# 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.

> AQA A-Level Physics (7408) — 3.10.6 Radionuclide Imaging and Therapy · Explain · 5 marks

## Mark scheme (5 marks)

1. A short half-life means the radioactive source decays quickly / activity falls quickly
2. So the patient is exposed to / receives a lower dose of radiation (reducing risk of damage to healthy tissue / cells)
3. Gamma radiation is weakly ionising / has low ionising power compared to alpha and beta
4. Gamma radiation is highly penetrating and can pass out of the body so it can be detected externally
5. Alpha and beta radiation would be absorbed inside the body / would not reach the detector, causing greater ionisation damage to internal tissue

## Key terms

- [radionuclide](https://www.gradenine.co.uk/glossary/radionuclide)
- [half-life](https://www.gradenine.co.uk/glossary/half-life)
- [gamma radiation](https://www.gradenine.co.uk/glossary/gamma-radiation)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-radionuclide-used-in-d33eb3bd) · Published by Druglandscape Ltd.