A hospital is deciding whether to use a radioactive isotope that emits alpha particles or one that emits gamma rays as a medical tracer to be injected into a patient's bloodstream. Explain why the gamma-emitting isotope is more suitable for use as a medical tracer than the alpha-emitting isotope.

OCR A-Level Physics A (H556) — 6.4 Medical imaging · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Medical tracers are radioactive isotopes injected into a patient's body. Detectors outside the body are then used to form an image of internal organs and tissues.

Model answer (5 marks)

1. Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue and be detected outside the body.
2. Alpha particles are the least penetrating type of radiation and would be absorbed by body tissue before reaching the detector.
3. Alpha particles are highly ionising, so they would damage or destroy healthy cells inside the body.
4. Gamma radiation has the lowest ionising power, so it causes less damage to healthy cells as it passes through the body.
5. The tracer isotope should have a short half‑life so that it decays away quickly after the scan, reducing the patient’s total radiation exposure.

Examiner tips

  • Use the exact terminology: ‘penetrating’, ‘ionising power’, ‘short half‑life’.
  • Show the logical sequence: penetration → detection, then safety → half‑life.

Common mistakes

  • Confusing alpha with beta radiation; alpha is not used for imaging.
  • Ignoring the importance of half‑life and over‑emphasising only penetration.
  • Using vague terms like ‘safe’ without linking to ionising power.

Mark scheme (5 marks)

  1. Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue and be detected outside the body.
  2. Alpha particles are the least penetrating type of radiation and would be absorbed by body tissue before reaching the detector.
  3. Alpha particles are highly ionising, so they would damage or destroy healthy cells inside the body.
  4. Gamma radiation has the lowest ionising power, so it causes less damage to healthy cells as it passes through the body.
  5. The tracer isotope should have a short half-life so that it decays away quickly after the scan, reducing the patient's total radiation exposure.

Key terms in this question

gamma ray · alpha particle · tracer

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