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Radioactive decay and the Mo-99/Tc-99m generator

Radioactivity falls at a constant rate per half-life, and the Mo-99/Tc-99m generator uses the long half-life of the parent (about 66 hours) to supply fresh Tc-99m every day with a short half-life (about 6 hours) and a gamma energy of 140 keV — nearly ideal for the gamma camera.

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Why it matters in the exam

The exam includes calculation questions. Units: the becquerel (Bq) — one decay per second; the curie (Ci) — 3.7×10¹⁰ Bq, so 1 mCi = 37 MBq. The decay formula: A = A₀ × e^(−0.693 t / T½); after one half-life 50% remains, after two 25%, and after 24 hours of Tc-99m (4 half-lives) about 6% remains. Example: 100 mCi of Tc-99m eluted at noon will be about 50 mCi 6 hours later. The effective half-life combines physical and biological: 1/Teff = 1/Tphys + 1/Tbiol, so it is always shorter than both.

In the generator, Mo-99 is bound to an alumina column and decays to Tc-99m; washing with saline ("milking") removes pertechnetate. In transient equilibrium — when the parent half-life is roughly 10 times the daughter's — daughter activity peaks about 23 hours after elution, so elution is daily. Quality checks for each elution: Mo-99 breakthrough (radionuclidic purity) — the accepted limit is 0.15 kBq Mo-99 per MBq of Tc-99m at the time of injection; and aluminium breakthrough (chemical purity), which disrupts labelling and forms colloids.

Emission types: alpha — a heavy particle, very short range, high local damage (Ra-223); beta minus — an electron, range of millimetres (I-131, Lu-177, Y-90); positron — annihilates and creates two 511 keV photons (F-18, Ga-68); gamma — a penetrating photon suited to imaging (Tc-99m). In electron capture and isomeric transition, an electron from an outer shell dropping to an inner shell emits a characteristic X-ray or an Auger electron. Cyclotron isotopes: F-18 (about 110 minutes), C-11 (about 20 minutes), N-13 (about 10 minutes); generator isotopes: Ga-68 from Ge-68 and Rb-82 from Sr-82.

Key points

• Bq — one decay per second; 1 mCi = 37 MBq.
• Tc-99m — about 6 hours, 140 keV.
• Transient equilibrium: Tc-99m peaks about 23 hours after elution.
• Mo-99 breakthrough — up to 0.15 kBq/MBq.
• 1/Teff = 1/Tphys + 1/Tbiol.

Summary table

In the full version for subscribers

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Sources

• Physics in Nuclear Medicine (Cherry, Sorenson, Phelps), 4th edition (2012) — Decay of radioactivity; Radionuclide generators
• IAEA Nuclear Medicine Physics: A Handbook for Teachers and Students, 2015 — Basic physics for nuclear medicine; Production of radionuclides
• European Pharmacopoeia: Sodium Pertechnetate (Tc-99m) Injection (Fission) Monograph, current edition — Radionuclidic purity limits

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.