Technetium-99m is a radioactive isotope used as a medical tracer. Explain why technetium-99m is well suited for use as a medical tracer, including reference to its type of emission, its half-life, and the difference between irradiation and contamination.

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).

Hospitals use radioactive tracers to produce images of the inside of the body. A patient is injected with a small amount of a radioactive substance, which is then detected by scanners outside the body. Technetium-99m has a half-life of 6 hours and is a gamma emitter.

Model answer (5 marks)

Technetium‑99m is a gamma emitter, so the gamma rays it emits can penetrate body tissue and be detected by external scanners.

Gamma radiation has the lowest ionising power of the common radiation types, so it causes less damage to healthy cells while passing through the body.

Its half‑life is 6 h – long enough for the tracer to distribute through the body and be imaged, but short enough that it decays away quickly after the examination.

The patient is irradiated, not contaminated. Irradiation is a temporary exposure to radiation that ends when the source decays or is removed. Contamination would mean radioactive atoms remain on or inside the body, continuing to decay and pose a hazard. The tracer is designed to be excreted, avoiding lasting contamination.

Examiner tips

  • Use the word "gamma emitter" and explain penetration. Mention low ionising power. State the 6‑h half‑life and its dual advantage. Distinguish irradiation (temporary) from contamination (persistent).

Common mistakes

  • Confusing gamma with alpha or beta emission. Forgetting to explain the difference between irradiation and contamination. Giving an incorrect half‑life or not linking it to imaging time.

Mark scheme (5 marks)

  1. Technetium-99m is a gamma emitter, so gamma radiation can pass through body tissue to be detected by scanners outside the body.
  2. Gamma radiation has the least ionising power, so it causes less damage to healthy cells as it passes through the body.
  3. The half-life of 6 hours is long enough for the tracer to travel through the body and be detected, but short enough that it decays away quickly afterwards.
  4. The patient is irradiated (not contaminated) by the tracer — irradiation is temporary exposure to radiation and ends once the source has decayed or been removed.
  5. Contamination is the unwanted presence of radioactive atoms on or in a material, which is hazardous because those atoms continue to decay; the tracer is designed to be excreted by the body to avoid lasting contamination.

Key terms in this question

half-life · irradiation

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