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Radioactive Dating



Radioactive clocks ...

Our ancestors measured the passing of time with water clocks or hourglasses. Nature has none of our modern watches. It measures time -like our ancestors - by using hourglasses provided by radioactivity. In the radioactivity hourglass upper part, that gradually empties, are decaying nuclei. 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. Direct carbon 14 dating of this painting was carried out by the TANDETRON laboratory in Gif-sur-Yvette, using a highly sensitive method able to measure extremely low amounts of radioisotopes.
CEA

Radioactive hourglasses are used to date the relics of bygone civilizations, by measuring the amount of Carbon-14, whose decay rate allows for precise age calculations. Carbon-14 is a radioactive carbon isotope present in the atmosphere, plants and living bodies. Radioactive dating can also be applied to the dating of rocks as old as the Earth, of coral and volcanic lava. Anything between a few hundred years to several billion years old can be dated. Archaeologists, geologists, physicists can choose between array of radioisotopes

Let us quote the words of one scientist: “We must appreciate the immense favour Nature has done us in providing a table of elements bursting with radioactive clocks capable of functioning over a wide range of timescales. These timescales are associated with elements of vastly differing physical and chemical properties, deeply embedded in many processes going from the inert to the living, from the scale of the mineral to that of our planet.”

Radiocarbon corrections for accurate dating
The changes that have occurred in the Earth’s magnetic field should be taken into account for Carbon 14. The magnetic field effectiveness as a shield against cosmic radiation can vary with time, influencing the amount of carbon 14 in the atmosphere. Other dating methods (using the uranium-thorium ratio, for example) reveal the amount by which the carbon 14 results need to be corrected. For instance, the correction is an approximate 3,000 years increase, for ages of the order of 20,000 years,.
Centre des Faibles Radioactivités (Laboratoire mixte CNRS-CEA)
Dating with carbon 14 is the best-known technique for measuring the age of archaeological or prehistoric artefacts. These measurements are made by comparing the amount of carbon 14 found in an ancient sample (a partially empty hourglass) with the amount present in a today sample (a full hourglass). The older the sample, the more carbon 14 will have decayed and the emptier the hourglass will be. Some corrections are necessary, since the amount of carbon 14 present in the atmosphere has varied over the last 40,000 years.

Sophisticated techniques rely on ‘the disruption of radioactive equilibrium’ that exists in radioactive families. A good example is the dating of coral. Living corals cannot absorb thorium 230 (the fourth descendant of uranium 238) in water, since thorium is insoluble in water. The moment the coral dies, however, the decay of radioactive elements begins to produce thorium 230 inside the coral. The amount of thorium 230 found in the dead coral allows to measure how long it have been dead.

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