Carbon dating

Carbon dating is used to find the age of of plants and animal remains less than 60,000 years old, to within a few years for very recent artifacts, or a few centuries for much older ones.

Natural carbon occurs as three isotopes: ¹²C, ¹³C and ¹⁴C. Unstable ¹⁴C decays with a half-life of 5,568 years. If you left 100 atoms of ¹⁴C on the shelf today, in 5,568 years you would have about 50 atoms of ¹⁴C left. The remaining 50 would have decayed into nitrogen. The important questions are: how do you know how many ¹⁴C atoms you had to start with, and how does this radioactive carbon get into living things?

It all starts high up in the atmosphere. Nitrogen from the air is constantly bombarded by secondary neutrons in cosmic ray showers. Infrequently, one of these neutrons penetrates deep into the nitrogen nucleus and converts it to ¹⁴C. This ¹⁴C then gets oxygenated into ¹⁴CO₂ which plants absorb through photosynthesis, and which gets into animals through the food-chain. The day the plant or animal dies, it stops absorbing ¹⁴C. From then on, this fixed number of ¹⁴C atoms is depleted as the years go by. The number of these ¹⁴C atoms at the time of death is approximately constant for any given type of plant or animal regardless of whether it died today or 10,000 years ago.

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