Explain why the half-life of a radioactive isotope used as a medical tracer must be neither too long nor too short.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Technetium-99m is a gamma-emitting isotope with a half-life of six hours. It is injected into patients so that doctors can monitor organs and detect disease.
Model answer (4 marks)
A half‑life that is too short means the isotope decays before it can travel through the body and be detected.
A half‑life that is too long means the patient is exposed to radiation for an unnecessarily long period of time.
The isotope must be a gamma emitter so that the radiation can penetrate out of the body and be detected externally.
A suitable half‑life means the isotope decays to a safe or stable product after the procedure, minimising long‑term harm to the patient.
A half‑life that is too long means the patient is exposed to radiation for an unnecessarily long period of time.
The isotope must be a gamma emitter so that the radiation can penetrate out of the body and be detected externally.
A suitable half‑life means the isotope decays to a safe or stable product after the procedure, minimising long‑term harm to the patient.
Examiner tips
- Use the exact wording from the mark scheme; each point counts as one mark.
- Show the cause–effect relationship: short half‑life → no detection, long half‑life → excess dose.
- Mention that gamma emission is required for external detection.
Common mistakes
- Confusing half‑life with overall decay time; writing ‘too short means no decay’ instead of ‘no detection’. Forgetting to state that gamma radiation is needed for external detection.
Mark scheme (4 marks)
- A half-life that is too short means the isotope decays before it can travel through the body and be detected
- A half-life that is too long means the patient is exposed to radiation / irradiated for an unnecessarily long period of time
- The isotope must be a gamma emitter so that the radiation can penetrate out of the body and be detected externally
- A suitable half-life means the isotope decays to a safe / stable product after the procedure, minimising long-term harm to the patient
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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