Radiocarbon Dating

Counting carbon 14 atoms in a bygone object to find its age

The most common of the radioactive dating techniques currently in use involves the isotope 14 of carbon, the radiocarbon. This radioactive isotope of carbon is present in the atmosphere in trace amounts, and in chemical processes is indistinguishable from normal carbon 12. As a result, animal and plant life regularly assimilate carbon 14 atom together with the usual carbon 12.

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. These neutrons, once freed, can interact with atoms of nitrogen 14 in air, causing the expulsion of a proton and the formation of carbon 14.

Formation of carbon-14 from atmospheric nitrogen.

One naturally assumes that the cosmic bombardment responsible for this transmutation remains constant over the millennia. The rate of cosmic rays which hit the Earth depends on two very slowly changing factors: the solar activity and the Earth's magnetic field. This latter serves as a shield against all cosmic radiation - when its strength goes down, the bombardment increases, as does the number of carbon 14 atoms.

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. 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.

The fundamental hypothesis in these estimations is that the rate of radioactive carbon existing when the organism was living would have been the same as the rate in a similar organism alive today. The ratio of the activities of the fossilized and living bodies then provides an age. The estimation assumes that the rate of formation of atmospheric carbon 14 has not changed since the days when the fossil was alive. This is not entirely true and it is necessary to readjust the time and make corrections.

ARTEMIS : a facility to count carbon-14 atoms.
When the remains to date are very old, the nuclei of carbon-14 become so rare that the observation of their decays becomes impractical. One has to count the carbon-14 atoms themselves. This is done in facilities designed for this purpose, made of a mass spectrograph associated with a small accelerator. Samples of a few milligrams of the vestige to date are introduced in the installation which allows to measure the isotopic ratios of the ordinary carbon and its radioactive isotope. The photograph shows the CEA ARTEMIS facility in Saclay (France).

The measurement of carbon-14 activity requires to collect a sufficiently large of the fossil. Obtaining such a sample can be tricky. 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. For ancient sample, it may becomes too low for an accurate measure.

One of the key breakthroughs of recent years has been the development of techniques sensitive enough to directly count the number of carbon 14 atoms present in a sample instead of counting their rare disintegrations. Thanks to a 'mass spectrometer' connected to a particle accelerator, physicists are able to count radiocarbon atoms at the rate of one in 1000 trillion (10 to the power -15), and thus go back 50,000 years in time. The key advantage is to require minute samples of fossil for the dating.

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.

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