A patient has been referred for a medical scan to investigate a suspected blockage in their digestive system. The medical team is considering injecting a radioactive tracer into the patient. Explain why a gamma-emitting isotope with a short half-life is most suitable for use as a radioactive tracer in this procedure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Radioactive tracers are used in hospitals to produce images of the inside of the body. The tracer travels through the body and is detected externally by specialist equipment.
Model answer (5 marks)
1. Gamma radiation is the most penetrating type of radiation.
2. This allows the gamma radiation to escape the body and be detected by external equipment.
3. Alpha or beta radiation would be absorbed in tissue and would not reach the detector.
4. A short half‑life means the isotope does not remain strongly radioactive in the body for long; the activity quickly falls after the scan.
5. The half‑life must still be long enough for the tracer to travel through the digestive system and be detected before it decays away.
2. This allows the gamma radiation to escape the body and be detected by external equipment.
3. Alpha or beta radiation would be absorbed in tissue and would not reach the detector.
4. A short half‑life means the isotope does not remain strongly radioactive in the body for long; the activity quickly falls after the scan.
5. The half‑life must still be long enough for the tracer to travel through the digestive system and be detected before it decays away.
Examiner tips
- Use the exact terms ‘gamma radiation’, ‘short half‑life’, ‘penetrating’, ‘external detector’.
- Show the logical sequence: penetration → detection → safety via short half‑life.
- Mention that the half‑life must balance safety and sufficient imaging time.
Common mistakes
- Confusing gamma with beta radiation; students often say ‘beta’ instead of ‘gamma’.
- Forgetting to explain why a short half‑life is desirable for safety and activity decay.
- Over‑generalising the half‑life requirement, e.g. saying it must be as short as possible without noting the need for sufficient imaging time.
Mark scheme (5 marks)
- Gamma radiation is the most penetrating type of radiation
- This allows the gamma radiation to be detected outside the body
- Alpha or beta radiation would be absorbed before leaving the body / would not reach the external detector
- A short half-life means the isotope does not remain strongly radioactive in the body for long / the activity quickly decreases after the scan
- The half-life must be long enough for the tracer to travel through the body and be detected before it decays away
Key terms in this question
radioactive tracer · half-life
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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