Carbon
dating is a variety of radioactive dating which is applicable only to matter
which was once living and presumed to be in equilibrium with the atmosphere,
taking in carbon dioxide from the air for photosynthesis.
Cosmic ray protons blast nuclei in the upper atmosphere, producing
neutrons which in turn bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment
produces the radioactive isotope carbon-14. The radioactive carbon-14
combines with oxygen to form carbon dioxide and is incorporated into the cycle
of living things.
The carbon-14 forms at a rate which appears to be constant, so
that by measuring the radioactive emissions from once-living matter and
comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made.
Presuming
the rate of production of carbon-14 to be constant, the
activity of a sample can be directly compared to the equilibrium activity of
living matter and the age calculated. Various tests of reliability have confirmed the value of carbon
data, and many examples provide an interesting range of
application.
Carbon-14 decays with a halflife of about 5730 years by the
emission of an electron of energy 0.016 MeV. This changes the atomic number of
the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the
atmosphere, a gram of carbon shows an activity of about 15 decays per minute.
The low activity of the carbon-14 limits age determinations to the
order of 50,000 years by counting techniques. That can be extended to perhaps
100,000 years byaccelerator techniques for counting the carbon-14
concentration.
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.
From the known half-life of carbon-14 and the number of carbon
atoms in a gram of carbon, you can calculate the number of radioactive decays
to be about 15 decays per minute per gram of carbon in a living organism.
Radioactive carbon is being created by this process at the rate of
about two atoms per second for every square centimeter of the earth's
surface." Levin
The rate of production of carbon-14 in the atmosphere seems to be fairly constant. Carbon dating of
ancient bristlecone
pine trees of ages around
6000 years have provided general corroboration of carbon dating and have
provided some corrections to the data.
