Mathematics
The formula for decay correcting is:
where is the raw, (uncorrected) count at time t, is the count at time 0 (the decay corrected counts), "e" is the "inverse natural log" function, "k" is the decay constant (or "lambda"), and "t" is the elapsed time.
The decay constant is where .693 is (approximately) the and "" is the half-life of the radioactive material of interest.
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