Radioactive Dating (OCR A Level Physics)

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Katie M


Katie M



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:

Radiocarbon Decay, downloadable AS & A Level Physics revision notes

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

Exam Tip

It is important to have good knowledge and understanding of Carbon dating; how it works and its limitations. 

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.