Living things and their products contain carbon. Like all elements there are som
ID: 1419428 • Letter: L
Question
Living things and their products contain carbon. Like all elements there are some atoms of carbon that are stable (C-12) and some that decay (C-14, among others). The carbon-14 Is produced -n the upper atmosphere when a neutron strikes nitrogen-14 atoms. Write the nuclear equation that produces carbon-14 and predict the second product of this reaction. Interpret the above graph to determine the half life of carbon-14. Radiocarbon dating requires material from previously living things no more than about 60 000 yrs of age. Why these two limitations?Explanation / Answer
a) Carbon-14 is produced in the upper layers of the troposphere and the stratosphere by thermal neutrons absorbed by nitrogen atoms. When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons. The resulting neutrons (1n) participate in the following reaction:
n + 146 N 146C + p
The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 49,000 ft) and at high geomagnetic latitudes.
b) From the curve, it follows that the activity ratio becomes 0.5 at approximately 5,730 years. Thus, the half life is approximately 5,730 years.
c) Radiocarbon dating is only possible in organic matter. This is because elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research. It was soon discovered that 14C's half-life was far longer than had been previously thought. Since living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, the ratio of C-14 to C-12 approaches that of the atmosphere.
After an organism has been dead for 60,000 years, so little carbon-14 remains left that accurate dating can not be established. Thus, radiocarbon dating requires material from previously living things no more than about 60,000 years of age. In fact, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
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