6.9.8 Radioactive Dating

• The isotope carbon-14 is commonly used in radioactive dating
• It forms as a result of cosmic rays knocking out neutrons from nuclei, which then collide with nitrogen nuclei in the air:

¹n + ¹⁴N → ¹⁴C + ¹p

• Plants take in carbon dioxide from the atmosphere for photosynthesis, including the radioactive isotope carbon-14
• Animals and humans take in carbon-14 by eating the plants
  • Therefore, all living organisms absorb carbon-14, but after they die they do not absorb any more

• The proportion of carbon-14 is constant in living organisms as carbon is constantly being replaced during the period they are alive
• When they die, the activity of carbon-14 in the organic matter starts to fall, with a half-life of around 5730 years
• Samples of living material can be tested by comparing the current amount of carbon-14 in them and compared to the initial amount (which is based on the current ratio of carbon-14 to carbon-12), and hence they can be dated

Reliability of Carbon Dating

• Carbon dating is a highly reliable ageing method for samples ranging from around 1000 years old up to a limit of around 40 000 years old
  • Therefore, for very young, or very old samples, carbon dating is not the most reliable method to use

• This can be explained by looking at the decay curve of carbon-14:

Carbon-14 decay curve used for carbon dating

• If the sample is less than 1000 years old:
  • The activity of the sample will be too high
  • So, it is difficult to accurately measure the small change in activity
  • Therefore, the ratio of carbon-14 to carbon-12 will be too high to determine an accurate age

• If the sample is more than 40 000 years old:
  • The activity will be too small and have a count rate similar to that of background radiation
  • So, there will be very few carbon-14 atoms remaining, hence very few decays will occur
  • Therefore, the ratio of carbon-14 to carbon-12 will be too small to determine an accurate age

• Carbon dating uses the currently known ratio of carbon-14 to carbon-12, however, scientists cannot know the level of carbon-14 in the biosphere thousands of years ago
• Therefore, this makes it difficult to age samples which are very old