Cosmic radiation generates neutrons in the atmosphere, which split nitrogen 14 into hydrogen and carbon 14. Plants consume carbon dioxide, including small quantities of radioactive C14 atoms. When those plants, the animals that ate them, and the animals' predators die, they stop consuming the C14 isotope, which decays to half its original amount in about 5730 years. Observing the number of decay events per minute in fresh and historic samples makes it possible to estimate the age of a sample.
• The radius of the yellow disk represents the amount of C14 left in a probe after the estimated number of years.
• The yellow line shows how the number of decay events per minute in one gram decreases during 50000 years.
• The blue line marks the number of decay events in fresh carbon from living matter.
• The red line marks the number of decay events in a sample.
• The green line shows the decay events corresponding to the estimated age.