A hospital uses a radioactive tracer to help diagnose a patient's condition. The tracer contains a gamma-emitting isotope. Explain why a gamma-emitting isotope is used as a tracer rather than an alpha-emitting isotope, and why an isotope with a short half-life is chosen for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Radioactive tracers are injected into patients and detected outside the body using specialist equipment. The choice of isotope used in a tracer is important for both effectiveness and patient safety.
Model answer (5 marks)
1. Gamma rays are highly penetrating and can pass through the body to reach an external detector.
2. Alpha particles are weakly penetrating and would be absorbed by body tissue before reaching the detector.
3. Alpha particles are highly ionising and would damage the patient's cells and tissues.
4. A short half‑life means the isotope’s activity falls rapidly after the procedure.
5. This rapid decay reduces the patient’s exposure to radiation and lowers the risk of harm after the test.
2. Alpha particles are weakly penetrating and would be absorbed by body tissue before reaching the detector.
3. Alpha particles are highly ionising and would damage the patient's cells and tissues.
4. A short half‑life means the isotope’s activity falls rapidly after the procedure.
5. This rapid decay reduces the patient’s exposure to radiation and lowers the risk of harm after the test.
Examiner tips
- Use the exact terms ‘gamma rays’, ‘alpha particles’, ‘penetrating’, ‘ionising’, ‘short half‑life’, ‘activity’, ‘radiation dose’.
- Show the causal link: penetration → detection, ionisation → damage, half‑life → dose reduction.
- Keep each point concise – one sentence per mark.
- Mention both effectiveness (detection) and safety (low dose).
Common mistakes
- Confusing gamma rays with X‑rays or saying ‘alpha rays’ instead of ‘alpha particles’.
- Failing to explain why a short half‑life reduces patient dose.
- Giving extra detail about decay modes that is not required for the marks.
Mark scheme (5 marks)
- Gamma radiation is highly penetrating / can pass through body tissue to reach an external detector
- Alpha radiation is weakly penetrating / would be absorbed by body tissue before reaching an external detector
- Alpha radiation is highly ionising, which would cause damage to the patient's cells / body tissue
- A short half-life means the activity / count rate of the tracer decreases rapidly after use
- This reduces the patient's exposure to radiation / reduces the risk of harm to the patient after the procedure is complete
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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