Carbon-14 is a radioactive isotope used in radioactive dating of ancient organic materials. Carbon-14 undergoes beta decay and has a half-life of approximately 5700 years. Explain why carbon-14 is suitable for dating ancient organic materials, and describe the hazards associated with handling a sample that has been contaminated with carbon-14.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Archaeologists use radioactive dating to estimate the age of organic artefacts such as bones and wood. Carbon-14 is continuously produced in the atmosphere and absorbed by living organisms. When an organism dies, the carbon-14 in its body begins to decay and is no longer replenished.
Model answer (5 marks)
Carbon‑14 has a half‑life of about 5700 yr, which is long enough that a measurable amount remains in organic material up to several tens of thousands of years old.
When an organism dies the atmospheric supply stops, so the number of ¹⁴C nuclei decreases by radioactive decay. By measuring the ratio of remaining ¹⁴C to the stable ¹²C (or to the initial ¹⁴C/¹²C ratio) and applying the decay law, the time elapsed since death can be calculated.
Handling a sample contaminated with ¹⁴C is hazardous because the isotope emits β‑particles. β‑particles have medium penetrating power and are stopped by a few millimetres of aluminium, but they are ionising and can damage living cells or DNA. If ¹⁴C atoms are on the skin or clothing they continue to emit β‑radiation, posing a long‑term health risk.
When an organism dies the atmospheric supply stops, so the number of ¹⁴C nuclei decreases by radioactive decay. By measuring the ratio of remaining ¹⁴C to the stable ¹²C (or to the initial ¹⁴C/¹²C ratio) and applying the decay law, the time elapsed since death can be calculated.
Handling a sample contaminated with ¹⁴C is hazardous because the isotope emits β‑particles. β‑particles have medium penetrating power and are stopped by a few millimetres of aluminium, but they are ionising and can damage living cells or DNA. If ¹⁴C atoms are on the skin or clothing they continue to emit β‑radiation, posing a long‑term health risk.
Examiner tips
- Mention the 5700‑yr half‑life first to justify suitability. Explain the decay‑law calculation briefly. State the medium penetration of β‑particles and the aluminium shield. Highlight ionising damage and long‑term contamination risk.
Common mistakes
- Confusing ¹⁴C with ¹⁴C decay products; forgetting the half‑life. Ignoring the need to shield β‑particles with aluminium. Under‑stating the biological hazard of ionising radiation.
Mark scheme (5 marks)
- Carbon-14 has a long enough half-life (approximately 5700 years) to remain detectable in ancient samples
- The ratio of decayed to undecayed carbon-14 nuclei can be used to calculate the age of the sample
- Beta particles have medium penetrating power and are stopped by a few millimetres of aluminium
- Radioactive contamination from carbon-14 on the skin or clothing means the radioactive atoms remain on the object, continuing to emit beta radiation and posing a long-term hazard
- Beta particles are ionising and can damage living cells or DNA, making contamination a health hazard
Key terms in this question
radioactive dating · half-life
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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