The carbon dioxide is absorbed by plants, and the plants are eaten by animals, thus contaminating every living thing on earth with radioactive carbon. As time passes, the C14 in its tissues is converted back into nitrogen.If we know what the original ratios of C14 to C12 were in the organism when it died, and if we know that the sample has not been contaminated by contact with other carbon since its death, we should be able to calculate when it died by its C14 to C12 ratio.PROBLEMS WITH RADIOCARBON DATING During the last 30 years, a new method of determining C14/C12 ratios has been developed.It uses accelerator mass spectrometry to determine the amounts of C14 and C12 in a small sample which is vaporised in the test.It is the supposed accuracy of the new method that allows measurements sensitive enough to date objects claimed to be more than twenty or thirty thousand years old.A recent test by the British Science and Engineering Research Council has shown that the accuracy of the new technique is greatly overrated.The radioactive carbon has six protons and eight neutrons in its nucleus, giving it a total atomic mass of 14.
Every 5,730 years, approximately half of this radioactive carbon spontaneously converts itself back into nitrogen by emitting an electron from a neutron.
When they strike ordinary atoms in the upper atmosphere, the cosmic rays smash them apart. Some of these neutrons then collide with nitrogen atoms.
This collision is less destructive than the initial collision that produced them.
Usually a proton is knocked out of the nitrogen atom's nucleus and is replaced with the neutron.
The proton takes an electron with it and becomes an atom of hydrogen.