Number of Half-Lives
Compute how many half-lives have elapsed, n = t/T½, and the fraction of radioactive material remaining, (1/2)ⁿ, from the elapsed time and the half-life. With each half-life the amount halves: after 1 half-life 50% remains, after 2 it is 25%, after 10 less than 0.1%. It is the intuitive way to assess how much of a source (or contamination) is left. Enter the elapsed time and the half-life.
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Number of half-lives
The most intuitive way to think about radioactive decay is to count half-lives: n = t/T½. With every half-life that passes, the amount of radioactive material drops by half, whatever the starting amount. One half-life leaves 50%; two leave 25%; three leave 12.5%; ten half-lives leave less than 0.1% (the rule of thumb holds that ~7 half-lives bring a source below 1%, at which point it gets treated as negligible in many settings). The remaining fraction is (1/2)ⁿ. This reasoning decides how long radioactive waste must sit in storage before it becomes safe, or when a radiopharmaceutical has lost its usefulness. Note that decay follows an exponential curve rather than a straight line - it approaches zero without ever reaching it exactly. Enter the elapsed time and the half-life.
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Decay Constant
Compute the radioactive decay constant, λ = ln(2)/T½, from the half-life (T½). The constant λ is the probability of a nucleus decaying per unit time — the larger it is, the more unstable the isotope and the shorter its half-life. It links the half-life (time for half the nuclei to decay) to the activity and to the exponential decay law. Enter the isotope's half-life.
Effective Half-Life
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.