A hospital uses a radioactive tracer to investigate the function of a patient's thyroid gland. The tracer emits gamma radiation and has a short half-life. Explain why a gamma-emitting source with a short half-life is suitable for use as a medical tracer inside the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Radioactive tracers are injected into patients and their movement through the body is monitored from outside using detectors.
Model answer (4 marks)
Gamma radiation can penetrate body tissues, allowing detectors outside the body to record the signal.
Alpha or beta particles are absorbed by tissues and cannot be detected externally.
A short half‑life means the isotope decays rapidly, so its activity falls to negligible levels soon after the scan.
This rapid decay reduces the total radiation dose to the patient, lowering the risk of cellular damage or cancer.
Alpha or beta particles are absorbed by tissues and cannot be detected externally.
A short half‑life means the isotope decays rapidly, so its activity falls to negligible levels soon after the scan.
This rapid decay reduces the total radiation dose to the patient, lowering the risk of cellular damage or cancer.
Examiner tips
- Use the exact phrase ‘gamma radiation can penetrate body tissues’ to secure the first point. Mention ‘alpha or beta would be absorbed’ for the second. Explain ‘short half‑life means activity decreases quickly’ and link to dose reduction for the last two points.
Common mistakes
- Confusing gamma with alpha/beta radiation. Failing to mention the dose‑reduction benefit of a short half‑life. Using vague terms like ‘low risk’ without linking to the half‑life.
Mark scheme (4 marks)
- Gamma radiation can penetrate through body tissues to reach detectors outside the body
- Alpha or beta radiation would be absorbed by body tissues and could not be detected externally
- A short half-life means the activity/radioactivity decreases quickly after use
- This reduces the radiation dose received by the patient, lowering the risk of cell damage or cancer
Key terms in this question
gamma radiation · tracer · half-life
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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