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

OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks · View as Markdown

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

Technetium-99m is injected into a patient and detected by a scanner outside the body. It has a half-life of 6 hours and decays into a stable, non-radioactive isotope that is safely excreted by the body.

Model answer (5 marks)

Technetium‑99m is a gamma emitter; the high‑energy γ‑rays penetrate body tissues and can be detected by a scanner outside the body.

Its half‑life is 6 h – long enough for the tracer to distribute through the body and be imaged, yet short enough that the activity falls to negligible levels before the patient is exposed for a long time.

Because the half‑life is short, the patient receives only a brief period of radiation, minimising long‑term risk.

Irradiation occurs when body tissues are exposed to external radiation without becoming radioactive themselves; the patient is irradiated but not contaminated.

Contamination would be the presence of radioactive atoms within the body. 99mTc decays to a stable isotope that is excreted, so any contamination is short‑lived and eliminated, reducing long‑term contamination risk.

Examiner tips

  • Use the exact terms "gamma emitter", "half‑life", "irradiation", "contamination". Show the two key advantages: penetration of γ‑rays and short half‑life. Explain both irradiation and contamination in one sentence each.
  • Keep the answer to 5–6 sentences; avoid extra detail such as decay scheme or chemical properties.

Common mistakes

  • Confusing gamma emission with beta emission. Using "short half‑life" but not linking it to reduced patient exposure. Mixing up irradiation and contamination or omitting one of them.

Mark scheme (5 marks)

  1. Technetium-99m is a gamma emitter, and gamma radiation is the most penetrating type, allowing it to pass through body tissue and be detected outside the body.
  2. A 6-hour half-life means there is sufficient time for the tracer to travel through the body and be detected before it decays away.
  3. The short half-life means the patient is not exposed to radiation for a long period of time, reducing the long-term risk to the patient.
  4. Irradiation is when an object is exposed to radiation from a source but does not itself become radioactive, so the patient's tissues are irradiated but not contaminated.
  5. Contamination is the unwanted presence of radioactive atoms on or in a material; because Technetium-99m decays into a stable isotope that is excreted, the risk of long-term contamination is minimised.

Key terms in this question

half-life · irradiation · isotope

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