How this calculator works
Apply one exponential decay law to a single independently decaying population. Initial and remaining quantities can represent atoms, activity or mass of the same parent species, provided they use the same units. Daughter products, sources, removal and multiple decay components are not included. A finite inverse age requires a positive remaining quantity.
Formula
N(t) = N0 exp(-lambda t); lambda = ln(2)/T; mean lifetime = 1/lambda.
Worked example
With N0 = 100, T = 10 s and t = 20 s, N(t) = 25. With t = 10 s, the remaining quantity is 50.
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Frequently asked questions
Is mean lifetime the same as half-life?
No. Mean lifetime equals half-life divided by ln(2), about 1.443 times the half-life.
Can a sample reach exactly zero?
The exponential model approaches zero but never reaches it at finite time.
Does this describe mixed isotopes?
Only if the measured quantity follows one decay constant. Mixed populations usually need a sum of exponentials.