A radiographer is deciding which radioactive isotope to use for two different medical procedures. One procedure requires a tracer to be injected into a patient so that internal organs can be imaged. The other procedure requires a source to be used externally to destroy cancerous cells inside the body. Explain why a gamma-emitting isotope with a short half-life is suitable for use as a medical tracer, and explain why a gamma emitter is also used in the treatment of cancerous cells.

OCR A-Level Physics B: Advancing Physics (H557) — 6.1 Capacitors · Explain · 5 marks · View as Markdown

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

Radioactive isotopes are used in medicine both for imaging the inside of the body and for destroying cancerous cells. The choice of isotope depends on the type of radiation it emits and its half-life.

Model answer (5 marks)

Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue without being absorbed and be detected outside the body.
Gamma radiation has the least ionising power, so it causes minimal damage to healthy cells as it travels through the body.
A short half‑life means the isotope decays quickly, so the patient is not exposed to radiation for a long period of time – the activity rapidly decreases after imaging.
The half‑life must be long enough for the tracer to travel through the body and be detected before it decays away.
In cancer treatment, gamma radiation from an external source can be directed at cancerous cells, which absorb the energy and are killed.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "most penetrating" and "least ionising power". Show the logical link between short half‑life and patient safety for imaging. Remember to mention both the penetration and the low damage for the tracer, and the directed energy for treatment.
  • common_mistakes
  • :
  • Confusing gamma with beta or alpha radiation. Failing to mention the short half‑life as a safety advantage for imaging. Using vague terms like "radiation" without specifying gamma or its properties.

Mark scheme (5 marks)

  1. Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue without being absorbed and be detected outside the body.
  2. Gamma radiation has the least ionising power, so it causes minimal damage to healthy cells as it travels through the body.
  3. A short half-life means the isotope decays quickly, so the patient is not exposed to radiation for a long period of time / the activity rapidly decreases after imaging.
  4. The half-life must be long enough for the tracer to travel through the body and be detected before it decays away.
  5. In cancer treatment, gamma radiation from an external source can be directed at cancerous cells, which absorb the energy and are killed.

Key terms in this question

half-life · tracer · cancerous cells

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