxicity and isotopes of elements. It can be seen that the toxicity of elements depends on their chemical form. The isotopes of elements are rare cosmogenic nuclides. Nuclear fuel reprocessing, as well as reactor accidents, may release them to the biosphere. The isotopes are used as a radioactive tracer in medical practice. 

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