A hospital uses a radioactive tracer to monitor blood flow through a patient's body. The tracer emits gamma radiation and has a half-life of six hours. Explain why a gamma-emitting source with a short half-life is suitable for this medical use.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Gamma radiation can penetrate the body and be detected externally.
Gamma rays cause the least ionisation inside the body, so they minimise damage to cells.
A short half‑life means the activity falls rapidly, so the patient is exposed to radiation for a brief period.
Thus the long‑term dose to the patient is reduced.
If the half‑life were long the tracer would remain active for a long time, increasing the risk of radiation damage.
Gamma rays cause the least ionisation inside the body, so they minimise damage to cells.
A short half‑life means the activity falls rapidly, so the patient is exposed to radiation for a brief period.
Thus the long‑term dose to the patient is reduced.
If the half‑life were long the tracer would remain active for a long time, increasing the risk of radiation damage.
Examiner tips
- Use the word ‘penetrate’ to show understanding of gamma rays. Mention ‘least ionisation’ to explain low tissue damage. Explain the effect of a short half‑life on patient exposure. Link the short half‑life to reduced long‑term dose.
Common mistakes
- Confusing gamma rays with alpha or beta radiation. Failing to mention the reduced ionisation of gamma rays. Not explaining the link between half‑life and patient dose.
Mark scheme (5 marks)
- Gamma radiation can penetrate through the body / tissues to be detected outside the body
- Gamma causes the least ionisation (inside the body), so causes least damage to cells/tissue
- A short half-life means the activity / level of radiation decreases quickly
- So the patient is not exposed to radiation for a long time / reduces long-term radiation dose to the patient
- A long half-life would mean the source remains active / patient stays radioactive for a long time, increasing risk
Key terms in this question
gamma radiation · half-life · radioactive tracer
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Radioactivity questions
- Explain why alpha radiation is more ionising than gamma radiation, and why alpha…
- A nuclear power station uses uranium-235 as fuel. During operation, the uranium-…
- A student investigates the count rate from a radioactive source over several wee…
- A technician is storing radioactive sources in a laboratory. One source emits al…