Dating with radioactivity

ARTEMIS/CEA The measurement of carbon-14 activity requires to collect a sufficiently large of the fossil. There are a trillion times less (10 to the power -12) of carbon-14 radioactive than carbon-12.

The radioactivity of a “fresh” gram of carbon is counted in counts per minute.

The atoms of carbon 14 then proceed to decay exponentially, with a half life of 5,700 years.

When much later, an archaeologist examines the remains (fireplace ashes, bones, plant remains), he can date the fossil by comparing the fraction of remaining radiocarbon nuclei to the fraction existing at the time the organism stopped absorbing carbon.

All living beings assimilate carbon dioxide molecules, a fixed but very small fraction of which contains carbon 14.

This assimilation stops upon the death of the organism, thus halting the absorption of any more carbon 14.This technique was first implemented in France at the center of the low radioactivity of Gif-sur-Yvette in France with an instrument called Tandetron.It has been replaced since 2004 by Artemis, a mass spectrometer capable of dating each year 4,500 samples of less than a milligram. At the bottom part, slowly filling up, are the nuclei resulting from these decays.Grotto radiocarbon dating Bison on the wall of the Niaux cave (in Ariège), drawn some 13,000 years ago.Our ancestors measured the passing of time with water clocks or hourglasses. It measures time -like our ancestors - by using hourglasses provided by radioactivity.In the radioactivity hourglass upper part, that gradually empties, are decaying nuclei.For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.Later called Ötzi the Iceman, small samples from his body were carbon dated by scientists.The carbon 14 present in the atmosphere is constantly renewed.The cosmic rays originating from the Sun collide with nuclei in the upper atmosphere and are capable of breaking off individual neutrons.


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