Half-Life of Common Radioactive Isotopes — Complete Reference Table
Radioactive half-lives span an absurd range. Polonium-214 decays in 164 microseconds. Uranium-238 takes 4.47 billion years — roughly the age of the Earth. Both follow the exact same exponential decay law; the only difference is the timescale. Here's a comprehensive reference table covering the isotopes you're most likely to encounter in physics, chemistry, medicine and geology.
Medical isotopes
These are the isotopes used in nuclear medicine for imaging and therapy. Their half-lives are chosen to be short enough that radiation exposure is limited, but long enough to be practically useful.
| Isotope | Half-Life | Decay Mode | Use |
|---|---|---|---|
| Technetium-99m | 6.01 hours | γ (isomeric) | Most widely used medical imaging isotope |
| Iodine-131 | 8.02 days | β⁻ | Thyroid imaging and cancer treatment |
| Fluorine-18 | 109.8 minutes | β⁺ | PET scans (as FDG) |
| Gallium-68 | 67.7 minutes | β⁺ | PET imaging of neuroendocrine tumors |
| Lutetium-177 | 6.65 days | β⁻ | Targeted radionuclide therapy |
| Cobalt-60 | 5.27 years | β⁻, γ | Radiation therapy, sterilization |
| Molybdenum-99 | 65.9 hours | β⁻ | Parent of Tc-99m generators |
Geological and dating isotopes
These long-lived isotopes are the clocks geologists and archaeologists use to date everything from ancient pottery to the oldest rocks on Earth.
| Isotope | Half-Life | Decay Mode | Dating Range |
|---|---|---|---|
| Carbon-14 | 5,730 years | β⁻ | Up to ~50,000 years (organic material) |
| Uranium-238 | 4.468 × 10⁹ years | α | Millions to billions of years (rocks) |
| Uranium-235 | 703.8 × 10⁶ years | α | Millions to billions of years |
| Potassium-40 | 1.25 × 10⁹ years | β⁻, EC | 100,000+ years (K-Ar dating) |
| Rubidium-87 | 4.97 × 10¹⁰ years | β⁻ | Oldest rocks, meteorites |
| Samarium-147 | 1.06 × 10¹¹ years | α | Very old geological formations |
| Thorium-232 | 1.405 × 10¹⁰ years | α | Age of the solar system |
Environmental and safety isotopes
These are the isotopes that matter most for nuclear accidents, fallout, and radiation protection. Their half-lives determine how long contaminated areas remain dangerous.
| Isotope | Half-Life | Decay Mode | Significance |
|---|---|---|---|
| Cesium-137 | 30.17 years | β⁻ | Major fission product, Chernobyl/Fukushima |
| Strontium-90 | 28.8 years | β⁻ | Bone-seeking, major fallout concern |
| Iodine-131 | 8.02 days | β⁻ | Short-term thyroid risk after accidents |
| Plutonium-239 | 24,110 years | α | Nuclear weapons and reactor fuel |
| Americium-241 | 432.2 years | α | Smoke detectors, industrial gauges |
| Radon-222 | 3.82 days | α | Natural radiation exposure in buildings |
| Tritium (H-3) | 12.32 years | β⁻ | Fusion fuel, watch dials, exit signs |
| Krypton-85 | 10.76 years | β⁻ | Fission product, atmospheric tracer |
Extremely short-lived isotopes
At the other extreme, some isotopes exist for fractions of a second. These are mostly studied in nuclear physics experiments and play roles in decay chains.
| Isotope | Half-Life | Decay Mode | Context |
|---|---|---|---|
| Polonium-214 | 164.3 μs | α | U-238 decay chain |
| Polonium-212 | 0.299 μs | α | Th-232 decay chain |
| Radon-220 (Thoron) | 55.6 seconds | α | Th-232 decay chain |
| Nitrogen-12 | 11.0 ms | β⁺ | Nuclear physics research |
| Lithium-4 | ~10⁻²³ seconds | p emission | Beyond the neutron drip line |
The span from 10⁻²³ seconds to 10¹⁰ years covers roughly 40 orders of magnitude. No other physical quantity in everyday use varies by that much. Yet the underlying physics is identical in every case — a quantum mechanical transition probability that happens to be wildly different for different nuclear configurations.
Why half-lives are what they are
The half-life of an isotope depends on the energy and mechanism of its decay. Alpha decay half-lives are extremely sensitive to the energy of the emitted alpha particle — the Geiger-Nuttall law shows that a small change in energy produces an enormous change in half-life. This is because the alpha particle must quantum-tunnel through the Coulomb barrier, and the tunneling probability is exponentially sensitive to the barrier height and width.
Beta decay half-lives depend on the energy available and on selection rules from angular momentum conservation. Forbidden transitions (where the angular momentum change is large) have much longer half-lives than allowed transitions. Gamma decay from metastable states (isomers) can also produce a wide range of half-lives depending on the multipole order of the transition.
Need to calculate remaining quantity or activity for any of these isotopes? Our calculator has 16 built-in presets.
Open Half-Life CalculatorFor the full decay chains showing how uranium and thorium isotopes transform step by step into stable lead, see our decay chain visualizer. For activity calculations in Becquerels and Curies, use the decay and activity calculator.