Calculate remaining activity and decay constant for a radioactive isotope.
Decay is exponential, so each half-life removes half of what remains rather than a fixed amount. Ten half-lives leave under a thousandth of the original activity, which is the usual rule for practical disposal. Isotope selection in imaging balances half-life against dose: too short and it decays before imaging, too long and the patient carries activity unnecessarily.
Half-Life/Decay
A = A₀ × (1/2)^(t ÷ t½); decay constant λ = ln2 ÷ t½
A = A₀ × (1/2)^(t ÷ t½); decay constant λ = ln2 ÷ t½ Decay is exponential, so each half-life removes half of what remains rather than a fixed amount. Ten half-lives leave under a thousandth of the original activity, which is the usual rule for practical disposal.
Isotope selection in imaging balances half-life against dose: too short and it decays before imaging, too long and the patient carries activity unnecessarily.
This calculator takes 5 inputs: Initial activity, Half-life, Half-life unit, Elapsed time, Elapsed time unit. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.