Half-Life
Radioactive decay N = N₀·2^(−t/T): amount remaining, decay time and half-life.
What to find:
The amount can be in nuclei, grams, percent or becquerels (activity decays the same way). Time and half-life use one unit. Examples: iodine-131 — 8 days, carbon-14 — 5730 years, radon-222 — 3.8 days.
The radioactive decay calculator finds how many nuclei (or how much substance) remain after a given time, how long it takes for a part to decay and what the half-life is.
Law of radioactive decay
N = N0 · 2−t/T
In each half-life T half of the nuclei decay: after T, 1/2 remains; after 2T, 1/4; after 3T, 1/8.
Example. Iodine-131 has a half-life of 8 days. Of 800 nuclei, 800 / 2³ = 100 remain after 24 days (3 half-lives).
| Isotope | Half-life |
|---|---|
| radon-222 | 3.8 days |
| iodine-131 | 8 days |
| caesium-137 | 30 years |
| carbon-14 | 5730 years |
| uranium-238 | 4.5 billion years |
Decay time: t = T · log2(N0/N). Carbon-14 decay is used to date ancient finds (radiocarbon dating).
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