A radionuclide decays at a fixed rate described by its half-life: after one half-life, half of the atoms remain; after ten, about 0.1%. Half-lives range from hours (technetium-99m, 6.01 h) to billions of years (uranium-238). Gamma energies are also fixed for each nuclide, which is why gamma shielding is specified for the named radionuclide.
Reviewed 2026-09-24
The decay law
Activity after time t is A = A₀ × (1/2)^(t / T½). It depends only on the nuclide, not on temperature, pressure or chemical form.
Common radionuclides
| Nuclide | Half-life | Main gamma or X-ray energy | Where you meet it |
|---|---|---|---|
| Technetium-99m | 6.01 h | 140.5 keV | Nuclear medicine imaging |
| Iodine-131 | 8.02 d | 364 keV | Thyroid therapy |
| Radon-222 | 3.82 d | Alpha emitter | Indoor air, from soil and stone |
| Cobalt-60 | 5.27 y | 1.17 / 1.33 MeV | Industrial radiography, irradiation |
| Caesium-137 | 30.1 y | 662 keV | Industrial gauges, calibration sources |
| Americium-241 | 432.6 y | 59.5 keV | Smoke detectors, gauges |
| Potassium-40 | 1.25 × 10⁹ y | 1.46 MeV | Naturally present in the body and food |
| Uranium-238 | 4.47 × 10⁹ y | Alpha emitter | Rocks and soil; nuclear fuel |
Half-lives and energies from NNDC NuDat. Caesium-137’s 662 keV gamma is emitted by its daughter barium-137m.
Why the nuclide matters for shielding
Am-241’s 59.5 keV gamma is easily attenuated by thin high-Z layers, while Co-60’s 1.17 and 1.33 MeV gammas need centimetres of lead or much more concrete. Data measured for one nuclide cannot be extrapolated to another; Radbar’s ALF material data, for example, are listed only for Am-241 and energies below 100 keV.
References
- NNDC NuDat 3, Brookhaven National Laboratory (half-lives and gamma energies)
- IAEA — Radiation, People and the Environment (2004)
This encyclopedia is for general and engineering reference and is not medical advice. Consult radiation protection or medical professionals for individual exposure assessment.