A patient has been injected with a radioactive tracer that emits beta particles. A second patient has been injected with a different tracer that emits gamma rays. A doctor wants to detect the radiation from outside the body to form an image of an internal organ. Explain why the gamma-emitting tracer is more suitable for this purpose than the beta-emitting tracer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Radioactive tracers are used in medicine to image internal organs. The tracer is injected into or swallowed by the patient, and detectors placed outside the body record the radiation emitted.
Model answer (5 marks)
1. Gamma radiation has the greatest penetrating power of the three types of radiation.
2. It can pass through body tissue and reach the detector outside the body.
3. Beta radiation has only medium penetrating power and would be absorbed by body tissue before reaching the external detector.
4. Gamma radiation has the least ionising power, so it causes less damage to body tissue as it passes through.
5. Beta radiation is more ionising than gamma, so using a beta tracer would cause more harm to healthy tissue.
2. It can pass through body tissue and reach the detector outside the body.
3. Beta radiation has only medium penetrating power and would be absorbed by body tissue before reaching the external detector.
4. Gamma radiation has the least ionising power, so it causes less damage to body tissue as it passes through.
5. Beta radiation is more ionising than gamma, so using a beta tracer would cause more harm to healthy tissue.
Examiner tips
- Use the exact terms ‘penetrating power’, ‘ionising power’ and ‘gamma rays’ as the mark scheme rewards these words.
- Show the logical sequence: greater penetration → external detection, lower ionisation → less harm.
- Keep each point brief and directly linked to the marks listed.
Common mistakes
- Confusing beta and gamma radiation or stating that beta has greater penetration; this loses marks.
- Failing to mention the ionising power difference, which is a separate mark.
- Writing a long paragraph instead of clear, numbered points, which can obscure the required points.
Mark scheme (5 marks)
- Gamma radiation has the greatest penetrating power (of the three types of radiation)
- Gamma radiation can pass through body tissue to reach the detector outside the body
- Beta radiation has medium penetrating power and would be absorbed by body tissue before reaching the external detector
- Gamma radiation has the least ionising power, so it causes less damage to body tissue as it passes through
- Beta radiation is more ionising than gamma, so using a beta tracer would cause more harm / damage to healthy tissue
Key terms in this question
gamma ray · beta particle · radioactive tracer
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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