Search This Blog

Saturday, September 26, 2015

What is Carbon dating?

Radioactive C-14 is produced in our atmosphere by the bombardment of N-14 by neutrons produced by cosmic rays. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things.

Carbon-14 decays with a half life of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute.

Presuming the rate of production of carbon-14 to be constant, the activity of a sample can be directly compared to the equilibrium activity of living matter and the age calculated. 

When living organisms die, their intake of 14C ceases, and the ratio of 14C / 12C   decreases as 14C decays. The period just before 9000 years ago had a higher 14C / 12C ratio by factor of about 1.5 than it does today.

Because the half-life of 14C is 5730 years, it is convenient to use the 14C / 12C ratio to determine the age of objects over a range up to 45,000 years ago.