Explain why a radionuclide used in medical imaging must have a short half-life and must emit gamma radiation rather than alpha or beta radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A radionuclide used for imaging must have a short half‑life so that the activity falls rapidly, reducing the total dose the patient receives and minimising the risk of radiation damage to healthy tissue.
It must emit gamma radiation rather than alpha or beta. Gamma rays are weakly ionising and highly penetrating; they can escape the body and be detected by external detectors, allowing a clear image. Alpha and beta particles are strongly ionising but have very short ranges in tissue; they would be absorbed inside the body, causing unnecessary damage and would not reach the detector.
It must emit gamma radiation rather than alpha or beta. Gamma rays are weakly ionising and highly penetrating; they can escape the body and be detected by external detectors, allowing a clear image. Alpha and beta particles are strongly ionising but have very short ranges in tissue; they would be absorbed inside the body, causing unnecessary damage and would not reach the detector.
Examiner tips
- State the two key properties – short half‑life and gamma emission – and explain the benefit of each.
- Use the exact terms ‘short half‑life’, ‘gamma radiation’, ‘weakly ionising’, ‘highly penetrating’ and ‘external detector’ as the mark scheme rewards.
- Keep the answer concise – 5 marks can be achieved in 3–4 short sentences.
Mark scheme (5 marks)
- A short half-life means the radioactive source decays quickly / activity falls quickly
- So the patient is exposed to / receives a lower dose of radiation (reducing risk of damage to healthy tissue / cells)
- Gamma radiation is weakly ionising / has low ionising power compared to alpha and beta
- Gamma radiation is highly penetrating and can pass out of the body so it can be detected externally
- Alpha and beta radiation would be absorbed inside the body / would not reach the detector, causing greater ionisation damage to internal tissue
Key terms in this question
radionuclide · half-life · gamma radiation
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →