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Show < p i . THE C- 14 IN ALL PLANTS AND ANIMALS GIVES CLUES TO THE DATE OF ANCIENT f INDS bsL:&%; 5,:: t LA- ll living things contain radioactive car- > on 14 ( C- 14). Because they do, scientists can test plant and animal remains from an archae- '; ological site and tell about how manv years - '- a go people lived there. This natural clock is " wound up'' by cos-mic ray neutrons that bombard the earth's lpper atmosphere, changing nitrogen 14 tr carbon 14 ( N- 14 f n = C- 14). This radioac- PI , tive isotope of carbon easily combines with >.; : .' a ; * < , iA4 >% *- r' 3 y;:*+,. oxygen to form carbon dioxide. In that form. the C- 14 enters all organisms as part of themL7:; normal oxygen- exchange process of all l. 5, living plants and animals. As long as a per-*$; son, a tree, a deer, or any other plant or ani-' * ma1 is alive, it continues to receive a steady,, supply of C- 14. When a plant or animal dies, oxygen-?;; exchange stops, and no more Cr14 is absorbe&$ si f 1 the almosphere, Then, the natural clock begins to " run down." The C- 14 begins to decay. In 5.570 years the plant or animal will have lost onehalf of its C- 14. That is why 5,570 years is called the half- life of C- 14. As GI4 decays it is changed back to nitm gm 14 and in the process emits or releases a beta particle. By measuring the number of beta particles being released, scientists can tel! about how long ago the plant or aairnal died. If a tree were cut down today, ea, ch gram of waad from that tree would release about 15 beta particles every minute. In 5,570 years a gram of wood fr6m that same tree would release about 7 $ beta particles . per minute. In another 5,570 years ( or half- life] the num-ber of beta particlei released would be about 3 %. The best- plant or animal remains t. o% wf far age by radiocarbon dating are charcoal, wood, bone, shell, and horn. Archae~ llogists usually find one or more of rhose materials at a prehistoric site. Samples are carefnlly selected and handled to avoid contamination. Then they ' are sent to special laboratories equipped with sensitive and accurate radio-carbon counters that can measure beta radi Zion from plants and animals that have been dead for as long as 5[ 1,900 years. When testing is completed, the archaeola-gist will have a fairly good idea of how long ago. a prehistoric hunter killed a deer for his dinner ox when a tree was cut down to use - as a beam for a dwelling or when the hunter himself died. - Knowing whkn iin event took place is just as'importaM as knowing what the event was and why it'bappened. No one will ever write a history book without dates in it. Archaeola-gists, too, need dates ta fell the story of the prehistoric peoples who lived in what is now |
