Half-life — Calculate

How do you calculate the half-life of an isotope from decay data or a decay curve in which background radiation has not yet been subtracted?

First subtract the background count rate from every reading to obtain the corrected count rate (the activity due to the source alone) at each time - this is necessary because background radiation does not decay away and would otherwise make the source appear to decay more slowly (or not reach zero). Using the corrected values (from a table or by plotting a corrected decay curve), the half-life is the time taken for the corrected count rate (or activity, or number of undecayed nuclei) to fall to half of a chosen value. This can be found by reading the time at which the corrected count rate has a certain value, then reading the time at which it has fallen to half that value, and subtracting the two times; repeating for a second halving and averaging improves accuracy.