A patient requires a medical scan to investigate blood flow through the heart. A radioactive tracer containing technetium is considered, but a doctor also considers using an alpha-emitting tracer instead. Explain why technetium is a more suitable choice as a medical tracer than an alpha-emitting isotope for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Technetium has a half-life of 6 hours and is a gamma emitter. It is injected into the bloodstream and detected by external sensors placed outside the body. The scan takes approximately 2–3 hours to complete.
Model answer (5 marks)
Technetium is a gamma emitter, and gamma rays are the most penetrating type of radiation.
Gamma radiation can pass through body tissue, allowing external sensors to detect it without needing to be inside the body.
Alpha radiation is stopped within a few centimetres of air and cannot penetrate body tissue, so it would not reach the external detectors.
Alpha particles are highly ionising and would damage healthy cells and tissues inside the body if used as a tracer.
Technetium’s half‑life of 6 h is long enough to allow a 2–3 h scan to be completed, yet short enough that it decays quickly, minimising long‑term radiation exposure.
Gamma radiation can pass through body tissue, allowing external sensors to detect it without needing to be inside the body.
Alpha radiation is stopped within a few centimetres of air and cannot penetrate body tissue, so it would not reach the external detectors.
Alpha particles are highly ionising and would damage healthy cells and tissues inside the body if used as a tracer.
Technetium’s half‑life of 6 h is long enough to allow a 2–3 h scan to be completed, yet short enough that it decays quickly, minimising long‑term radiation exposure.
Examiner tips
- Use the word ‘gamma emitter’ and ‘alpha emitter’ early. Mention penetration and ionising power. Include the half‑life reasoning. Keep each point concise and directly linked to the marks.
Common mistakes
- Confusing gamma with X‑ray or calling alpha radiation ‘penetrating’. Forgetting to mention the half‑life or its suitability for the scan duration. Using vague terms like ‘safe’ without explaining why.
Mark scheme (5 marks)
- Technetium is a gamma emitter, which is the most penetrating type of radiation
- Gamma radiation can pass through body tissue to be detected by sensors outside the body
- Alpha radiation is stopped within a few centimetres of air / cannot penetrate body tissue
- Alpha radiation is highly ionising, which would damage healthy cells / tissue inside the body
- Technetium's half-life of 6 hours is long enough to complete the scan but short enough to decay away quickly, reducing long-term radiation exposure to the patient
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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