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Establishing the relative age of various
materials with the use of carbon-14. This involves measuring the amount of
14C and of 12C and comparing the measured ratio to
the one established by the production of 14C in the upper
atmosphere by cosmic rays. When an organism is alive the 14C/
12C ratio in its biomass is constant (because of constant
atmospheric 14C production, difussion to the lower atmsophere,
absorption by organisms, and because of constant 14C
radioactive decay) but when the organism dies the ratio begins to
change--at a very predictable rate. Therefore knowing the
carbon-14/carbon-12 ratio now in some artifact gives a very good
measure of how long the carbon has been "dead." The object however must
obviously have organic material either in it or on it for this method to
work. Also the lenth of time one can "look back" is limited because the
amount of carbon-14 must be detected with some certainty for the age to be
known with confidence, and the longer the sample has been dead the less
14C is present because it is continually decaying.
[New
Scientist; v274; 18; 1996.] [Scientific American; v150; 24; 1996.]
[Bowman, Sheridan, Radiocarbon dating--Interpreting the past. British
Museum Press; 1995.]
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