A doctor suspects that a patient has a bone tumour. The patient is given a radioactive tracer that accumulates in areas of high bone activity. A gamma camera placed outside the body detects the radiation and builds up an image showing where the tracer has collected. Explain why a gamma-emitting tracer with a half-life of several hours is suitable for this procedure, rather than an alpha-emitting tracer with a half-life of several years.

OCR A-Level Physics B: Advancing Physics (H557) — 6.4 Medical imaging · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A doctor suspects that a patient has a bone tumour. The patient is given a radioactive tracer that accumulates in areas of high bone activity. A gamma camera placed outside the body detects the radiation and builds up an image showing where the tracer has collected.

Model answer (5 marks)

1. Gamma radiation is highly penetrating, so it can pass through body tissue and reach the external detector.
2. Alpha radiation is poorly penetrating and would be absorbed by tissue before reaching the detector.
3. Alpha particles are highly ionising, so an alpha‑emitting tracer would damage healthy cells inside the body.
4. A half‑life of several hours keeps the tracer active long enough for it to distribute and be imaged, but it decays quickly so the patient is not exposed for long.
5. A half‑life of several years would leave the tracer radioactive in the body for a long time, increasing the risk of prolonged radiation damage.

Examiner tips

  • Use the exact terms ‘gamma radiation’, ‘alpha radiation’, ‘penetrating’, ‘ionising’, ‘half‑life’. Show the logical sequence: penetration → detection, then safety → half‑life. Keep each point concise and directly linked to the marks.
  • common_mistakes
  • :
  • Confusing alpha with beta radiation. Failing to mention the safety aspect of the half‑life. Using vague phrases like ‘more radiation’ instead of ‘highly ionising’.

Mark scheme (5 marks)

  1. Gamma radiation is the most penetrating type of radiation, so it can pass out through body tissue to reach the detector outside the body.
  2. Alpha radiation is the least penetrating type of radiation and would be absorbed by body tissue before reaching the detector.
  3. Alpha radiation is highly ionising, so an alpha-emitting tracer inside the body would cause significant damage to healthy cells / internal tissue.
  4. A half-life of several hours means the tracer remains active long enough to travel through the body and be detected, but then decays away quickly so the patient is not exposed to radiation for a long time.
  5. A half-life of several years would mean the tracer stays radioactive inside the patient for a very long time, causing prolonged irradiation and increased risk of harm.

Key terms in this question

half-life · radioactive tracer

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