Carbon radioactive dating with

Once amounts of carbon-12 and carbon-14 are measured in an object, the ratio of carbon-14 to carbon-12 is compared with the ratio of these isotopes in a sample of similar material whose age is known. Using radioactive dating, with carbon-14, scientists can estimate the age of the object. 

Natural carbon contains chiefly the stable isotopes C (-98.89%) and C (-1.10%) (Rosier and Lange 1972). Radioactive dating with C depends upon the fact that trace amounts of radioactive (-10 % of total C) are created from nitrogen gas in the upper atmosphere through the reaction... 

Radioactive dating is a means of determining the age of a dead plant or animal by comparing the amount of a radioactive isotope in it with the amount of the same radioactive isotope in a living organism. Carbon-14 is often the isotope used for this purpose. Most carbon atoms are the nonradioactive carbon-12 isotope. The radioactive carbon-14 isotope is formed in the atmosphere as a result of random neutron capture, as shown here ... 

Because radioactive isotopes seem to decay at very constant rates, they can be used as clocks. One of the first radioactive dating techniques involved the use of the radioisotope carbon-14. Carbon-14 is produced in the upper atmosphere when neutrons (produced by cosmic rays from space) collide with nitrogen-14 molecules in the reaction shown below ... 

Carbon-14 Dating. The earth is bombarded by an almost constant flux of cosmic rays, which in the stratosphere generate many particles, including neutrons, 0n1 which react with 7N14 nuclei to form radioactive C-14 by the (n, p) reaction, i.e. the emission of a proton ppx. 

Not all examples of elemental notation will be written with the same amount of information. Notice that the first two examples show only two numbers. The number in the upper-left corner is the mass number, so, for example, the carbon sample shown is of the isotope called carbon-14, which, as you may know, is used for radioactive dating. The number in the lower-left corner is the atomic number. This number is not always given, because the elemental symbol already tells you the identity of the element, so the atomic number represents redundant information. If you needed to know... 

In its elemental form, carbon can be fonnd as diamond, the hardest naturally occurring substance graphite, an excellent Inbricant or as a fullerene (or buckyball ). Althongh five isotopes are known, only C-12 (98.9 per-centnatural abundance) and C-13 (1.1 percent) are stable. Nevertheless, the presence of trace amounts of C-14, a radioactive isotope (with a half-life of 5,730 years), permits carbon dating of historical objects. 

Half-Life and Radiocarbon Dating The half-life of a radioactive nuclide is the time it takes for half of the parent nuclides in a radioactive sample to decay. The presence of radioactive carbon-14 (with a half-life of 5730 years) in the environment provides a natural clock by which to estimate the age of many arfifacfs and fossils. All living things contain carbon-14. When they die, the carbon-14 decays with its characteristic half-life. A measurement of the amount of carbon-14 remaining in a fossil or are artifact can therefore reveal ifs age. 

Carbon is naturally present in the forms of different isotopes. As indicated in Table 4.1, c is predominant and represents 98.9% of the total. "<1, commonly used for radioactive dating of ancient objects, is so scarce in nature that it cannot be detected with an ordinary mass spectrometer. C constitutes approximately 1.1% of the total carbon and is detectable by mass spectrometry. The peak corresponding to the isotopomer of C at m/z -e1 compared to that of C is as abundant as the number of carbon atoms of the ion. 

Most CO and CO2 in the atmosphere contain the mass 12 isotope of carbon. However, due to the reaction of cosmic ray neutrons with nitrogen in the upper atmosphere, C is produced. Nuclear bomb explosions also produce C. The C is oxidized, first to CO and then to C02 by OH- radicals. As a result, all CO2 in the atmosphere contains some 0, currently a fraction of ca. 10 of all CO2. Since C is radioactive (j -emitter, 0.156 MeV, half-life of 5770 years), all atmospheric CO2 is slightly radioactive. Again, since atmospheric CO2 is the carbon source for photos5mthesis, aU biomass contains C and its level of radioactivity can be used to date the age of the biological material. 

Examples of isotopes are abundant. The major form of hydrogen is represented as H (or H-1), with one proton H, known as the isotope deuterium or heavy hydrogen, consists of one proton and one neutron (thus an amu of 2) and is the isotope of hydrogen called tritium with an amu of 3. Carbon-12 ( C or C-12) is the most abundant form of carbon, though carbon has several isotopes. One is the C isotope, a radioactive isotope of carbon that is used as a tracer and to determine dates of organic artifacts. Uranium-238 is the radioactive isotope (Note The atomic number is placed as a subscript prefix to the element s symbol—for example, —and the atomic mass number can be written either as a dash and number fol-... 

Obviously this wide distribution of the 14C formed in the atmosphere lakes time it is believed to require a period of 500-1000 years. This time is not. however, a deterrent to radiocarbon dating because of two factors die long half-life of I4C and the relatively constant rate of cosmic-ray formation of l4C in the earth s atmosphere over the most recent several thousands of years. These considerations lead to the conclusion that the proportion of 14C in the carbon reservoir of the earth is constant, and that the addition by cosmic ray production is in balance with the loss by radioactive decay. If this conclusion is warranted, then the carbon dioxide on earth many centuries ago had the same content of radioactive carbon as the carbon dioxide on earth today, Thus, radioactive carbon in the wood of a tree growing centuries ago had the same content as that in carbon oil earth today. Therefore, if we wish to determine how long ago a tree was cut down to build an ancient fire, all we need to do is to determine the relative 14C content of the carbon in the charcoal remaining, using the value we have determined for llie half life of 14C. If the carbon from Ihe charcoal in an ancient cave has only as much 14C radioactivity as does carbon on earth today, then we can conclude that the tree which furnished llie firewood grew 5730 30 years ago. 

C-14 dating was discovered by Libby11 and co-worker [2], The cosmic ray flux has been fairly constant over prehistoric and current time and provides a small but almost constant supply of 6C14, at a rate averaged over the whole atmosphere of about 2.2 atoms cm-2 s 1. The radioactive 6C14 will bind to oxygen in the atmosphere to form radioactive carbon dioxide, but will decay, with a half-life fi/2 = 5730 years, by emitting an electron (or "fj ray") and an electron antineutrino ... 

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