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The nuclei of radioactive atoms are unstable. They break down and change into a
completely different type of atom. This is called radioactive decay. For example,
carbon-14 decays to nitrogen-14 when it emits beta radiation.
It is not possible to predict when an individual atom might decay. But it is possible
to measure how long it takes for half the nuclei of a piece of radioactive material
to decay. This is called the half-life of the radioactive isotope.
There are two definitions of half-life, but they mean essentially the same thing:
1. the time it takes for the number of nuclei of the isotope in a sample to halve
2. the time it takes for the count rate from a sample containing the isotope to
fall to half its starting level
Different radioactive isotopes have different half-lives. For example, the half-life of
carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.
It is possible to find out the half-life of a radioactive substance from a graph of
the count rate against time. The graph shows the decay curve for a radioactive
The decay curve for a radioactive substance
The count rate drops from 80 to 40 counts a minute in two days, so the half-life is
two days. In the next two days, it drops from 40 to 20 - it halves. In the two
days after that, it drops from 20 to 10 - it halves again - and so on.