Free Radioactive Decay Calculator
Calculate remaining activity, number of atoms, and mass of a radioactive isotope after a specified time period.
Bq
days
days
Remaining Activity
125,000 Bq
Fraction Remaining12.5000 %
Fraction Decayed87.5000 %
Half-Lives Elapsed3.00
Remaining Activity vs Elapsed Time
Radioactive Decay Law
Radioactive decay follows first-order kinetics. The activity (and number of atoms) decreases exponentially with time.
Formula
A(t) = A0 × e^(-lambda × t) = A0 × (1/2)^(t/t½)
where lambda = ln(2)/t½ is the decay constant. After one half-life, 50% remains. After two, 25%. After ten, only 0.098% remains.
Example Calculation
I-131 (t½ = 8 days) with initial activity 1 MBq after 24 days.
- 01Half-lives elapsed = 24 / 8 = 3.0
- 02Fraction remaining = (1/2)³ = 0.125 = 12.5%
- 03A(24) = 1,000,000 × 0.125 = 125,000 Bq
Frequently Asked Questions
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